Some apparent problems in a high-profile study of ultra-processed vs unprocessed diets

Update 2021-01-27 13:11 UTC: Added the word "apparent" to the post title. That should have been there before.

Update 2021-01-26 14:28 UTC: With the permission of Kevin Hall I have reproduced his responses in line below, in italics and bracketed by [KH... / ...KH]. I believe that these responses adequately address all of the points that I made in the original post.

Update 2021-01-22 23:21 UTC: I have received an extensive response from Kevin Hall, the lead author of the study under discussion here. This addresses the great majority of the points that I raised in this post. I will attempt to incorporate a version of those responses in a forthcoming update, but I wanted to get this acknowledgement of that response out there as soon as possible.

Update 2021-01-21 13:52 UTC: Clarified my use of the terms "processed" and "ultra-processed". In the first version I wrote "I will mostly follow the authors' use of the terms "processed" and "unprocessed" to distinguish between the two". This was sloppy on my part; the authors use "ultra-processed" consistently throughout the article. They mostly use "PROC" and "UNPROC" (or variations on those terms) in the data files, presumably for the easy visual contrast between the two, and that was what I wanted to convey. I also changed "processed" to "ultra-processed" in the title of the post.

(Preamble: This post appears simultaneously with this post by Ethan and Sarah Ludwin-Peery, who have some questions about patterns in the data associated with the article that is discussed. They got in touch with me via Ivan Oransky at Retraction Watch, who is also writing about this today. I recommend that you read their analysis first, not least because it provides a much more comprehensive introduction to the study. Here I discuss a variety of other apparent problems with the same article, which I found while Ethan and Sarah got on with sorting out the mystery of the daily weight changes. Although the contents of this post do not need a great deal of statistical training to follow, they are—as is often the case with post-publication data-based forensics—not exactly a thrill-a-minute ride, but I hope that I have made the implications of each set of points reasonably clear.)

This post looks at an article that first appeared in May 2019 describing a randomised controlled nutrition study. The authors claimed that people who were allowed to eat as much as they wished of a diet based on either "ultra-processed" or "unprocessed" food(*) consumed around 500 kcal/day more on the ultra-processed diet, and gained an average of 0.9 kg (2 lbs) in weight in two weeks, compared to people on the unprocessed diet, who lost an average of 0.9 kg in the same period. The same 20 participants ate both diets, in a randomised order. Importantly, the amount of macronutrients (protein, fat, and carbohydrates) provided in the meals was closely matched across diets, as was the number of calories offered (logically, since calories are a linear function of the macronutrients). That is, the principal claim of the study is that the mere fact that the food was ultra-processed, versus unprocessed, caused people to consume 500 kcal/day more and thus gain, rather than lose, weight in a controlled in-patient setting.

Perhaps not surprisingly, this study has attracted a lot of attention. It has already been cited more than 360 times according to Google Scholar. The National Institutes of Health (NIH), which funded and conducted the study, put out an extensive news release about it, and the story was covered by both Science and Nature, as well as the BBC, the Guardian, the Washington Post, and many other major media outlets.

Here is the full APA-style reference of the article. For the first time since the appearance of the 7th edition of the APA Publication Manual (which says that we now have to list up to 20 authors’ names in a reference) I'm actually going to need an ellipsis to omit some of the 25 authors:

Hall, K. D., Ayuketah, A., Brychta, R., Cai, H., Cassimatis, T., Chen, K. Y., Chung, S. T., Costa, E., Courville, A., Darcey, V., Fletcher, L. A., Forde, C. G., Gharib, A. M., Guo, J., Howard, R., Joseph, P. V., McGehee, S., Ouwerkerk, R., Raisinger, K., ... Zhou1, M. (2019). Ultra-processed diets cause excess calorie intake and weight gain: An inpatient randomized controlled trial of ad libitum food intake. Cell Metabolism, 30(1), 67–77. https://doi.org/10.1016/j.cmet.2019.05.008

The article is published on an Open Access basis; you can find the full text here (PDF, 2 MB) or a fuller version, including the Supplemental Information, here (PDF, 23 MB). A small erratum, correcting a number of minor issues, was published on August 6, 2019; all of the issues mentioned in that erratum are already corrected in the PDF files, so you don't need to keep that to hand while reading the article.

