Why diets don’t work.

by Michelle

Most diets seem to succeed in the short-term, and fail in the long-term. This is not a new, or even particularly controversial, observation among researchers:

“There are two indisputable facts regarding dietary treatment of obesity. The first is that virtually all programs appear to be able to demonstrate moderate success in promoting at least some short-term weight loss. The second is that there is virtually no evidence that clinically significant weight loss can be maintained over the long-term by the vast majority of people.”

Confronting the failure of behavioral and dietary treatments for obesity, Garner & Wooley, 1991

“Although weight loss can usually be achieved through dietary restriction and/or increased physical activity, the overwhelming majority of people regain the weight that they have lost over the long-term.”

The Defence of Body Weight: A physiological basis for weight regain after weight loss, Sumithran & Proietto, 2013

“Of course, we can all endorse the call for a healthier lifestyle, but we must be realistic about what it can and cannot accomplish – including that it is not by itself an effective approach to long-term obesity treatment.”

An Inconvenient Truth about Obesity, Schwartz, 2012

More in-depth analysis of the failure rate of dieting can wait for another post. The question I’m asking here is, if diets fail for some proportion of people, which they indisputably do, why is that? What is the reason? What are the specific mechanisms at work?

The usual assumption among non-researchers about why diets fail is that when a dieter regains weight, it must be because they stopped dieting, which is in turn attributed to things like not having enough willpower, personal and moral failure, gluttony and laziness, or being too ignorant to know better.

These are assumptions which reflect the mythology of our culture: that anyone, if they try hard enough, can be anything they want — and therefore that weight is entirely a choice, a product of effort and moral character. This story centres the individual, their behaviour, their character traits, and their moral attributes as the cause of fatness in the first place, and the reason why weight is regained following a diet.

But these explanations are not satisfactory to me, nor, as you will see, are they reflected in the scientific literature.

To explore other answers, I haphazardly gathered peer-reviewed articles, spanning a range of more than 30 years, that investigated or discussed the various reasons why weight loss produced by dieting is not maintained long-term.

Here is what they theorize about why diets fail.

1. Behavioural relapse, a.k.a. “going off the diet”

The earlier papers on the failure of dieting focused on behavioural factors, since dieting was, at the time, a relatively new and exciting behavioural intervention for “obesity.” (By the mid-20th century, dieting as a popular pastime was not new, but as a subject of medical research, it was still fairly novel.) Researchers assumed that when someone could not sustain weight loss, it must’ve been due to a breakdown in their new behaviours — people must have gone back to eating more and moving less, just as is popularly assumed.

However, the researchers tended not to lean so heavily on moral explanations for this relapse. One study suggested that the fault lay with lack of scholarly attention to the maintenance phase of behavioural change in designing weight loss plans. This was further complicated by the fact that no one can avoid eating entirely, which makes dieting quite different from other behavioural interventions like smoking cessation programs and abstinence from alcohol.

Alongside this were proposed cultural and commercial pressures to eat, especially calorie-rich and highly palatable foods. There also appeared to be few natural rewards provided by dieting once the intervention phase ended — apparently nothing, not even thinness, feels as good as food tastes.

The researchers were not very optimistic about the usefulness of dieting if it only resulted in regaining weight. An illuminating quote from the conclusion of one paper:

“Research on humans suggests that the deleterious effects of obesity are exerted primarily during periods of weight gain…Its medical consequences may be unfortunate enough that if people cannot maintain weight loss, they would be better off not trying to lose weight!”

Behavior Modification in the Treatment of Obesity: The problem of maintaining weight loss, Stunkard & Penick, 1979

Another paper suggested that culprits for the breakdown of dieting behaviours were negative moods, emotional stress, social pressures to eat more, as well feelings of intense hunger that prompted overeating. But an interesting quote from this same article hints of more than purely behavioural factors:

“The obvious reason for weight regain after weight loss treatment is that participants return to inappropriate eating and exercise habits. These habits need not be as bad as pretreatment habits to cause regain, because metabolic factors may make it easier to regain after a period of dietary restriction…The pattern of relapse and regain appears to be the result of a war between the will and physiologic demands over which self-control appears relatively powerless.”

Why Treatments for Obesity Don’t Last, Goodrick & Foreyt, 1991

So even in cases where behavioural relapse were implicated, researchers seemed to acknowledge that other factors contributed to that relapse (like stress, biological and cultural pressures to eat, and increased hunger), or to the weight regain itself (metabolic changes.)

2. Lowered energy expenditure

Reduced calorie intake and weight loss, it turns out, cause some interesting changes to the body that result in expending fewer calories. In animal studies, changes include decreased body temperature, less spontaneous activity, and lowered resting metabolic rate (the amount of energy the body uses while at rest.)

Reduced total energy expenditure and, possibly, lowered resting metabolic rate after diet-induced weight loss have also been observed in humans. (Conversely, humans who gain weight above their baseline weight through eating have been observed to have an increased resting metabolic rate.)

A person who gains weight would be expected to expend more energy just due to their increased body mass, thus requiring more energy to physically move and biologically maintain it. The same, but in reverse, is true for someone who loses weight – less energy is required to maintain a smaller body.

But the changes in energy expenditure resulting from dieting have been described as “disproportionate,” meaning that they were greater than the changes expected for the amount of weight gain or loss, indicating that some compensatory mechanism meant to restore preferred weight may exist.

In other words, a person who lost weight to reach 150 lbs. may expend fewer calories just existing than someone who has always weighed 150 lbs. And someone who purposely gained weight to reach 150 lbs. may use more calories to maintain their weight than the person who has always weighed 150 lbs.

