Which diet?

Thursday July 17 2008

The Atkins diet is “safe and far more effective than a low-fat one”, reports the Daily Mail , quoting a two-year study conducted in the canteen of the Nuclear Research Centre in Israel. The staff provided suitable dishes for three different diets, and the researchers looked at how much weight was lost on the low-carb regime compared with either a conventional calorie-controlled low-fat diet or the “Mediterranean diet”, which encourages plenty of vegetables, fibre, white meat, fish and unsaturated fats, such as olive oil.

Those on the conventional low-fat diet lost an average of 2.9kg (6.5 pounds) over the two years – compared with 4.4kg (10 pounds) for those on the Mediterranean diet and 4.7kg (10.3 pounds) on the “low-carb” diet.

Fad diets come and go. The “Atkins diet” in this study discouraged animal fats, and all participants had intensive dietary and exercise advice. The consistent message promoted by these researchers and others is that that more than one dietary approach, according to individual preferences and metabolic needs, may work as long as the effort is sustained.

Where did the story come from?

Dr Iris Shai from Ben-Gurion University of the Negev in Israel and international colleagues from Germany and the US carried out this research. The study was supported by the Nuclear Research Center Negev, the Dr. Robert C. and Veronica Atkins Research Foundation, and the S. Daniel Abraham International Center for Health and Nutrition in Israel. It was published in the peer-reviewed medical journal: The New England Journal of Medicine .

What kind of scientific study was this?

This was a randomised trial comparing the effectiveness and safety of three weight-loss diets. The researchers randomly allocated 322 moderately obese individuals with an average age of 52 years to one of three diets: a low-fat, calorie-restricted diet (conventional diet), a Mediterranean, restricted-calorie diet, or a low-carbohydrate, non-restricted-calorie (Atkins type) diet. Most of the participants were male (86%), and the average body mass index (BMI) was 31kg/m2.

The trial took place between July 2005 and June 2007. The researchers selected people between the ages of 40 to 65 years with a BMI of 27 or more, or those who had type 2 diabetes or coronary heart disease, regardless of age and BMI. Those who were pregnant, breast feeding or who had cancer, bowel, kidney or liver problems were excluded from the trial.

The researchers used an intensive programme of dietary advice, motivation and exercise, based on the materials used in the US diabetes prevention program. Each diet group was assigned a registered dietician who led 90-minute group education sessions at weeks one, three, five, and seven and thereafter at six-week intervals, for a total of 18 sessions. In order to maintain equal intensity of treatment, the workshop format and the quality of the materials were similar among the three diet groups, except for instructions and materials specific to each diet strategy.

The low-fat, calorie-restricted diet was based on American guidelines. It aimed to reduce energy intake to 1500kcal per day for women and 1800kcal per day for men, with 30% of calories from fat, 10% of calories from saturated fat and an intake of 300mg of cholesterol per day.

The moderate-fat, calorie-restricted, Mediterranean diet was based on the recommendations of two other researchers, Willett and Skerrett. It encourages the consumption of vegetables and suggests replacing red meat with poultry and fish. Energy intake was restricted to 1500kcal per day for women and 1800kcal per day for men, with a goal of no more than 35% of calories from fat. The main sources of added fat were 30–45g of olive oil and a handful of nuts (five to seven nuts, <20g) per day.

The low-carbohydrate, non-restricted-calorie diet was based on the Atkins diet. It aimed to provide 20g of carbohydrates per day for the two-month induction phase and immediately after religious holidays, with a gradual increase to a maximum of 120g per day to maintain the weight loss. The intakes of total calories, protein and fat were not limited. However, the diet was modified from the usual Atkins diet in that participants were told to choose vegetarian sources of fat and protein, and to avoid trans fat.

Participants were weighed every month, had fasting blood tests done at the start and at six, 12 and 24 months for cholesterol and other lipids, inflammatory biomarkers and insulin. Their blood pressure and waist circumference was also recorded.

What were the results of the study?

After one year, 95.4% of the participants were still in the study, and this fell to 84.6% after two years.

There were differences between the groups in the amount of fibre and types of fat consumed which were related to their diets. The Mediterranean-diet group consumed the largest amounts of dietary fibre and monounsaturated fat. The low-carbohydrate group consumed the smallest amount of carbohydrates and the largest amounts of fat, protein and cholesterol.

The mean weight loss was 2.9kg (6.5 pounds) for the low-fat group, 4.4kg (10 pounds) for the Mediterranean-diet group, and 4.7kg (10.3 pounds) for the low-carbohydrate group. The researchers also report that the lipid profile (the ratio of total cholesterol to high-density lipoprotein – ‘good’ cholesterol) improved more in the low-carbohydrate group than in the low-fat group. They added that “among the 36 subjects with diabetes, changes in fasting plasma glucose and insulin levels were more favourable among those assigned to the Mediterranean diet than among those assigned to the low-fat diet”.

What interpretations did the researchers draw from these results?

The researchers conclude that the “Mediterranean and low-carbohydrate diets may be effective alternatives to low-fat diets. The more favourable effects on lipids (with the low-carbohydrate diet) and on glycaemic control (with the Mediterranean diet) suggest that personal preferences and metabolic considerations might inform individualised tailoring of dietary interventions”.

What does the NHS Knowledge Service make of this study?

This is a well-conducted and reliable study that has followed a discrete group of workers for two years, with good follow up rates for this type of study. The researchers acknowledge a few limitations:

  • The study enrolled few women and there was a tendency for women to lose more weight on the Mediterranean diet. The researchers suggest that this needs further investigation.
  • There was little data from people with diabetes, making the data on this subgroup less reliable.
  • Self reporting of dietary intake may have led to some inaccuracies. However, the researchers did validate their questionnaire and used computer entry to minimise the amount of missing data.

Overall, although the setting was unique and the intervention was intensive, this study does suggest a model that might be applied to workplace settings. It is possible that similar strategies to maintain adherence to diets could be applied in other countries.

Analysis by Bazian
Edited by NHS Choices