Diets weighed up

Thursday February 26 2009

“Low carbohydrate diets, such as Atkins, do not work any better than old fashioned calorie counting,” The Daily Telegraph reported. The newspaper said that researchers have found that diets in which starchy foods like potatoes and pasta are restricted work no better than diets that have no carbohydrate restrictions.

This large, well conducted study tracked over 800 people assigned to different low-calorie diets for over two years. The weight loss by those on a high carbohydrate diet was not significantly different from those on a low carbohydrate diet. This study appears to show that as long as total calories are reduced, the restriction of specific parts of a diet, such as carbohydrates, fat and protein, does not have an effect.

Sticking to diets can be difficult and the participants in this study were supported by regular counselling sessions. Although the low carbohydrate diet in this trial had a target of a maximum 35% carbohydrates, most people did not achieve this. In addition, this is higher than the targets that some Atkins-style diets promote. As the Atkins Diet was not specifically tested, it is not possible to say how it would perform. What is known, is that weight loss diets should be healthy and balanced. Efforts to lose weight are more effective with increased physical exercise.

Where did the story come from?

This study was carried out by Dr Frank Sacks at the Department of Nutrition in the Harvard School of Public Health and colleagues from Brigham and Women’s Hospital, Boston, the Pennington Biomedical Research Center of the Louisiana State University System, Baton Rouge, and the National Heart, Lung, and Blood Institute, Bethesda.

The study was funded by the National Heart, Lung, and Blood Institute and the National Institutes of Health and published in the peer-reviewed The New England Journal of Medicine .

What kind of scientific study was this?

This was a randomised controlled trial conducted at two centres, one in Boston and one in Baton Rouge, in the US.

The researchers explain that the effectiveness of diets in which major changes to how much protein, fat or carbohydrate (known as macronutrients) is eaten has not been established. They also say there are few studies that look at weight loss after one year. This study was aimed at investigating weight change from reducing calories and changing the proportions of these three macronutrients over a two-year period.

The researchers recruited overweight adults between 30 and 70 years of age, with a body mass index (BMI) between 25 and 40. Using questionnaires and interviews, they excluded anyone who had diabetes or unstable heart disease, those on medications that affect body weight and those judged insufficiently motivated. This resulted in 811 suitable people of an average age of 52 and BMI of 33, who were mostly women (62%).

The participants were randomly allocated to one of four diets with a set amount of calories per day. Calorie allowances ranged from 1,200 to 2,400Kcal per day, and were calculated for each individual based on how much weight they needed to lose. Each of the four diets had different amounts of energy derived from fat, protein and carbohydrates.

The first diet (low fat and average protein diet with the highest level of carbohydrate) was aimed at giving an individual 20% of their energy from fat,15% from protein and 65% from carbohydrate. People on the second diet (low fat and high protein with the second highest level of carbohydrate) had 20% of their energy from fat, 25% from protein and 55% from carbohydrate. The third diet (high fat and average protein, with the third highest level of carbohydrate) delivered energy by 40% fat, 15% protein and 45% carbohydrate. The fourth diet (high fat and high protein with the lowest level of carbohydrate) comprised 40% fat, 25% protein and 35% carbohydrate.

The study was designed so that the researchers who measured the outcomes were unaware which diet each participant was on. Efforts were made to maintain this blinding by using similar foods for each diet. The prescribed foods were heart healthy and all the participants were offered group and individual dietary advice for the two years. Group sessions were held once a week, three out of four weeks during the first six months and then two out of every four weeks from six months to two years. Individual sessions were held every eight weeks for the entire two years. The participants were also set goals for physical activity (90 minutes of moderate exercise per week). This was monitored by questionnaire and by an online self-monitoring tool.

The researchers measured the change in body weight after two years in two main comparisons: low-fat versus high-fat diets and average-protein versus high-protein diets (this approach pooled results across the different carbohydrate levels). They also compared body weight in the groups who followed the highest and lowest carbohydrate content diets. In addition to weight, other measures of heart health, such as blood pressure and cholesterol, glucose and insulin levels, were also examined.

What were the results of the study?

After the first six months, people on each diet had lost an average of 6kg, which was about 7% of their body weight. After this, the participants gradually put weight back on over the next 12 months. After two years, all diets groups had achieved a similar weight loss, an average of 3kg.

People on the 65% carbohydrate diet lost an average of 2.9kg and those on the 35% carbohydrate diet lost 3.4kg. At the end of the programme, between 14% and 15% of the people in each group had lost at least 10% of their body weight.

What interpretations did the researchers draw from these results?

The researchers conclude that they “did not confirm the previous findings that low-carbohydrate or high-protein diets caused increased weight loss at six months” and go on to say that “reduced-calorie diets result in clinically meaningful weight loss regardless of which macronutrients they emphasise”.

They say that diets that can be tailored to individual patients on the basis of their personal and cultural preferences may have the best chance for long-term success.

What does the NHS Knowledge Service make of this study?

The main finding of this study is that all four diets were equally successful in promoting weight loss and that this could be maintained to some extent over two years. The study has several strengths:

  • How full the participants felt on the diets (satiety), hunger, diet satisfaction and attendance at group sessions were similar for all the diets. As these aspects are thought to affect how successfully people lose weight in trials such as this, this suggests the results of this trial were not affected in this way.
  • The diets also improved vascular risk factors such as cholesterol and fasting insulin levels, and this suggests that they were having important clinical effects. With the large sample size and the fact that few people dropped out of the study, the researchers were able to show statistical significance of any small changes in weight.
  • The population varied in age, income and included a relatively large percentage of men for this type of study. This improves its relevance for a wider population.

The authors report that most trials of intensive behavioural counselling and dietary advice show relatively small changes in weight over the long term. This is partly because, as in this study, participants often have difficulty achieving the goals for calorie and macronutrient intake.

The conclusions from this study are limited by the fact that not all its participants managed to achieve the proportions of macronutrients prescribed. The authors used the difference in cholesterol levels to estimate the carbohydrate content in the diets. This difference, between the lowest and highest carbohydrate groups, turned out to be 6% of energy instead of the planned 30%. This suggests not many people achieved the macronutrient change. As the Atkins Diet aims to achieve even lower carbohydrate intake than what was tested here, it is not possible to say whether it would be better or worse.

This study was well conducted and provides evidence that aiming for an overall reduction in calories is as effective as trying to alter particular parts of the diet like protein or carbohydrates.