As European waistlines continue to expand, research into the optimal diet for weight loss and keeping slim continues. As in all areas of research, ideas evolve and develop in surprising ways, often challenging accepted wisdom. Here are some of the recent developments in obesity research.
In the past when people were using up plenty of calories going about their daily business it was possible to eat a diet with pretty much any nutritional profile and remain slim. The problem nowadays is that many people are extremely sedentary making it possible to overeat on not so much food. For example it is easy to prepare tasty, filling meals for an active person who needs about 2,500 calories a day. It is much more difficult to prepare tasty, filling meals for someone who needs only 1,800. This is why a major focus for scientists is to find out how to increase the satiating power of the diet – so people get full on fewer calories.
So how can the diet become more satiating? The main areas under debate here are:
- The glycaemic index – which measures the rate of absorption of carbohydrate foods into the body
- Whether we should be consuming more or less protein
The glycaemic index
The glycaemic index (GI) has been used for some years as a tool to assess how quickly carbohydrate-containing foods are digested and absorbed in the body. Slowly digested carbohydrates have a low GI and are thought to be more ‘filling’, whereas quickly digested carbohydrates have a high GI and are deemed less satisfying. However in real life people eat foods as part of a meal and rarely on their own. Previous work has demonstrated that, the GI predicts, with reasonable accuracy, the relative blood glucose responses of mixed meals of the same composition but consisting of different carbohydrate foods (1). However new evidence challenges this view and suggests that when a meal is consumed, there is no relationship between the GI of the individual foods and the overall effect of the meal on the body (2). For example the addition of small amounts of fat inhibits stomach emptying and slows digestion dramatically. Perhaps the GI is not a good way of classifying how carbohydrates affect hunger and satiety.
The nutrients ratio
The ratio of fat, carbohydrate and protein in the diet is also another big area of debate. One of the biggest multicentre trials in Europe found that in a free living situation overweight people eating as much as they wished of a low-fat, high carbohydrate diet, spontaneously lost weight (3). Whether the carbohydrate portion of the diet came from starch or sugar was not important -- they all lost weight. But what about the protein fraction?
In the wake of the Atkins diet, the role of protein has come to the fore. Protein is known to act quickly and powerfully to inhibit hunger (4), but current advice for weight control emphasises a moderate protein, low-fat, high carbohydrate, diet (4). For active people, perhaps a high carbohydrate diet is ideal, because carbohydrate is the preferred fuel for active muscles, but for more sedentary people a diet where some of the carbohydrate is replaced by protein (but not on the scale of the Atkins diet) may be more successful (5). To find out if this is the case a new large scale EU project which will compare low and high protein slimming diets is being planned.
So what advice can we still rely on?
It is generally agreed that it would be a whole lot easier for everyone to keep slim if they were more active. This does not have to involve going to the gym or jogging every morning, just building some activity like walking or cycling into the daily routine. This way a person can enjoy more calories without putting on an ounce. A low-fat diet would still seem to be the best advice along with encouraging moderation, variety and the inclusion of enough fruit, vegetables, lean meat, wholegrain cereals and low-fat dairy products.
- FAO/WHO (1998) Food and nutrition paper 66. Carbohydrates in Human Nutrition. Rome
- Flint A, Moller BK, Raben A, et al (2004) The use of glycaemic index tables to predict glycaemic index of composite breakfast meals. British Journal of Nutrition 2004 91:979-89
- Saris WHM, Astrup A, Prentice AM et al (2000 ) Randomised controlled trial of changes in dietary carbohydrate/fat ratio and simple vs complex carbohydrate on body weight and blood lipids. International Journal of Obesity 24:1310-1318
- World Health Organisation (1997) Obesity; preventing and managing a global epidemic. Geneva
- Skov AR, Toubro S, Rønn B, Holm L, Astrup A. (1999) Randomized trial on protein versus carbohydrate in ad libitum fat reduced diet for the treatment of obesity. International Journal of Obesity 23:528-536.