[KH... Importantly, another erratum was published in October 2020 and is available here. The correction relates one of the questions raised below and we realize that the updated data and code were not yet deposited on the OSF website. We will do so. ...KH]

This study has already been the subject of a comment on PubPeer by Edward Archer, who is, I think it is fair to say, a prolific critic of the way that much nutritional research is carried out. I am not a nutrition scientist, so this blog post will mostly concentrate on the data and statistics of the study. I do have one or two small methodological questions too, but these are based only on my 60 years of experience of consuming food and 40 or so of preparing it, rather than any understanding of how nutrition studies are run.

The study

The authors recruited 20 volunteers, 10 male and 10 female, and kept them under observation for 28 days in an in-patient environment at the NIH Clinical Center in Bethesda, Maryland. The data show that between one and four people were in the facility at any point between the first admission on April 17, 2018 and the last recorded data collection on November 19, 2018.

Participants spent 14 days on each of two diets, one described as "ultra-processed" and the other as "unprocessed". The diets were presented on a 7-day rotation, so each participant ate the same meal twice, 7 days apart. Although the purpose of the study was to examine the effect of an "ultra-processed" diet, and that term tends to be used in nutrition science with a specific meaning that is different from "processed" (it's complicated), I will use the terms "processed" and "unprocessed" to distinguish between the two, which I hope will avoid any confusion that might be caused by the fact that "ultra-processed" and "unprocessed" both start with the same letter. The participants were randomised to receive the processed diet first (N=10, 6 male, 4 female) or the unprocessed diet first (N=10, 4 male, 6 female); after 14 days on one diet they immediately switched to the other, as shown here.

Timeline of participants in the study. Reproduced from Figure 1 of Hall et al.'s article.

This study seems to have been a substantial undertaking. The participants spent 28 days in a highly controlled environment. The study was invasive, with subcutaneous sensors to monitor glucose levels as well as multiple finger stick blood testing operations daily. I like to imagine that the participants were handsomely compensated for taking a month out of their lives in the name of science; certainly the budget for the study must have run well into six figures.

The study code and data

The authors have made their data and SAS analysis code available in an OSF repository here. There are two datasets, named ADLDataSAScode and ADLDataSAScode1, each in its own ZIP file. The only difference between these seems to be that ADLDataSAScode1, which was uploaded on August 20, 2019 (three months after the article was first published online, which was on May 16, 2019), contains one extra data file, and the code has been extended with a few lines to produce a table from that file (more on this later). All of the analyses in this post refer to the ADLDataSAScode1 dataset.

Screenshot of the timestamps of the OSF repository for the study. A full-size version of this image is available as part of the supporting files for this post (see "My code and data", below).

The SAS code is not, as one might have hoped, a run-once script that generates all of the tables and figures from the article. Indeed, as supplied, the main script file (ADLDocumentation1.sas) produces two runtime errors at line 61 because the variables created within the SAS data file DLW at lines 42 and 43 are lost when this file is overwritten twice at lines 45 and 46. It seems that the code is best regarded as a collection of "building blocks" of code that can be run individually, possibly with minor modifications to use different subsets of the data. However, for completeness, I patched up the code so that it would run without error messages, and also to include both the original and adjusted analyses of the figures from Table 3D (see "The adjusted weight data", below), and ran it in SAS University edition. I have made the resulting code ("Nick-ADLDocumentation1.sas") and output ("(Annotated) Results_Nick-ADLDocumentation1.pdf") files available online (see "My code and data", below).

The exact length of the study

An issue that stands out immediately when one looks at any of the data files containing daily records is that there seems to be a fencepost error. Participants spent 14 days on each of two diets, with no break in between; their weight at the start of day 1 was the baseline for the first phase (processed or unprocessed diet, assigned at random), and their weight at the start of day 15 was the baseline for the second phase, when they received the other diet. It would seem, therefore, that they should have been weighed 29 times—once at the very start of the study, and then 28 more times after eating a day's worth of meals each time—but there are only 28 daily weight records for each participant. That is, we apparently do not know the effect on their weight of the last (14th) day of the second diet, because the last measurement of their weight on that second diet was apparently the one made on the morning of the 14th day (their 28th in the study), before they proceeded to eat their food and undergo whatever other measurements were performed on that day. This seems to make little sense, from the standpoint of either study design or ethics. Why feed your participants the controlled diet on the last day if you are not going to collect weight data from them relating to that day?

In fact this problem seems to be exacerbated because, as the data files deltabc and deltabw show, the difference in weight retained for each participant on both diets was the difference between their weights at the start of the first and 14th day on that diet. That is, even for the first diet that each person followed, their final weight was the weight at the start of the 14th day in the study, not that at the start of the 15th day; and the effect of the meals that they consumed on the 14th day of the study is also essentially disregarded.

