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expert reaction to study looking at calories, time-restricted eating and weight loss

A study published in Annals of Internal Medicine looks at the effect of time-restricted eating on body weight. 


Dr Jacopo Scotucci, Research Associate and Honorary Endocrine Consultant, University of Cambridge and Cambridge University Hospital, said:

Does the press release accurately reflect the science?

“Overall yes, in my opinion.  I am only not sure about the fact that this study has important clinical implications, for the reasons stated below.


Is this good quality research?  Are the conclusions backed up by solid data?

“I think the research is of acceptable quality, although only preliminary and conclusions needs to be backed by stronger evidence.  I am saying this in view of the design limitations highlighted in the press release (e.g. small number of participants, with or without type 2 diabetes), compounded by a non-broadly representative population (93% women, 93% Black), as well as other aspects that make the data more difficult to interpret (lack of detailed data on participants’ adherence to prescribed diet, participants’ calorific need estimated with a formula but not confirmed experimentally, limited assessments of glycaemic profile, including having inadvertently not collected HbA1c).  Therefore, I do not think that significant conclusions can be drawn from this study, and more research is needed to confirm or refute this preliminary data.


How does this work fit with the existing evidence?

“There is currently little and conflicting evidence, so hard to say.  As I mentioned, more rigorous research probably needed.


What are the implications in the real world?  Is there any overspeculation?

“As I stated above, I do not think there are significant real-world implication from this study alone, for the reasons highlighted before.  Also, given that we know already that restricted time eating can be a viable and effective option for some patients regardless of the underlying mechanism of action (e.g. based on their life and work circumstances, simplicity, ease of delivery, lower cost than other diets, etc.), I do not see this piece of information significantly altering clinical practice.”


Prof Nita Forouhi, MRC Epidemiology Unit, University of Cambridge, said:

“The study findings indicate that there is no benefit of TRE over usual eating pattern for weight loss, when calorie intake is matched in the two groups, but the scientific understanding of this topic is not done and dusted yet, as many unanswered questions still remain.

“The key strength of this trial is that it enabled teasing out the effect of TRE independent of calorie intake, while tightly controlling what is eaten and when it is eaten by providing all food consumed for 12 weeks.  However, there were several problems that may limit reaching firm conclusions beyond the issues of small sample size and short duration already acknowledged by the authors.  There were protocol violations.  It is questionable whether randomisation worked effectively, with several parameters not equally distributed in the two study groups.  The authors acknowledged that participants had substantially greater average weight in the usual eating pattern group (103.7 kg) than the TRE group (95.6 kg) and tested its impact in ancillary analysis, but other factors were ignored, such as notable differences in the two groups for markers of glucose and insulin, cholesterol and systolic blood pressure.  Energy intake information is not provided, but it seems that physical activity energy expenditure was notably reduced in the TRE group, with on average 34 minutes lower time spent active and around 430 lower activity counts per day compared with the usual eating pattern group.  The generalisability of the findings to men and other ethnic groups will need exploring given that 93% of the sample were Black women.

“The study’s findings for the secondary outcomes are inconclusive, for instance the results for improvements in markers of glucose control seemed to be greater in the TRE group but the sample size was not likely adequate to pick up the between-group differences.  The measurement of glycated haemoglobin would have been useful but was omitted in error though it was prespecified.  The use of continuous glucose monitoring is now readily available and would have contributed meaningfully to understanding a more granular effect of the two interventions on whether and how much the glucose levels change over timing of eating, but this was not included.

“As the authors and the Editorial state, TRE can be a useful strategy because it can help to lose weight without the need for counting calories which is not an easy exercise.”


Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:

“This a well controlled trial of effects on bodyweight of time restricted eating (TRE) between 08:00 and 18:00 compared with unrestricted eating between 08:00 and midnight.  The participants were predominantly obese women of African ancestry.  The calorie content of both diets was similar and portion controlled foods were provided for the participants.  After 12 weeks there were no statistically significant differences between treatments in weight change – both groups lost weight (2.3 kg in TRE group vs 2.6 kg).  This indicates that there is no advantage of TRE.  Previous analyses of observational studies suggested the TRE was associated with lower body weight.  The authors conclude that it is more important at to focus on reducing calorie intake. cHowever, for some people, TRE may help them reduce their calorie intake.

