Both low-fat and low-carb diets work equally well to help people lose weight over two years, new research has shown. In addition, the low-carb diet may also boost levels of ‘good’ cholesterol, although it’s not clear how much difference this would make to your overall health.

What do we know already?

The quest to find a diet that helps people lose weight goes back decades. In recent years, there’s been much discussion about the merits of low-fat diets, compared with low-carbohydrate diets.

Research shows the low-carb diet works well for short-term weight loss, but there isn’t enough evidence to show how well the two types of diet work in the long-term, and what the effects are on people’s health, for example their cholesterol levels.

In addition, it’s been recognised that weight loss programmes are better if they include some support to help people make changes to their eating and exercise habits. This is often called behavioural therapy, because it works through changing people’s behaviour. Group support also seems to help.

A new study has compared people who followed a weight-loss programme over two years, all of whom had behavioural therapy. Half the people followed a low-carb diet (where they could eat as much as they liked, but were only allowed small amounts of carbohydrate – just 20 grams a day at the start of the study), and half followed a low-fat diet (where they were limited to between 1,200 and 1,800 calories a day, only one-third of which could be from fat).

The researchers measured people’s weight after two years, and also other factors such as cholesterol levels, blood pressure, bone strength, and body fat percentage.

What does the new study say?

People on both diets did equally well in losing weight. After two years, they had lost on average 7 kilograms (just over a stone), or the equivalent of 7 percent of their body weight. This amount of weight loss is enough to have positive effects on the health of overweight people.

There were few differences in the other health measures. The group who’d had the low-carb diet had much better levels of ‘good’ (HDL) cholesterol than people on the low-fat diet.

How reliable are the findings?

This was a well-run study, looking at 307 people with a body mass index of 30 to 40 (classed as obese). Quite a lot of people (77) dropped out of the study, which makes the final weight loss results slightly less reliable.

However, people were equally likely to have dropped out of either diet group. So, the results comparing the effects of the diets are likely to be reasonably reliable.

Where does the study come from?

The researchers were from the University of Colorado Denver, Denver, Colorado; Washington University, St. Louis, Missouri; and the University of Pennsylvania, Philadelphia, Pennsylvania. The study was published in the medical journal Annals of Internal Medicine. It was funded by the US National Institutes of Health.

What does this mean for me?

If you need to lose weight, this study shows that low-carb and low-fat diets are likely to work equally well. You may find it easier to lose weight as part of a supervised weight-loss programme that can help you break bad habits and make healthy changes to your diet and lifestyle.

It isn’t clear how important the difference in ‘good’ cholesterol is, because the study didn’t measure long-term results, such as whether people went on to get heart disease. However, higher levels of HDL cholesterol are usually linked to better heart health.

What should I do now?

If you are concerned about your weight, you can make an appointment to see your GP. He or she can tell you how much weight you need to lose, and may be able to recommend a local programme to help you.