Participant delta weights according to the data files deltabw (top; an Excel filter has been applied to show only values near to the start and end of each diet period) and deltabc (bottom). It can be seen that the retained weight change for each participant and each diet is the difference between their weight at the start of the 14th day on that diet and the start of the first day on that diet, apparently representing making a span of 13 rather than 14 days. The same pattern holds for every participant.

[KH... Participants were admitted the afternoon before the study began. An overnight fasted body weight measurement was collected the next morning (day 1) which served as the fiducial point for the weight change calculations during the next 14 days on the first diet. On the morning of day 15, subjects were weighed which served as the fiducial point for weight change calculations on the alternate diet that was provided after an oral glucose tolerance test (OGTT). Fasted body weight measurements were then collected each morning including day 29 when the final OGTT was performed after which the subject was discharged. Thus, there were 29 fasted body weight measurements for each subject corresponding to the fiducial markers on days 1 and 15 prior to delivery of each diet and 14 days thereafter. However, the reported body weight changes in the manuscript correspond to days 1-14 of the first diet period and days 15-28 of the second diet period as shown in Figure 3A of the manuscript as described as the weight changes on each respective day on the diet. It would have been possible to report body weight changes corresponding to days 1-15 of the first diet period and days 15-29 of the second diet period, but we thought this would have been confusing to readers. ...KH]

Which days did participants spend in the respiratory chamber?

Participants spent one day per week in a respiratory chamber to enable their energy expenditure to be studied in detail. The article states that "On the chamber days, subjects were presented with identical meals within each diet period, and those meals were not offered on non-chamber days" (p. 72), which makes sense from an experimental control point of view, in that all participants would have consumed the same food on that day. The article's Supplemental Information [PDF, 21MB] further states (on pp. 15, 16, 17, 37, 38, and 39) that the chamber day was day 5 of each weekly meal rotation, corresponding to days 5 and 12 of each participant's time on each diet.

However, the great majority of the records in the data file chamber appear to contradict this. I looked for precise matches between the recorded energy intake on the chamber days and the records for each participant in the dailyintake file, and found exactly one match for each participant and chamber day. Support for the idea that these matches are not coincidental is provided by the fact that the calendar dates of each record of the matched pairs (one in chamber and one in dailyintake) are identical. The matched records imply that of the 80 chamber days (20 participants x 2 diets x 2 chamber days per diet), only 7 took place on day 5 of the weekly meal rotation (whereas 2 were on day 1, 24 on day 3, 3 on day 4, 31 on day 6, and 13 on day 7). Furthermore, of the 40 pairs of chamber days within the same diet, 15 were on different meal rotation days within the pair (e.g., for participant ADL002 on the unprocessed diet, the chamber days were 3 and 8, corresponding to the third and first days of the meal rotation, respectively), meaning that the participant would have eaten different meals on their two chamber days for a given diet in 37.5% of cases. It is difficult to reconcile these records with the claims in the article and supplemental information.

[KH... The article and supplement do not claim that “participants did indeed all spend days 5 and 12 of each diet in the chamber”. Rather, the main manuscript describes that participants spent one day each week in the respiratory chambers but does not specify the days of the week. The Supplementary Materials provide information about the rotating 7-day menu of meals provided on each diet and the chamber days were listed as occurring on day 5 of each week. This was not intended to indicate that the chamber days only occurred on day 5 but rather that the meals provided during the chamber days were prespecified and did not vary between subjects on the same diet no matter what day the chamber days occurred. The clinical protocol (available on the OSF website) indicates in Appendix A that the proposed schedule (page 34) had chamber days planned for days 3 and 10 on each diet. However, the protocol also notes on pages 13-14 that “Every effort will be made to adhere to the proposed timelines, but some flexibility is required for scheduling of other studies, unanticipated equipment maintenance, etc. Scheduling variations will not be reported.” Thus, while chamber days varied to accommodate such scheduling challenges, the meals provided on the chamber days were constant within each diet. ...KH]

Counting the calories

The data file dailyintake contains information about the amount of calories and individual nutrients consumed by the participants on each day. The total number of calories consumed is reported to two decimal places, but the individual readings of calories for protein, fat, and carbohydrates that sum to that total are reported to six decimal places, which on visual inspection do not appear to contain any regular patterns (which might correspond to, say, recurring decimals).

Extract from dailyintake file, showing six digits of precision for macronutrient calorie counts. Some columns have been reduced to zero width to enable the image to fit on this web page.

