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expert reaction to study looking at Mediterranean diet, levels of fat and body weight

The effect of a high-fat Mediterranean diet on body weight was examined in a randomised controlled trial published in the Lancet Diabetes & Endocrinology in which the authors reported an unrestricted-calorie, high-vegetable-fat Mediterranean diet was not associated with weight gain when compared to an unrestricted-calorie, lower-fat diet in older people at risk of cardiovascular disease. A Before the Headlines analysis accompanied these comments.


Prof. Susan Jebb, Professor of Diet and Population Health, University of Oxford, said:

“This paper describes a large randomised controlled trial to compare dietary advice given to three groups of people in Spain with type 2 diabetes or at high risk of cardiovascular disease. Each group was allowed to eat ad libitum, with no energy restriction. The result is that the weight changes are tiny. The strongest finding from the whole study is that to successfully lose weight you need to limit overall food  (calorie) intake – this is far more important that the particular macronutrient composition.

“However, the main focus of this paper is on the fat content of the diet and the effect on body weight. Here the conclusion is clear – where fat intake between groups only varied from 37 – 42% energy (all higher than the UK average of 35%), the fat content of the diet had minimal effect on body weight, about 0.1 kg per year.

“The paper also shows how hard it is to conduct dietary intervention studies as people find it difficult to follow the advice they receive. Here participants met with a dietitian every 3 months yet the changes they achieved were very small. Fat intake at the start was about 40% of total energy. The two ‘intervention’ groups were advised to follow a Mediterranean diet with the goal of increasing fat intake to more than the initial 40% energy. To do this, participants were provided with either 1 litre olive oil per week (to cover the needs of the whole household) or 30g nuts per day. They managed to increase the fat content of their diet from 40 to 42% fat.

“Usually the ‘control’ group in a trial represents no change, or usual care, but here the control group were advised to adopt a low fat diet, containing less than 30% energy from fat, representing a substantial change from their usual 40% fat diet.  In practice, they only managed to reduce their fat intake from 40% to 37% – a long way from the 30% the investigators aimed to achieve and still higher than the fat content of the UK diet (35%).  This was not a low fat diet.

“So, overall, the differences between groups in terms of fat intake were relatively small and the weight changes, unsurprisingly, not very different. The average weight loss after 5 years was tiny, just -0.604, -0.88 and -0.402 kg in the control, olive oil and nut group respectively – a loss of about 125 grams of weight per year. Interestingly, the greatest difference in weight is seen between the two ‘high fat’ groups following a Mediterranean diet with the ‘control’ group in the middle. So, if you increase the fat content of your diet with foods high in mono-unsaturated fat you may, or may not, lose a teeny bit more weight than a ‘not-quite-so-high-fat’ diet.

“It is impossible from this study to draw any conclusion about the impact of the low fat diets on body weight since each group consumed more than the UK average (35% fat) and way more than the World Health Organisation recommendation (30% fat).”


Dr Gunter Kuhnle, a nutritional scientist at the University of Reading, said:

“This is a very interesting study and its publication just a few days after the controversial report by the National Obesity Forum is very timely by providing actual evidence, but it’s important to be clear that this study does not suggest that if we ate more fat we wouldn’t gain weight. While the PREDIMED study did not investigate extreme low fat or low carbohydrate dietary patterns, it showed that both a reduction in fat and carbohydrate can be used successfully for weight maintenance and that there is no clear superiority.

“The PREDIMED study confirms that a simplistic approach to diet is short-sighted and unlikely to work. Fat, carbohydrates and protein all have a place in our diet and it is wrong to demonise one or the other. Moreover, they have to be seen in the context of a wider dietary pattern and the approach within PREDIMED – to investigate dietary patterns – is very relevant and provides us with a wealth of new data.

Is this good quality research?  Is it robust science?

“The PREDIMED study is a well-known and well conducted dietary intervention study investigating the effects of a Mediterranean diet. In common with long term studies which investigate different dietary patterns, it is of course difficult to ensure and assess compliance. The authors address this to some extend and report very high compliance to the Mediterranean diets – it is however not as clear how well participants adhered to the control low-fat diet. The main endpoint of the study was heart-disease and not weight change – this should be taken into consideration when interpreting these results.  Overall, this is however a well conducted study.

