Weight loss diets compared

Several newspapers reported on a recent study that compared four popular commercial weight loss diets: the Slim.Fast Plan, Weight Watchers Pure Points Programme, Dr Atkins' New Diet Revolution, and Rosemary Conley's "Eat Yourself Slim" Diet & Fitness Plan. The newspapers approached the study differently, most focusing on the Atkins diet and variously reporting that it is only as effective as other commercial diets, that it and the Weight Watchers diets are the best ways to lose weight, and questioned its safety.

The study was set up to examine the nutrient intake of the four diets, including carbohydrate, fat and protein intake, as well as vitamins and minerals. It found that all the diets caused weight loss while continuing to give adequate nutrition. However, it did not conclude that one diet was better, and found there to be no significant difference in weight loss between the diets. The main conclusion of the study is that although the diets are all nutritionally adequate, people with specific imbalances in the body, such as low iron or folate levels, would benefit from personalised dietary guidance. This seems sensible advice at the current time.

Where did the story come from?

Dr Helen Truby of Children’s Nutrition Research Centre, at the Royal Children’s Hospital, Queensland, Australia, and colleagues from universities in the UK, Northern Ireland and Australia, carried out the research. The study was funded by the BBC and was published in the (peer-reviewed) Nutrition Journal.

What kind of scientific study was this?

This was a randomised controlled trial designed to investigate the macro- and micronutrient intake from four commonly used commercial weight loss diets over an eight-week period.

Participants were recruited through media advertising across the UK and Ireland, and assigned to attend one of five different regional centres. The researchers selected a total of 293 adults (73% female) aged between 18 and 65 years (average age 40), who had a BMI in the overweight or obese range, and were not currently dieting. They had excluded those with coronary heart disease, diabetes, liver or respiratory failure, gout, those taking cholesterol or blood pressure medication, those with depression, substance or alcohol abuse, eating disorders, cancer, known metabolic conditions causing overweight, previous stomach or weight-loss surgery and those with malabsorptive digestive conditions.

The participants were split evenly at random across four groups, each of which followed a different weight loss diet: the Slim.Fast Plan, Weight Watchers Pure Points Programme, Dr Atkins' New Diet Revolution, or Rosemary Conley's "Eat Yourself Slim" Diet & Fitness Plan. Some were also randomly allocated to a control group.

Before the study began, the participants completed a seven-day food and activity diary. The food diary was filled in by estimating the weight of foods and drink they consumed using pictures as guidance. The activity diary was aimed at covering minute-by-minute activities throughout the day and gave the participants a wide list of activities to which they could allot time to. Food diaries were analysed and nutritional intake determined by professional dieticians or using the WinDiets software for nutrition analysis. The researchers assessed energy expenditure by dividing the activities into sleeping, light, moderate or vigorous activity and looking at the time spent in each to calculate a total metabolic equivalent value (MET). They also calculated the MET per week and gave each participant a physical activity level (PAL) score. The predicted total energy expenditure (pTEE) was defined for each person and took into account their PAL score, age, sex, height and weight. Attempts were made to take into account any possible errors introduced by the participants’ reports by using a calculation comparing reported energy intake (rEI) with pTEE to see which energy intake reports seemed ‘plausible’ based on their energy expenditure.

The Weight Watchers and Rosemary Conley diets involved group sessions, the Slim.Fast group were given the meal replacement shakes and guidance pack, and the Atkins group received a copy of the Atkins book but no further support. The control group continued their normal diet and activity for the duration of the trial, with the promise that after completion they could receive six months of the diet of their choice, free of charge. No individual dietary counselling was given to any participant. At the end of the study, assessments of energy intake and energy expenditure were repeated using the same method used at the start of the study.

What were the results of the study?

Between study groups or study centres there were no differences in terms of BMI, age, waist circumference or smoking rates. Overall 18% of the participants did not complete the study, with most of those being controls (23% in this group) who did not wish to delay a weight loss attempt. Before the study began, 76% returned completed food and activity diaries, and 74% returned them at the end of the study.

At the beginning of the study, there were no significant differences between the five groups’ dietary intake and energy expenditure. Average nutrient intake across the entire group was 42% carbohydrates, 37% fat, 16% protein, and 5% alcohol. Micronutrient intake (vitamins and minerals) met recommended daily intake (apart from potassium, which was 95% of that recommended).

At the end of the study, energy intake had fallen significantly in all the groups. Weight loss also occurred in all the groups with an average of 5.2kg in the Atkins group, 4.7kg in the Weight Watchers group, 3.7kg with Slim.Fast, 4.0kg with Rosemary Conley and 0.4kg in controls, across the eight weeks. Although all the diet groups had significant weight loss compared to the control group, there was no significant difference in weight loss between the groups.

There was an apparent shift in nutrient intake with the diets, for example, those on the Atkins diet had an 11% increase in protein intake, and 10% increase in fat intake compared to a 29% fall in carbohydrate intake. Overall, the Atkins diet gave a 30% reduction in total daily energy intake, Weight Watchers a 38% reduction, and the Slim.Fast and Rosemary Conley diet a 37% reduction.

When comparing micronutrient intake with that at the beginning of the study, the control group did not change whilst the other diets showed a significantly decreased intake of various minerals. Those on the Atkins diet showed decreases in folate, potassium, magnesium, calcium and iron. Weight Watchers had decreases in riboflavin, niacin, potassium, magnesium, calcium, zinc and iron. Those on the Rosemary Conley diet had decreases in potassium, magnesium and zinc, and those on Slim.Fast had reduced niacin. Selenium intake significantly increased in the Atkins group and zinc intake significantly increased in the Slim.Fast group.

What interpretations did the researchers draw from these results?

The researchers concluded that the diets caused significant changes in macronutrient intake with a decrease in energy intake with all diets. They say that health professionals and the public should be reassured of the nutritional adequacy of the diets tested, but that people with particular deficits of certain micronutrients should receive tailored advice.

What does the NHS Knowledge Service make of this study?

This was a well-designed and conducted randomised trial and based on its findings the conclusion that the researchers arrive at is a sound one. The news reports have interpreted the findings in various ways. A few points to consider:

  • Many of the news reports have focused on the ‘safety’ of these diets. However, the study only assessed this in terms of estimated macro- and micronutrient intake from the foods. The effects in terms of health or change in body chemistry have not been examined.
  • The study did not look at the longer-term effects of the diets, such as how sustainable the weight loss or dietary changes are, and whether there is any difference in these longer-term effects between the groups.
  • Despite what the newspapers report, the study did not conclude that any one diet is better than any other, as there were no significant differences in weight loss or total energy intake between the groups.
  • Food intake and energy expenditure were only assessed over two, weeklong periods and therefore may not be representative of all other times. There is also the possibility that errors were introduced by the participants estimating these - although the authors took care to account for this.
  • There were a number of people who did not complete the study or complete the diaries, this number was significantly higher in the control group, and this affects the reliability of the findings.
  • Despite being overweight, all the participants were in good health. The nutritional requirements of people who do not fall into the criteria for acceptance into the study may be different, particularly with vitamin and mineral requirements.

The main conclusion of this study for both professionals and the public is that each of the diets was nutritionally adequate but that people who have specific imbalances in the body, such as low iron or folate levels, would benefit from personalised dietary guidance. This seems sensible advice at the current time.

NHS Attribution