It is not clear how such numbers could have been generated, however, as the process for calculating the amount of calories consumed presumably ought to have been a fairly simple multiplicative one, based on estimates of the numbers of grams of protein, fat, and carbohydrates in the uneaten portions of each food that was offered, after deducting an estimate of the amount of water. (Edward Archer's comment on PubPeer mentions this issue, and suggests that using a bomb calorimeter might have been a better way to measure energy intake, although this doesn't seem to address the split into macronutrient types.) The authors report that the diets were designed and analyzed using ProNutra software, made by Viocare of Princeton, NJ. I wrote to Viocare to ask how this software calculate calories from macronutrients—for example, whether it uses the Atwater values of 4.0 kcal/g for protein and carbohydrates and 9.0 kcal/g for fat, and whether it typically generates long mantissas in its output. Its founder and president, Rick Weiss, sent me this reply: 

ProNutra’s standard nutritional database is from USDA which we load into ProNutra with the resolution as USDA provides. Typically a research group using ProNutra would round off to the decimal place that they need. So I agree, seeing a value to the 6th decimal doesn’t make sense. The analysis of calories from macronutrients does use Atwater values.

[KH... More specifically, ProNutra uses specific Atwater factors which can deviate from the general values of 4.0 kcal/g for protein and carbohydrates and 9.0 kcal/g for fat. Therefore, the assumption immediately below is invalid. ...KH]

But if the calories per gram are always integers, the presence of six decimal places of precision in the macronutrient information of every meal would seem to imply that the authors calculated the amount of food that was (a) served and (b) remained uneaten to the nearest microgram, which would require rather a lot of effort.

[KH... The six decimal points for the macronutrient kcals in the data files are easily explained. The data for the total energy consumed and the percentage from each macronutrient were provided to 2 decimal places. For example, 15.68% of energy consumed as protein and a total energy intake of 2003.47 kcal. Therefore, the kcal provided from protein was calculated to six decimal places in the data file as follows: 2003.47*0.1568 = 314.144096 kcal from protein. ...KH]

I also wonder what was done in the case of processed snacks, where one would expect the authors to have simply used the nutrition information provided by the manufacturers.

[KH... The assumption that we used manufacturer provided nutrition information is not correct. As indicated in the manuscript, nutrient information was obtained from the USDA standard reference databases or if an item was not found in that database, we pulled from the Food and Nutrition Database for Dietary Studies, (also through the USDA). ...KH]

For example, on four days of the processed diet, three participants (ADL006 on days 3 and 4, ADL007 on day 8, and ADL015 on day 9) are recorded in the data file intakebymeal as having consumed 403.14 kcal in snacks, with 42.007956, 202.218222, and 158.933010 kcal coming from protein, fat, and carbohydrates respectively (these amounts are precisely identical on all four days). The chances that three people left exactly the same amount of snack food unfinished on a total of four occasions would seem to be negligible, so this duplication presumably corresponds to these participants having completely finished the contents of the same combination of snack packages on each day. But the nutrition information for each of these packaged snacks reports the amount of macronutrients with a precision of 1 g, so the calories from each of these macronutrients ought also to be an integer (a multiple of 4 or 8), unless the authors perhaps contacted the manufacturers and obtained analyses down to the microgram level.

Three different participants, four different days, identical snack consumption.

[KH... Indeed, ADL006 consumed the same snack items on days 3 and 4 as did ADL007 on day 8 and ADL015 day 9. From the mass consumed (grams), the subjects did finish the entire package of the snacks (28g, 39 g and 113 g for peanuts, cheese & peanut butter crackers, and applesauce, respectively). As explained above, we did not use manufacturer provided nutrition information, but rather nutrition information from the USDA database. Specific Atwater factors were used for the applesauce and the peanuts, whereas general Atwater factors were used for the cheese & peanut butter crackers. As also explained above, the six decimal points in the reported macronutrient kcals resulted from multiplying the macronutrient percentages by the total energy consumed. ...KH]

A further problem here is that these records show that the three participants in question consumed more calories in the form of fat versus carbohydrates from their snacking on these four days, but substantially fewer calories from protein versus carbohydrates. The only processed snack in the image on p. 24 of the Supplemental Information that has more calories from fat than from carbohydrates is the 28 g package of Planters salted peanuts (see my file snacks.xls), but this also has more calories from protein than from carbohydrates. I have not been able to identify any combination of packaged snacks that would get even close to the proportions of calories from protein, fat, and carbohydrates that is reported for these four participants, especially given the presumed constraint of counting only entire packages.