“The limitations of the study are its relatively small number of participants and limited proportion of men.  A further limitation is that the study was not designed to reduce calorie intake but it turned out that the estimated energy requirements used to calculate the amounts required in the experimental diets caused weight loss in both groups.  My calculations based on the weight loss suggest the participants were habitually consuming more calories (about 205 kcal/day) than the estimated requirement before the trial.”


Dr Katarina Kos, Senior Lecturer in Diabetes and Obesity, University of Exeter, said:

“Researchers in this study compared effects of different eating patterns by studying a group of obese primarily black prediabetic women eating 80% of their calories by lunchtime (1pm) within the window of 8am to 6pm for food intake as time-restricted eaters (TRE), with women who ate the same amount of calories between 8am and midnight and about 50% with the evening meal.  To be able to ensure no difference between calorie intake in the two groups, all main meals apart from snacks were provided by the researchers as packaged meals and allowances of snacks and calories from drinks restricted.  Food diaries and assessments of physical activity were used in combination to ensure similar energy expenditures.  This was a good attempt to ensure stable calorie intake in free living / non hospitalised individuals, as this is naturally difficult to ensure.

“The study did not find a difference in weight loss between the two groups.  Whilst the researchers try to explain why limiting the food intake to earlier in the day could be beneficial as found in other TRE studies, they felt that this was to be attributed to a reduction in calories with TRE.  Of note however is that other group of women in this study also lost weight which was not dissimilar and by 0.3kg somewhat superior to the time limited eaters.  There is no account of whether a weight loss per se was attempted in any group, though it is known that participation in research focuses the mind which may be the explanation why weight loss was observed in both groups.

“Whilst there is no evidence of superiority between approaches, I suggest for people to choose an eating pattern which is most agreeable to them and helps them with reducing their snacking in the long-term.”


Dr Adam Collins, Associate Professor of Nutrition, University of Surrey, said:

“Time-restricted eating (TRE) has emerged primarily as a form of intermittent fasting.  A TRE protocol incorporates an extended “fast” every 24 hours, instructing participants/patients to reduce their eating window to <12 hours, often around 8-10 hours.  This shortening of the eating window can be achieved synchronously (bringing breakfast later and dinner earlier) or by shifting the eating window to either early (eTRE) or late (lTRE).  TRE and the timing of the eating window also overlap with chrononutrition, considering chronobiology and the circadian effect on meal handling.

“When it comes to intermittent fasting, we know from previous evidence that alternate-day fasting (ADF) or 5:2 models of intermittent fasting, by their nature, create an overall calorie deficit and lead to significant weight loss, comparable to or better than traditional daily calorie restriction.  Similarly, previous studies on TRE have commonly observed weight loss as an outcome, despite no prescribed calorie restriction, likely due to changes in diet behaviour due to restricting the eating window.

“This new study specifically focuses on whether meal timing alone creates differences in weight loss when controlling for calories.  In this regard, the primary outcome of interest is weight loss.  The headline finding that TRE does not magically lead to more weight loss sounds sensational but is also obvious.  While there is some suggestion that metabolic rate and the energy cost of digesting and absorbing food may be influenced by meal timing, studies have failed to show any clear differences, partly due to methodological issues in reliably measuring these.

“What is more interesting and relevant about this study is that it goes some way to address a key question: whether the benefits of intermittent fasting (in this case TRE) are solely due to people losing weight, i.e., what are the independent metabolic effects of “fasting.”  The RCT can help answer this due to the careful matching of calories and macronutrient intake of the TRE and control (UEP) groups, through the use of supplied packaged meals.  However, this would focus more on the secondary outcomes of the study, namely the metabolic parameters (insulin, blood lipids, glucose, and glycemia).  These are known to change favourably in response to weight loss (reduction of adiposity), particularly in those overweight, with obesity, diabetes/prediabetes, or those with cardiometabolic risk factors (dyslipidemia, hypertension, etc.).