However, other studies of weight loss in humans have not demonstrated the effect of lowered resting metabolic rate, which leaves the question open.

A nod to weight diversity from the last study linked:

“Body weight in adults is remarkably stable for long periods of time. In the Framingham Study the body weight of the average adult increased by only 10 percent over a 20-year period. Such a fine balance is evidence of the presence of regulatory systems for body weight. Whatever the mechanism (or mechanisms), the weight at which regulation occurs differs from one person to another, and these differences are almost certainly due in part to genetic and developmental influences.”

Changes in Energy Expenditure Resulting from Altered Body Weight, Leibel, Rosenbaum, and Hirsch, 1995

3. Fat storage and insulin sensitivity

Another physiological change produced by weight loss is increased insulin sensitivity. This is generally considered a good thing, but it may also leave people vulnerable to weight regain. We may need to go back to a little high school biology to cover this one adequately.

Insulin is a hormone that the pancreas releases into your bloodstream. Insulin’s main life goal is to act like a key that allows glucose, also flowing through your bloodstream, into your cells, which then use the glucose for energy.

When a person’s cells become resistant to insulin, the glucose can’t get into the cells — it then builds up in the blood, eventually causing high blood sugar. Meanwhile, the cells switch to using fat for fuel.

With weight loss, cells become more sensitive to insulin, which allows glucose to enter the cell once more. Those cells use that glucose, and the fat that would otherwise be used for energy is directed back into storage, which may spell weight gain.

Experimental research in humans has indeed demonstrated that increased insulin sensitivity following weight loss from dieting predicts the amount of weight the person will eventually regain. The researchers are careful to point out that increased insulin sensitivity, alone, is not enough to cause weight regain, but in combination with lowered energy expenditure (see above) and increased food intake (see below), it certainly helps.

From this same paper:

“Following weight reduction, there is a 95% failure rate for obese individuals to stay weight-reduced more than 4 years (5). After obese subjects undergo weight reduction, metabolism shifts to favor weight regain…These metabolic phenomena result in the shunting of lipid fuels away from oxidation in muscle to storage in adipose tissue, and in the setting of positive energy balance, increases in body weight and percent body fat occur.”

Weight Regain Following Sustained Weight Reduction is Predicted by Relative Insulin Sensitivity, Yost, Jensen, and Eckel, 1995

4. Increased appetite

During and after weight loss, levels of several hormones involved in appetite regulation change significantly.

Hormones that promote feelings of fullness and inhibit food intake (including leptin, peptide YY, GLP-1, cholecystokinin, and amylin) are decreased with weight loss. Meanwhile, ghrelin, a hormone that stimulates hunger, is increased, along with reported food preoccupation and appetite.

Again, these responses may indicate the existence of a regulatory mechanism intended to restore preferred body weight:

“Taken together, these findings indicate that in obese persons who have lost weight, multiple compensatory mechanisms [encourage] weight gain…Furthermore, the activation of this coordinated response in people who remain obese after weight loss supports the view that there is an elevated body-weight set point in obese persons and that efforts to reduce weight below this point are vigorously resisted.”

Long-Term Persistence of Hormonal Adaptations to Weight Loss, Sumithran et al., 2011

In addition to feeling hungrier, weight-reduced people show a stronger preference for high-calorie (high sugar and high fat) foods. There are also changes in brain activity patterns indicating that weight-reduced people are more responsive to food rewards, while brain areas associated with controlling one’s food intake are less active.

The hypothalamus, a part of the brain that may act as a “brake” on the homeostatic tendency toward weight gain, shows decreased activity in people who have lost weight, which affects both food foraging behaviour and metabolism to favour eating more and regaining weight.

5. Genetic predisposition to gain weight

It has long been understood that body weight has a significant genetic component.

Research in pairs of identical twins shows that there is also a significant genetic component to weight loss, including how much and what type of fat is lost, and the rate of fat burning relative to use of glucose for energy.

On the other side of the coin, population studies of twins have shown an association between dieting attempts and subsequent weight gain, which probably reflects a pre-existing tendency to gain weight that is powerful enough to counteract weight loss attempts.

From that study:

“The poor success in weight maintenance after dieting predisposes individuals to the vicious cycle of frequent dieting attempts and weight regain. The relation between weight cycling and subsequent weight gain is well described in the literature. Part of the weight gain occurring in young adults may be regarded as physiologic, and is likely to occur independently of attempts to lose weight.”

Weight-loss attempts and risk of major weight gain: a prospective study in Finnish adults, Korkeila et al., 1999

Another study using twin data indicates that some of the weight gain may also be due to dieting itself, independent of genetics.


As you can see, moral explanations for weight regain leave a lot to be desired. They reflect lazy thinking. A person’s drive to eat, combined with their tendency to regain lost weight, is clearly more dependent on physiology than on moral corruption, or even simple ignorance.

Biology drives behaviour. It also primes the body to most efficiently exploit that behaviour. What is often interpreted as weakness of will and greediness by our culture is actually the result of a complex orchestration of genetic, homeostatic, metabolic, hormonal, and neurological processes influencing us to eat, restore lost weight, and ultimately survive.

And a final quote:

“…metabolic conditions after weight loss may not be the same as they were prior to gaining the weight in the first place. Instead of working in our favor to prevent weight gain, biology becomes one of the driving pressures that underlie weight regain.”

Biology’s response to dieting: the impetus for weight regain, MacLean et al., 2011

If you’ve ever regained weight after a diet, you are in very good company. Most dieters regain the weight. You are not lazy, stupid, or greedy. You did not fail — on the contrary, your body worked hard to save you.