[KH...  The combination of foods that result in these proportions of calories from protein, fat, and carbohydrates was indicated above: 28g, 39 g and 113 g for peanuts, cheese & peanut butter crackers, and applesauce, respectively. 

As an approximate calculation using general Atwater factors, we have:

• Peanuts 28 g providing 163.8 kcal, 6.63 g protein, 13.9 g fat, 6.02 g carbohydrates
• Cheese & Peanut butter crackers 39 g providing 191.88 kcal, 4.21 g protein, 9.55 g fat, 23.01 g carbohydrates
• Applesauce 113 g providing 47.46 kcal, 0.19 g protein, 0.11 g fat, 12.74 g carbohydrates

When summed, these snacks provide 403.1 kcal, 11.03 g protein (44.12 kcal using general Atwater factor), 23.56 g fat (212.04 kcal using general Atwater factor), 41.77 gm carbohydrates (167.08 kcal using general Atwater factor). Thus, most of the total calories come from fat, followed by carbs, and then protein. ...KH]

[Nick: Aaaarggggghhh. When preparing the spreadsheet that I used to try and determine a possible combination of snacks, I somehow entered 2 kcal/g instead of 9 kcal/g for fat. <homer_Doh!.gif > When I correct this, even using the manufacturers' approximate nutrition information, the combination leading to 403 pops right out at me. Apologies for my incompetence on this point. ]

Nutrition information for Planters salted peanuts snack package (source), showing total grams of protein, fat, and carbohydrate. The corresponding calorie amounts would be protein, 7 x 4 = 28 kcal; fat, 14 x 9 = 126 kcal; carbohydrates, 5 x 4 = 20 kcal.

The participants

Participants are identified in the data by sequentially numbered labels from ADL001 through ADL021. That represents a span of 21 unique values, but there are no records with the label ADL011. Whether this is due to an error in assigning a label or a participant dropping out is not clear; however, there is no mention in the article of anyone dropping out of the study.

[KH... ADL011 declined to participate in the study after their successful screening visit when they were assigned their subject number. No participants dropped out or were withdrawn from the study after admission. ...KH]

Participant ADL006 (male) had a baseline BMI of 18.050 kg/m², which is below the minimum specified in the inclusion criteria on pp. e1–e2 of the article (18.5 kg/m²). That is, on the authors' own terms it seems that he ought to have been excluded from the study.

[KH... This participant met inclusion criteria at their screening visit, but their starting BMI was lower once admitted for the study. ...KH]

Participant ADL020 (female) had a baseline BMI of 26.853. During her 14 days on the unprocessed diet she consumed an average of just 836 kcal/day and lost a total of 4.3 kg (9.4 lbs) in weight, accounting on her own for nearly a quarter (23.7%) of the total weight loss of the sample on the unprocessed diet. On day 12 of the same diet she obtained 22% of her calories (128 kcal out of 578 kcal total) from carbohydrates, which was the lowest daily percentage of any participant on any day on either diet in the entire study, whereas on the next day, day 13, she obtained 62% of her energy intake (602 kcal out of 962 kcal total) calories from carbohydrates, which was the highest daily percentage of any participant on any day on either diet in the entire study. This combination of extraordinary weight loss, very low levels of energy intake, and highly variable eating patterns make me wonder how much we can generalise from this participant to a broader understanding of the effects of different types of diet on the wider population. It seems to me that some kind of Hawthorne-type effect may have been present here.

[KH... The limitations of our study regarding generalizability were discussed in the manuscript. It is well-known in human nutrition research that individual subjects have large day-to-day diet variability and that there is large individual variability in weight loss. ...KH]

Errors in the data for individual participants

ADL002

The data file intakebymeal contains one record for every meal consumed by participants during the study (breakfast, lunch, dinner, and one record for all of the snacks that they took) containing an assortment of nutritional information about that meal, including the type of diet that the participant was following on that day (and, hence, at each meal). For participant ADL002, however, something strange seems to have happened. The three meals (but not the snacks) that he consumed on days when he was on the processed diet are marked with the "unprocessed" diet flag, and vice versa, for all 14 days of each diet.

Extract from data file intakebymeal showing that participant ADL002 apparently consumed unprocessed meals and processed snacks on the same day. Some columns have been reduced to zero width to enable the image to fit on this web page.