Are the conclusions backed up by solid data?

“The authors report a small reduction in body weight for the low-fat as well as for the Mediterranean diet groups, which is not surprising as following a specific diet often results in weight loss. The differences between the groups are however rather small and barely statistically significant – they are not enough to make any recommendation for or against an individual diet for the purpose of weight loss.

How does this work fit with the existing evidence?

“The results agree with existing evidence that a controlled diet often results in weight loss. It also agrees with existing evidence that shows no clinically relevant differences in long-term weight change between low carbohydrate and low fat diets.

Have the authors accounted for confounders?  Are there important limitations to be aware of?

“As with any long-term dietary intervention studies, compliance can be a problem. Over five years, participants might consciously or unconsciously change their diet, especially following media reports on specific foods. The authors did try to address this as much as possible.

There is a higher drop-out rate in the non-standard Mediterranean diet groups (Mediterranean diet with nuts and control) and it is not clear why.

Is the main result being reported the finding related to the original hypothesis or an incidental finding?

“It is a secondary endpoint of the study, so yes.

What are the implications in the real world?

“The results of this study clearly show that there are many different approaches to weight control and that a one-size-fits-all approach is not necessary. The data clearly show that weight maintenance can be achieved with different types of dietary interventions.

“The differences in fat and carbohydrate intake between different groups were rather modest and therefore the results don’t support either a low-fat or low-carbohydrate approach to weight loss. It is also noticeable that even in the low fat group fat intake was still above current dietary guidelines.

“While the study reports differences in energy intake between the groups, the methods used are not very suitable to determine energy intake accurately and the results need to be interpreted very carefully.

The press release says “More than 40 years of nutritional policy has advocated for a low-fat diet but we’re seeing little impact on rising levels of obesity” – is that true, and if so, do we know why?

“The statement is true, but the question is whether the dietary guidelines did cause an increase in obesity – and this has never been properly confirmed. There have been many changes to our environment in the past 40 years – most jobs have become less physically demanding, energy-rich food has become cheaper – and to focus on one single macronutrient is misleading. This study actually confirms that a low-fat diet can result in weight-maintenance and weight loss.”


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

“This paper claims that it is possible to consume a diet with a high proportion of the energy from fat without gaining weight.  This is something already known but not widely accepted and as the authors point out there is still a fear that advice to raise the upper limit of fat intake from 35% to 42% energy would promote obesity. However, it is important not to confuse a high proportion of energy from fat in the diet with a high intake of fat. It is possible to have a high proportion of energy from fat but with a modest energy intake and remain weight stable, but it is more difficult to maintain energy balance if you eat a lot of fat. Very low fat diets (less than 15% of the energy) definitely lead to weight loss because they are very restricted and often bulky have been the rationale behind some popular slimming regimes (e.g. the Hip and Thigh Diet by Rosemary Conley) but these have never been advocated in dietary guidelines.

“Both the background diet of the participants and the intervention diets were high in fat – indeed they contained more fat than is currently consumed in the UK (currently 35% energy, which is close to the dietary reference intake). The proportion of energy derived from fat fell from 39% to 37% energy on the conventional “low fat” diet whereas on the Mediterranean diet it rose slightly from 39.2 to 41.2% energy and on the Mediterranean diet plus nuts it increased from 39.4 to 41.5%: these changes are trivial and so it is not surprising there were no meaningful effects on weight gain after 5 years – a difference of 0.67 kg lower (about 1.5 lbs) on the Mediterranean diet versus control compared with an increase of 0.4 kg (about 1lb) on the Mediterranean diet plus nuts versus control.

“The context of the Mediterranean diet is important and it would be wrong to interpret this study as showing that eating lots of healthy fats does not result in weight gain. It remains sensible to use oils in moderation and get most of the fat from plant and marine sources. It is worth reflecting that both Greece and Spain have some of the highest rate of obesity in Europe, and Greece consumes the highest proportion of fat in Europe (46% energy). Although the Mediterranean diet is perceived to be a diet rich in fruit and vegetables with lots of olive oil, in the past most of the food energy was derived from plant based foods such as cereals, legumes, nuts and pulses with only small amounts of meat, fish and dairy produce. It seems likely that this former Mediterranean diet probably contained less fat (30-35%) energy and it is this version of the Mediterranean diet, as typified by the DASH diet, that has formed the backbone of current dietary guidelines.”