“Yet, similarly, these parameters have also been shown in previous studies to change in response to “intermittent fasting”.  Indeed, there is much existing mechanistic evidence to demonstrate the metabolic benefits of meal timing and fasting.  Our own RCT of continuous energy restriction versus intermittent energy restriction (5:2), matched for weight loss, showed benefits of metabolic handling following intermittent fasting not seen in the other group, particularly in terms of lipid handling.

“The failure of this new study to show any differences in these metabolic parameters is likely due to the study being underpowered for these variables, especially given that many of these are not sensitive enough markers of change (e.g., fasted insulin, glucose).  Failure to see differences is also majorly impacted by the inclusion of diabetic and non-diabetic individuals within the groups, which will likely obscure differences in glycemia that may be seen.

“Nevertheless, this remains a useful study to confirm the usefulness of TRE as a potential intervention for favourable dietary change but also eludes to other benefits of TRE that warrant further study.”



Antoni R, Johnston KL, Collins AL, Robertson MD. Effects of intermittent fasting on glucose and lipid metabolism. Proc Nutr Soc. 2017 Aug;76(3):361-368. doi: 10.1017/S0029665116002986. Epub 2017 Jan 16. PMID: 28091348.

Antoni R, Johnston KL, Collins AL, Robertson MD. Intermittent v. continuous energy restriction: differential effects on postprandial glucose and lipid metabolism following matched weight loss in overweight/obese participants. Br J Nutr. 2018 Mar;119(5):507-516. doi: 10.1017/S0007114517003890. PMID: 29508693.

Antoni R, Robertson TM, Robertson MD, Johnston JD. A pilot feasibility study exploring the effects of a moderate time-restricted feeding intervention on energy intake, adiposity and metabolic physiology in free-living human subjects. Journal of Nutritional Science. 2018;7:e22. doi:10.1017/jns.2018.13


Prof Naveed Sattar, Professor of Cardiometabolic Medicine/Honorary Consultant, University of Glasgow, said:

“This relatively small trial seems well done and tells us what we expected – that there is nothing magical about time restricted eating on weight change other than effects to reduce caloric intake.  In the end, we cannot reinvent physics – less calories in, however achieved, means lower weight over time.  If time restricted eating helps some people eat less calories than they would otherwise, great.”


Prof Tricia Tan, Professor of Metabolic Medicine & Endocrinology, Imperial College London, said:

“This is an interesting, if small, study which adds some extra information to what we know about time restricted eating.  The study does suggest that the principal way in which TRE works is to restrict the amount of calories eaten, thus driving weight loss.  This finding is consistent with other studies.

“The best method for weight loss is one that you can sustain long term, and TRE is a very simple method for people to reduce the number of calories they eat, thus helping with weight loss.

“It is important that people undertaking TRE do not compensate for the time restriction by eating more during their chosen time window as this may lead to less effective results.

“The study may not completely be applicable to people in the UK considering TRE as it was conducted in the US with a different mixture of ethnic backgrounds that we would not usually find in the UK, and most participants were women.  However, it is likely that we would find similar results if we did a similar study here.”


Comments on the paper:

“Time restricted eating is a popular method nowadays for people who are seeking to lose weight, and this paper does add some extra information to our knowledge about this method.

“In this paper, the authors did an experiment where they assigned participants to a 10 hour TRE vs a usual pattern of eating, but controlled the calories and food eaten, so that all participants ate the same number of calories each day over 12 weeks.  This diet was not designed to test the effect of a reduced calorie diet.

“Participants had obesity and were in the main ‘pre-diabetic’.

“The key finding was that both groups lost the same and small amount of weight.

“The paper’s results are consistent with the view that the feature that makes TRE work is the calorie restriction rather than the time restriction per se.