It is not at all clear how this could have happened, because one would expect the data to have been recorded directly at the end of the day in question (either in a spreadsheet or directly into the ProNutra software) such that the type of diet would either have been completed automatically by the system, or obvious based on the records from the preceding day. Certainly one would expect the snacks for any given day to have the same diet code as the three meals. (I believe that the three meals have the wrong diet code and the snacks have the right one, rather than the reverse, based on the fact that the dailybw and dailyintake files both show ADL002 being on the processed diet for the first 14 days of the study and the unprocessed diet for the last 14 days, whereas intakebymeal shows "unprocessed" as the diet for the breakfast, lunch, and dinner records for the first 14 days, and "processed" for the last 14 days.)

[KH... This error was previously discovered and an erratum was published in October of 2020 that corrected this error and is available here. We realize that we have yet to update the files in the OSF website to correct this previously identified error and apologize for the delay. ...KH]

ADL010

Participant ADL010 has a baseline (day 1, unprocessed diet) weight of 91.97 kg in the data file deltabw but 93.17 kg in the data file baseline. This affects, at least, the results shown in Table S1. If 91.97 kg is the correct weight then the Total mean for weight is correct but the Male mean (79.2 reported, 79.0 actual) and Male SE (6.6 reported, 6.5 actual) are not. If 93.17 kg is correct then the Male mean and SE are correct, but the Total mean (78.2 reported, 78.3 actual) isn't. I have not evaluated the effect of this discrepancy on the headline results of the study, but given that the total weight loss of all 20 participants on the unprocessed diet was 18.07 kg, a difference of 1.40 kg would seem to be potentially quite important.

ADL010's weight on day 2 (versus day 1) is recorded as 93.17 kg in deltabw, so one possibility is that for this participant only, the copying process that generated the baseline table somehow picked up the day 2 value rather than the day 1 value. Interestingly, according to that same file, this participant's weight fell back again to exactly 91.97 kg on day 3, which seems like quite a strong yo-yo effect.

Weight of participant ADL010 in the data files baseline (top) and deltabw (bottom)..

[KH... The baseline information in Table S1 contains body composition measurements obtained by DXA. All of the subjects except ADL001 and ADL010 had their first DXA measurement on day 1, but ADL001 and ADL010 were measured on day 2. For ADL001, their body weight measurements were the same on days 1 and 2, but ADL010 had different weights on these days. Therefore, the body weight measurement on day 2 for ADL010 was included in the baseline information to correctly correspond to the day of the DXA measurement. ...KH]

As with several other issues raised in this blog post, it is not clear how this discrepancy could have arisen with any kind of systematic processing of the study data from raw observations. If values were copied manually across the various data files, one wonders how many other transcription errors might be lurking.

Other oddities in the data

As mentioned above, the data file intakebymeal contains a record for each meal (plus snacks), with information such as macronutrient and total calories, free water consumption, the total mass of the food consumed, etc. Meanwhile, the data file dailyintake has a record for each day's consumption for each participant, broken down similarly. One would therefore expect the values in the four records in intakebymeal to sum to the values in the corresponding record in dailyintake. Curiously, however, this is not the case. Indeed, while the energy intake (EI) field in dailyintake matches the sum of the per-meal EI values in intakebymeal to within 0.05 kcal in every case (once the diet code error for participant ADL002, discussed above, has been corrected), the calories for protein, fat, and carbohydrates from the four meal records each day frequently sum to a total that is some way from the equivalent values in the daily record.

Per-meal (top, with sum for all four meals under "Total") and per-day intake for participant ADL001 on the first day of the processed diet. Note that while the total energy intake ("EI") from the meals is identical to within 0.01 kcal, the total for each of the macronutrients (protein, fat, and carbohydrates) is different by between 9 and 26 kcal. Some columns have been reduced to zero width to enable the image to fit on this web page.

[KH... We have been able to reproduce this problem using the data from several subjects and it appears to be an issue with the ProNutra software. We have contacted the manufacturer to identify the reason for the problem but have yet to receive a reply. However, we agree with the blogger that the magnitude of the discrepancy is very small (tens of calories) and we note that it does not affect the primary study outcome of total energy intake. This issue may be related to the next problem below. ...KH]

A related problem is that, within intakebymeal, the three macronutrient calorie observations for a meal frequently do not sum to the overall energy intake from that same meal. A spectacular example of this is the dinner of participant ADL005 on day 3 of the unprocessed diet, where the macronutrient calories sum to 1235.54 kcal, but whose total energy content is shown as 1720.21 kcal—a net discrepancy of about 484.67 kcal.

Per-meal total and per-macronutrient calories for participant ADL005 on day 3 of the unprocessed diet. Some columns have been reduced to zero width to enable the image to fit on this web page.

