A team of international scientists analysed a number of studies to determine how effective popular diets were for weight loss and lower blood pressure, when undertaken by overweight or obese adults. They published their findings in the BMJ in April 2020. Currently the full study is open access and available to read online.
The scientists examined the findings of 121 randomised international trials which had followed 21,942 patients, average age 49 years, who were following a named diet or control diet. The patients were asked to report weight loss and changes in their heart disease risk factors and were followed up for 3 months or longer. The studies were very different and the scientists had to account for this in their analysis. They chose to group diets into two types: those that were low carbohydrate or low-fat were grouped together and those that they termed moderate macronutrient. These were similar to low fat, but with decreased carbohydrate and slightly increased fat. The patients were following well-known diets including Atkins, DASH (Dietary Approaches to Stop Hypertension) and Mediterranean. If a diet was similar to a branded diet, but not branded, it was labelled Atkins-like or Mediterranean-like.
Eligible studies could provide exercise requirements, but it was not essential. There could be some meal replacement products, but the diet had to mainly use whole foods and not drugs. Control diets included usual diet, dietary advice where participants could be given dietary guidelines, professional consultation, reading materials and following a low fat diet with or without advice on reducing calories. The usual diet was used as a reference to compare the other diets to. When compared with the usual diet, low carbohydrate and low fat diets offered weight loss of around 4-5kg and reduction in hypertension at 6 months. Those diets that were labelled moderate micronutrient produced slightly less in terms of weight loss and blood pressure reduction. Atkins, Zone and DASH produced the best effects on blood pressure and weight loss. None of the diets improved good HDL cholesterol significantly. All of these results were taken at the 6 months mark.
After 12 months, weight loss had slowed among all the diets and the heart benefits were no longer significant apart from the Mediterranean diet. Although there were some study limitations, the scientists felt that their thorough analyses had helped to ensure the confidence of the results. Their findings showed that regardless of the type of diet chosen, the results were more or less the same, with no statistically significant better results from one type of diet. At 6 months, most of the diets had provided some weight loss and some improvement on heart disease factors.
The scientists felt that it was more important to decide how to maintain weight loss rather than focusing on specific types of diet. They suggested that national guidelines which include the encouragement to eat more vegetables, fruit and whole grains and cutting back on sugar, alcohol and salt is the right kind of advice to encourage people to eat more healthily.
In 2019, government statistics suggested that 28.7% of people in England are classified as obese (body mass index of 30 or above). There are 35.6% who are overweight (BMI between 25-30) but not obese, which means that over 65% people in England need to lose some weight to maintain a healthy lifestyle and reduce risk factors. It is therefore important that government guidelines are simple to understand and follow.
Obesity has been linked with mental health issues, including depression and low self-esteem, cancer of the breast and bowel, heart disease and type 2 diabetes. Before starting a diet, people are advised to consult their GP.
Johnston, B.C., et al., Comparison of dietary macronutrient patterns of 14 popular named dietary programmes for weight and cardiovascular risk factor reduction in adults: systematic review and network meta-analysis of randomised trials. BMJ, 2020; m696