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Why diets fail



a personal body weight scale flanked by fresh fruits and vegetables

I call them the 'diet wars': the ongoing controversy over which diet is most effective for long term weight loss. Unfortunately, regardless of what type of diet they are following, most people lose the most weight within the first few months of a diet and then slowly put the weight back on. Just why the weight is regained has been something of a mystery: dieters will respond to dietary questionnaires or keep food records that indicate that they cut calories by a certain amount and that they maintain that lower caloric intake - even while their weight initially drops and then slowly creeps back up.

So what's really going on? Are people following a specific type of diet more likely to regain the initial weight lost?

Researchers with the National Institute of Diabetes and Digestive and Kidney Diseases as well as those from the Department of Medicine at Stanford University analyzed dietary and individual weigh-in data from a study known as DIETFITS (Diet Intervention Examining the Factors Interacting with Treatment Success), a randomized weight loss trial lasting 1 year (Obesity 2019;27(3):420-426).

The study randomly assigned participants to a low-fat or low-carbohydrate diet: both groups were instructed (with counseling from dietitians) on how to reduce their intake of either fat or carbohydrates, respectively, to levels below 20 grams per day for the first 8 weeks of the trial. After the initial 8 weeks the participants were instructed to slowly add back fats or carbohydrates in increments of 5 to 15 grams per day until they reached, in the authors' words, "the lowest level of intake they believed could be maintained indefinitely."

Note that the participants in this study were not told to cut calories, but only were instructed to cut their fat or carbohydrate consumption.

The participants reported on their dietary intake with questionnaires that were administered at 3, 6, and 12 months. These questionnaires included 3 days: 2 weekdays and 1 weekend day for each questionnaire. With these recalls the authors could assess the number of calories the participants said they were consuming as well as their levels of fat or carbohydrate intake.

In an interesting wrinkle, the authors used a sophisticated mathematical formula that included such variables as metabolic rate, physical activity level (as reported by the participant), body weight, change in physical activity, age, sex, and height to calculate the required caloric intake for the amount of change in body weight during a designated time interval in the study. In essence, given the reported body weight for an individual and their change in weight as well as their level of physical activity, the authors were able to reverse-engineer how many calories the participants had to have been consuming. (This is over-simplified. )

The authors' mathematical analysis revealed that although both the low-fat and low-carbohydrate groups cut their total caloric intake significantly at the start of the study, leading to a fairly significant weight loss, their actual caloric intake gradually increased over the following year - leading to regaining weight. Over the first three months, for example, the participants cut their caloric intake by an average of about 800 calories per day, but from months 3 to 6 their caloric intake increased until they were consuming only about 300 calories per day less than baseline. The participants' personal dietary intake questionnaires, however, suggested that their caloric intake had not increased from their initially low levels.

Neither the low-fat nor the low-carbohydrate dieters could maintain their initial weight loss - not necessarily because of the type of diet they were on, but because they simply couldn't sustain the reduction in calories that drove their initial weight loss. The authors theorize that the difference between the reported intake and actual intake may have been driven by unreported snacking or even an easing up on measuring the specific amounts of foods.

What this means for you

Rather than dieting just to lose weight, choose a healthy diet you can live with that leads you to consume more low-caloric-density foods (that is, more volume for your caloric buck). The Mediterranean Diet is one such lifestyle that we know from extensive research is good for you, and when you eat more fruits, vegetables, whole grains, and legumes, you can consume much more food - keeping you more satisfied - than you might on a low-fat or low-carb diet.

First posted: April 10, 2019