A total of 639 of the 2,240 participant x day x meal records in intakebymeal suffer from this problem, whereas none of the records in dailyintake do. Put simply, a large number of the per-meal macronutrient values in the intakebymeal data file appear to be incorrect. Interestingly, all of these discrepancies are on the positive side—that is, when the reported overall energy intake differs substantially from the total of the energy intake from the macronutrients, the former is always larger— suggesting that whatever process is responsible for these discrepancies might not be entirely random.

[KH...  We noticed this problem with the meal data (and not the daily data) when preparing our correction published in Cell Metabolism in October of 2020.  We identified that this was an error in the ProNutra software that listed the fraction of calories coming from all three macronutrients as 0% while correctly providing a value for the total calories for the following food items:                 

Garlic, raw

Lemon juice, fresh squeezed

NutriSource Fiber

OLD FOODS- Oil, olive (Nina)

Oil, olive

Oil, olive (Nina)

Oranges, raw

Pepper, black (Monarch)

Salsa (del Pasado)

Tomatoes, raw

We contacted the manufacturer of ProNutra at the time, but we have yet to receive a satisfactory explanation for this error.  Nevertheless, we corrected these data in the erratum published in Cell Metabolism in October of 2020. We realize that we have yet to update the files in the OSF website to correct this previously identified error and apologize for the delay. ...KH]

The adjusted weight data

I mentioned earlier that the OSF repository for the project contains two ZIP files. The second of these, uploaded after the article was published, includes an extra data file named deltabcadj14, and the SAS code has been extended with a few lines that analyse this file. This code seems to be quite important as it claims to generate the results for figure 3D of the article, which presents what are arguably the headline findings of the study: a mean weight gain of 0.9 kg per participant on the processed diet and a mean weight loss of 0.9 kg on the unprocessed diet. The code file contains this comment:

Update1: Body composition changes presented in Figure 3D are adjusted for 14 days because the body compositions were not measured exactly 14 days apart. In the previous version of SAS code and data, such adjustment was not provided. Here we have updated the SAS code at the section "data for figure 3D" and added a dataset "DeltaBCadj14"; 

It is not clear what adjustments were performed to make this new data file. The extra code provided merely re-runs the comparisons of before/after weight, fat mass, and fat-free mass for the two types of diet, using the adjusted data. When the new code is run, it produces results for the mean weight loss and gain that are around 20% different from the originals; had these numbers been available when the article was published, the authors would presumably have reported a mean gain of 0.8 kg (to one decimal place) on the processed diet and a mean loss of 1.1 kg on the unprocessed diet.

Comparison of pre- and post-study weights (first two lines of each panel, for the processed and unprocessed diets, respectively) and fat/non-fat mass, using the original (top) and adjusted (bottom) data. The output from the original data file contains a descriptive label for each line, which I have removed here to allow the figures in the tables to appear in the same size font for both images.

Interestingly, the sample size for fat mass and fat-free mass on the unprocessed diet is higher with the adjusted data than the original data. The data file deltabc is missing these values for participant ADL002, whereas deltabcadj14 is not. Thus, whatever the adjustment process was, it seems to have extrapolated or interpolated in some way whatever data relating to fat mass might have been missing for this participant, such that he could now be included. (I assume that fat-free mass is calculated as weight minus fat mass, so that only one missing value needs to have been inferred in this way.)

I wonder if this adjustment might be an attempt to compensate for the issue that I raised earlier under the heading "The exact length of the study". But if that is the case, it is not clear why it would be necessary to adjust the values for both diets for each participant. After all, the start of day 15 of the study—the day on which the participants changed to the other diet—ought to correspond to exactly 14 days after they were weighed on day 1. (See also my section "The exact length of the study", above.)

The article states (p. e2) that participants were weighed at 6am every day. If it turns out that they were weighed substantially later on day 1 (or earlier on the last day), the question then arises of whether they skipped one or more meals on that day, although there are records for every scheduled meal in intakebymeal. On the other hand, if they were weighed only an hour or so late, the adjustment hardly seems necessary, especially since the Welch Allyn Scale-Tronix 5702 weighing scale that was used for the study has a precision of only 0.1 kg (a fact that I confirmed by e-mail correspondence with the manufacturer; see also Ethan and Sarah's post, which explores the consequences of this constraint in more detail). The adjusted values are reported to 10 or more decimal places, which—assuming that the adjustment was indeed a function of the difference between the actual elapsed time from the first to last measurement, and exactly 14 days—suggests that the time at which participants' weight and fat mass was measured must have been recorded to a very high degree of precision indeed.