“Unfortunately, the study is rather small, and it may be that there are differences between the TRE studied and the usual eating group, but the number of participants was not sufficient to demonstrate a difference.

“Another key issue that may restrict its applicability to people in the UK was that the participants were disproportionately female and the vast majority of participants were Black Americans.”


Prof Keith Frayn, Emeritus Professor of Human Metabolism, University of Oxford, said:

“The finding that time-restricted eating results in no more weight loss than expected for the reduction in calories taken in should surprise nobody.  If time-restricted eating were to have an effect on body weight beyond calorie restriction, that would imply that it ‘ramps up metabolism’, for which there is not a shred of evidence in humans.  That is not to say that time-restricted eating is not a good way to lose weight: it clearly helps some people to reduce their calorie intake.  But this study shows that there is no ‘magic’ effect beyond that.  It should be noted, though, that this is a relatively small study, of just 41 people, predominantly self-reporting as Black, and some with diabetes, and of only 12 weeks duration: convincing evidence against some special effect of time-restricted eating will only come when this study is repeated in larger numbers of people and over a longer time-period.

“On the other hand, it seems plausible that time-restricted eating might improve glucose control and cardiovascular risk factors.  This study found no evidence to support that, but again is probably neither large nor long enough to determine that for certain.”



‘Effect of Isocaloric, Time-Restricted Eating on Body Weight in Adults With Obesity: A Randomized Controlled Trial’ by Nisa M. Maruthur et al. was published in the Annals of Internal Medicine at 14:00 UK time on Friday 19 April 2024.

DOI: 10.7326/M23-3132



Declared interests

Dr Jacopo Scotucci: “No conflicts.”

Prof Nita Forouhi: “No conflicts of interest.”


Prof Tom Sanders: “Member of the Science Committee British Nutrition Foundation.  Honorary Nutritional Director HEART UK.

Before my retirement from King’s College London in 2014, I acted as a consultant to many companies and organisations involved in the manufacture of what are now designated ultraprocessed foods.

I used to be a consultant to the Breakfast Cereals Advisory Board of the Food and Drink Federation.

I used to be a consultant for aspartame more than a decade ago.

When I was doing research at King’ College London, the following applied: Tom does not hold any grants or have any consultancies with companies involved in the production or marketing of sugar-sweetened drinks.  In reference to previous funding to Tom’s institution: £4.5 million was donated to King’s College London by Tate & Lyle in 2006; this funding finished in 2011. This money was given to the College and was in recognition of the discovery of the artificial sweetener sucralose by Prof Hough at the Queen Elizabeth College (QEC), which merged with King’s College London. The Tate & Lyle grant paid for the Clinical Research Centre at St Thomas’ that is run by the Guy’s & St Thomas’ Trust, it was not used to fund research on sugar. Tate & Lyle sold their sugar interests to American Sugar so the brand Tate & Lyle still exists but it is no longer linked to the company Tate & Lyle PLC, which gave the money to King’s College London in 2006.”


Dr Katarina Kos: “I have no conflict of interest.”


Dr Adam Collins: “Currently researching:

  • Mathematical modelling of weight loss
  • Use of Protein in TRE protocol
  • Independent metabolic effects of carbohydrate and energy restriction – piloting a novel intermittent low carb diet. 
  • TRE in the treatment for Diabetes in Saudi.

Conflict of interest wise (besides the University of Surrey):

Consulting for Form Nutrition ( as Head of Nutrition – they are also supporting the Protein and TRE study (above).”


Prof Naveed Sattar: “No COI for this topic.”


Prof Tricia Tan: “I do not have any direct CoI but would like to declare the following:

I am a shareholder and consultant for Zihipp ltd a spinout from Imperial College London.

I have participated in a medical advisory board for Astra Zeneca.”


Prof Keith Frayn: “I am the author of a forthcoming back on calories: ‘A calorie is a calorie’ (Piatkus, Jan 2025), as well as several books on metabolism”

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