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

“This study provides pretty consistent evidence of its main conclusions. Over five years, they didn’t find any important differences between people from this population advised to eat either of two variations of a Mediterranean diet, or a diet with reduced fat, in terms of average weight change or average waist measurements.

“There are, however, a few points to bear in mind. The population that was studied consisted of older people living in Spain, who either had type 2 diabetes, or several other risk factors for heart disease and strokes, such as smoking, being overweight or obese, or having high blood pressure or cholesterol levels. Because they lived in Spain, it’s plausible that their diet before the trial began was rather different, possibly more Mediterranean, than the diet of people in other parts of Europe. Over 90% of the people in the trial were overweight or obese before the study began. Would the results have been similar in other populations, who had grown up on other diets or were younger or less overweight? We can’t tell from this study. It’s plausible that the conclusions would be similar in other populations – but this study doesn’t tell us.

“It’s quite striking that, despite being given regular dietary advice and (for those on the Mediterranean dietary advice) being given free extra virgin olive oil or free nuts, the actual changes in diet were not huge (on average). Diets did move in the planned directions, and the changes were statistically significant. But, for instance, in the control group (given advice to reduce fat intake), although the researchers aimed to reduce the percentage of their calories obtained from fat to below 30%, it fell only from 40% to 37.4%. Getting people to make large long-term changes in diet is not easy.

“One issue that does muddy the interpretation is that the way the control participants were treated did change during the trial. Originally they had less frequent dietary advice than did those on the Mediterranean diets, so that their treatment differed from the others. That difference could have changed the effectiveness of the advice, or it could have been a reason for differences (or lack of differences) between the groups in terms of outcomes, which would have nothing to do with what they actually ate. Participants who entered the control group later were given dietary advice as frequently as those on the other diets. That was a sensible change to make, but it’s not really possible to tell how big an effect all this may have had on the results.

“Finally, can I just clear up one point about the nature of the study. This was not the analysis of the primary endpoint of this randomized trial – that was about cardiovascular disease. Sometimes researchers report some other outcome, because they happened to notice something interesting while analyzing the data, and that can lead to severe problems in interpreting the statistics. But that did not happen here – looking at changes in weight and waist size was always something they planned to do. That does increase confidence in the conclusions – though questions do remain about how they might apply in other populations.”


‘Effect of a high-fat Mediterranean diet on bodyweight and waist circumference: a prespecified secondary outcomes analysis of the PREDIMED randomised controlled trial’ by Ramon Estruch et al. published in the Lancet Diabetes & Endocrinology on Monday 6 June 2016. 


Declared interests

Prof. Susan Jebb is employed by the University of Oxford and receives no personal funding from the food industry.  Susan Jebb is conducting research into the treatment of obesity, some of which include support from WeightWatchers, Slimming World and the Cambridge Weight Plan.  Susan was the independent Chair of the Public Health Responsibility Deal Food Network and was a science advisor to the Foresight obesity report. From 2007-10 she was the principal investigator for a research study funded by the food industry to investigate the potential for a functional beverage to help weight loss. The results of this work have been published:

Dr Gunter Kuhnle: – Employment: Associate Professor at the University of Reading.

– Grant funding: Investigation of links between polyphenol intake and health – EU, Mars, Horizon.

– Appointments: EFSA Working group – risk assessment of soy isoflavones.

– Memberships: British Mass Spectrometry Society, British Nutrition Society, Registered Nutritionist (Reg. Nr. 8236); 2011 to 2012 member of ‘Biomarker group’ at ILSI Europe.

– Other financial interests: Vineyard owned by family.

Prof. Tom Sanders: Prof Tom Sanders is a Scientific Governor of the charity British Nutrition Foundation, member of the scientific advisory committee of the Natural Hydration Council (which promotes the drinking of water), and honorary Nutritional Director of the charity HEART UK. Prof. Tom Sanders 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. Kevin McConway: “I have no relevant interests to declare.”

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