[KH... The question about the precision of the body weight measurements is addressed in our response to the blog post by Ethan and Sarah Ludwin-Peery. These apparent high-precision body weight measurements and the statistical anomalies noted by Ethan and Sarah Ludwin-Peery are explained by subtracting pre-weighed pajamas worn during the body weight measurements as described in the manuscript Method Details section. ...KH]

Two questions arise from this operation:

• First, it would be interesting to know what the adjustment process was. It seems to have been quite powerful, because some of the differences between the original and adjusted values are substantial. For example, for participant ADL014, the loss in weight on the unprocessed diet has been adjusted from 0.10 kg to 0.95 kg, and for ADL005 the equivalent loss has gone from 0.26 kg to 1.79 kg; participant ADL019's gain of 0.30 kg on the unprocessed diet has been adjusted to a loss of 0.24 kg, while participant ADL021's loss of 0.30 kg on the processed diet has been adjusted to a gain of 0.16 kg. These changes appear to affect principally the fat-free mass rather than the fat mass, which in numerous cases (8 out of 20 on the processed diet, 2 out of 19 on the unprocessed diet) is identical to two decimal places after adjustment. For example, participant ADL010's original weight gain of 3.60 kg on the processed diet becomes 2.69 kg in the adjusted file, but his fat mass did not change at all.
• Second, if the authors believe that these adjusted figures provide a better estimate of the effects of the diets, one might wonder why they have not submitted a correction, updating the claims about weight loss that featured in the abstract of their article, rather than allowing this important new information to languish in an OSF repository. Otherwise it is not clear what the point of performing these "adjusted" analyses was.

[KH... The results in the published manuscript correspond to the unadjusted data and code that was originally deposited on the OSF website. The adjustments in the second file on the OSF website were performed to address the fact that the DXA body composition measurements were not performed on exactly at the same time points for all subjects. Furthermore, subject ADL002 was missing one DXA measurement during the unprocessed diet period. The adjusted data attempt to estimate the mean changes in body composition that would have occurred had the DXA measurements been aligned on day 14. To do this, we calculated the slope of the best fit regression line to the fat mass measurements over each diet period to estimate the fat mass change on day 14. The DXA measurement at the end of the first diet period was also used as the fiducial measurement for the start of the second diet period and subject ADL002 contributed only 2 fat mass measurements during the unprocessed diet period. The corresponding body weight measurements on those days were used to calculate the fat-free mass estimates by subtracting the estimated fat masses on those aligned days. This explains the minor differences between mean results reported in the original file deposited in OSF (which correspond to the results published in the manuscript) and the first updated file. The mean results are not materially different between these analyses, and the adjusted data merely address the potential criticism that the DXA measurements were not all conducted on the same days in all subjects. The reported data in the manuscript are not in error. ...KH]

Conclusion

Hall et al.'s article seems to have had a substantial impact on the field of nutrition research. However, both Ethan & Sarah's post and this one raise a number of concerning questions about the reliability of this study. There seem to be problems with the design, the data collection process, and the analyses. I only looked at about half of the 23 data files, so there may be other problems lurking. I hope that the authors and the editors of Cell Metabolism will take another look at this study and perhaps consider issuing a correction of some kind.

[KH... A correction was published in Cell Metabolism in October of 2020 and is available here.

This correction regards an error described by the blogger that we previously independently discovered. Many of the other questions raised above are the result of misinterpretations of the data and the study. We hope that we have now clarified these issues. One remaining question appears to involve the ProNutra software used to calculate the individual macronutrient amounts, but the discrepancies are very small and do not affect the primary study outcome. ...KH]

My code and data

I have made my R analysis code, which reproduces most of the results reported above, here. Some of my results can probably best be checked by examining the data files in a spreadsheet, so my code also includes a loop (which you need to enable, following what I hope are clear instructions) that will export the original SAS data files to CSV format. Also included at the same location is a spreadsheet file named snacks.xls which summarises the nutrition information for the snacks that were served on the processed diet, plus the OSF screenshot and the SAS code and results files mentioned earlier.

Acknowledgements

Thanks to Andrew Althouse and James Heathers for help with the analyses, and to Ethan and Sarah Ludwin-Peery for sharing their discoveries about the Hall et al. article and some very interesting discussions about what it all might mean.

Note on copyright

I believe that the reproduction of two images in this post (Figure 1 of Hall et al.'s article and the Planters nutrition information label) constitute fair use.

Footnotes

(*) I have put these terms in quote marks to emphasise that they have a specific technical meaning. I don't know if that it a good idea, though; perhaps it looks like I am putting Dr Evil-style air quotes around them. That isn't my intention.