Diets, fast and slow

Many people say that dieting is hard, and that keeping the lost weight off is even harder. But we don’t need to take their word for it, the science of weight loss backs up their claims. One review^[1], containing evidence gathered during most of the 20th century, found that only about 15% of obese people were successful in keeping a significant amount of weight off long-term through dieting!

Why is dieting so hard? Maybe it’s because the way we go about dieting isn’t quite right. Government agencies in the U.S., U.K., and Australia all recommend an initial, gradual weight loss strategy of one to two pounds a week. Gradual weight loss is recommended for several reasons. It allows more time for people to form the habit of eating fewer calories. Also, crash diets with very low daily caloric intake can sometimes lead to nutritional deficiencies. Plus, it’s claimed that crash diets lead to larger rebounds after the diet is over, which makes sense considering that very low calorie diets have been seen to lower basal metabolic rate^[2].

But is this true? There is certainly some evidence to suggest that it is. For instance, one study^[3] found that counselling overweight and obese women to make small changes in their diet with the goal of 1800 calories per day succeeded in slow, steady and sustainable weight loss that persisted even after a year. A control group that provided counselling encouraging a 1200 calorie/day diet ended up with regaining weight after a year.

However, there is disagreement about whether or not slow weight loss actually leads to better outcomes. One review^[4] of the literature suggests that faster initial weight loss was associated with better long-term weight maintenance, as long as proper support is provided. A recent trial^[5], which encouraged older, obese women to decrease daily caloric intake to 1200 while increasing physical activity, found that the people who achieved more rapid weight loss both lost more weight and kept more weight off. However, this trial was not randomized, so it’s hard to say whether the rapid weight loss caused the increased success in weight maintenance or not.

Sustained rapid weight loss is a bit more common in clinical settings. Protein-sparing modified fasts^[6] (PSMFs) are sometimes prescribed to very obese patients, and have calorie intakes of 500-600 kcal per day. For these patients, PSMFs can have better long-term weight loss than other approaches. While there is a rebound, as with any diet, adherence is fairly high and people remain motivated because they can see the scale moving. These patients, as they tend to be very heavy, also tend to face substantial and often immediate health risks if weight is not lost.

The purpose of this study was to address the relative lack of evidence comparing rapid and gradual weight loss. By randomizing rapid versus slow weight loss, the researchers hoped to answer whether or not the speed of initial weight loss actually affects keeping the weight off.

Guidelines currently suggest that slow weight loss is preferred to rapid weight loss, since rapid weight loss may not lead to sustainable results over time. The evidence, however, is not clear-cut on this matter.