November 14, 2018

expert reaction to study looking at low carbohydrate diets and weight maintenance

Research published in the BMJ  demonstrates that lowering dietary carbohydrate increased energy expenditure during weight loss maintenance, which may improve the success of obesity treatment.

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

“This study allocated overweight or obese people to three different levels of carbohydrate intake after they had lost 12 percent body weight on an energy restricted diet containing 45 percent energy as carbohydrate.  The test diets contained 20, 40 and 60 percent of energy from carbohydrate with correspondingly high proportions of fat (60, 49 and 20%); the diets were followed for 20 weeks and body weight was stable, changing less that 1 kg and with no difference between test diets.

“The claim in the press release that a low carbohydrate diet would lead to a weight loss of 10 kg over three years is therefore not supported by the results of this study.

“The primary outcome – which was total energy expenditure measured using an isotopic method – suggested that it was 209 kcal per day higher on the low compared to high carbohydrate diet.  Over 20 weeks if this difference was maintained it would be predicted to result in a weight change of 3-4 kg, which did not occur!

“However, there were no changes in the secondary outcome – resting energy expenditure (measured by oxygen consumption at rest).  This might suggest that the differences were due to physical activity levels, but these did not appear to differ.

“The reasons for the higher energy expenditure on the low carbohydrate diet are uncertain but could be an artifact of the method used to measure energy expenditure, as very low carbohydrate high fat diets induce different pathways for fat oxidation (peroxysomal oxidation which generates more heat as opposed to mitochondial oxidation).  Alternatively, the subjects on the low carbohydrate group may have been eating a little more to maintain energy balance, as weight did not change.

“The low carbohydrate diet also contained a horrendous amount of saturated fat (20.9 percent energy versus the recommended amount) which would increased blood cholesterol levels and risk of heart disease.”

Prof Naveed Sattar, Professor of Metabolic Medicine, University of Glasgow, said:

“This seems like an important finding BUT I would be highly cautious as the interpretation of the results seems quite incorrect.  Whilst the authors suggest they controlled diets in the study, given there was no apparent difference in weight change in the three different groups of carbohydrate intake, it can only mean that the low carbohydrate group ate more calories than the other groups – this is the only way that their weights could have stayed similar since how many calories are burned must broadly equal how much is taken in.  This brings the other point that what really matters is weight change and we already know from much larger and longer trials that weight loss is broadly similar whatever diet one takes (low carb or low fat or others) as long as one sticks to the diet – multiple trials show this.  The current study reports on what trialists call ‘surrogate’ outcomes and the key goal is weight change.

“So in short, I don’t believe this work changes anything and nor does it convince me that low carb diets are meaningfully better to relevant health outcomes.  The overwhelming evidence does not support this.”

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

“This is a well conducted study of a long duration, however it is not a weight loss study but a weight maintenance study.  Subjects who lost or gained weight in the 20 weeks of study duration were excluded from the analysis.

“The three diets offered to the participants were matched for calorie and protein content and were given for the purpose of weight maintenance.  The low carbohydrate diet had the lowest glycaemic index (GI 30 versus 49) and as expected participant with the highest insulin secretion at the beginning (at risk of diabetes), benefited from it most.  Research suggests that diets with low glycaemic index have the benefit of reduced cravings and this is in keeping with a lower hunger hormone ghrelin which was detected in the low carbohydrate group.

“Noteworthy is that the basic metabolic rate was no different between participants of the three diets and it remains unclear how to explain the difference of total energy expenditure e.g. by fidgeting or other factors not measured or accounted for in this study.  It is not appropriate to make calculations on potential weight loss with these diets as this study did not assess weight loss.  It is unrealistic to expect weight loss with no change in calorie content of a diet.  I am confident however, to continue to recommend a low GI diet to people with pre diabetes and a low GI diet including some reduction in calories (such as saturated fats) to those with diabetes.”

Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“I think this is a good, well-designed study, and the statistics have been dealt with appropriately.  In that case, why are the researchers careful to say only that low carb diets ‘could’ or ‘might’ help to people to maintain weight loss?  The researchers make this clear in their report.  They describe several limitations of their study, and the most important for me is that the people that were studied got a carefully controlled diet that they didn’t have to prepare themselves, and they were studied only for a relatively short period in terms of maintaining weight loss (20 weeks).  Even if they didn’t all keep to the diet exactly, that’s not the same as expecting people to stick closely to dietary recommendations off their own bat for long periods, and incorporate the recommendations in their own choices on cooking and eating out.  That’s one of the reasons why the researchers have to be cautious about the implications of their findings for public health recommendations on what we should eat.

“A couple of points about things in the press release.  It says that the meals all had “a similar protein and fat content”.  Well, the proportion of calories from protein was similar (close to 20%) in all the diets.  But since the energy from food has to come from carbohydrates, fat and protein, and the protein contribution didn’t change, those who had diets low in carbs obviously had to have diets that were higher in fat to get the same total amount of calories.  Those who got 60% of their calories from carbs got only 20% from fat, and those who got only 20% from carbs got 60% from fat.  What was similar about the fat in the different diets was the proportion of saturated fat in the fat component (about a third in each case).  And because the different diets had to be made up from foods that may have differed in other ways than just their carbohydrate and fat content, it remains possible that these other differences were responsible for some part of the observed differences in energy expenditure.  That’s why the press release refers to “other unmeasured factors” and why the research report says that further research is needed to investigate this possibility before detailed recommendations on diet can be made.”

‘Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial’ by Cara B Ebbeling et al. was published in the BMJ at 16:30 UK time on Wednesday 14 November 2018.

Declared interests

Prof Tom Sanders: “Honorary Nutritional Director of HEART UK.  Scientific Governor of the British Nutrition Foundation.  He is now emeritus but when he was doing research at King’s 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.  Tom also used to work for Ajinomoto on aspartame about 8 years ago.  Tom was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain.  Tom has previously acted as a member of the Global Dairy Platform Scientific Advisory Panel and Tom is a member of the Programme Advisory Committee of the Malaysian Palm Oil Board.  In the past Tom has acted as a consultant to Archer Daniel Midland Company and received honoraria for meetings sponsored by Unilever PLC.  Tom’s research on fats was funded by Public Health England/Food Standards Agency.”

Prof Naveed Sattar: “No COI.”

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

Prof Kevin McConway: “Kevin McConway is a trustee of the SMC.”