Study on fasting and dieting suggests why diets fail -- and why a weekly fast might work | Cornell Chronicle (2023)

Fasting for one day does not lead to overeating the next day, or the following week, or to more than a temporary weight loss, reports a new Cornell study. The findings help explain why diets fail.

When people fast or diet and then eat freely, "we found they do not increase their food intake to compensate for a day without eating," said David Levitsky, professor of nutrition and psychology, whose new study was published in the journal Physiology and Behavior March 22.

However, they eventually regain their weight back anyway, he said, because metabolic rate is related to body weight. "Since you weigh less after fasting or dieting, your metabolism is slower, and so you regain the weight with normal eating," said Levitsky, a Stephen H. Weiss presidential fellow.

The results defy the notion that fasting or dieting leads to gorging later; they also counter the idea that people have a genetically determined set point weight.

However, the study suggests a new strategy for losing weight. Although chronic lifestyle changes (eating healthier foods and getting more exercise) are preferable ways to lose weight, Levitsky said, a weekly fast might be another way to go.

Since it takes 10 to 14 days to recover the body tissue lost from a one-day fast, "Going without food for one day each week should produce a significant reduction in body weight over time," Levitsky said, now that we know that "fasting does not lead to overeating, and total recovery of body tissue does not occur within the week."

In his study, 22 women, divided into three groups, ate meals Mondays through Fridays at the Cornell Human Metabolic Research Unit for four weeks. During the first week, all groups ate all they wanted. Each of the following three Mondays, one group was chosen to fast, one to consume only 1,200 calories, and one to eat unrestrained. For the rest of the week, all the women were free to eat as they liked. During each session, participants were weighed, and all food consumed was measured.

For those who fasted or ate a restricted diet on Monday, body weight declined significantly the following day, though it did not change for the unrestrained eaters.

By the end of the week, the fasters and dieters had recovered their lost weight (but not all their body tissue) -- without any increase in food consumption beyond normal levels. The fact they were burning fewer calories due to their slowed metabolism accounted for the weight gain; the paper estimates that it would take 10 days for the ratio of fat, bone and muscle to return to pre-fast levels.

"The women regained their weight within a week, because weight is a sloppy measure of body tissue -- weight includes body water, gastrointestinal tract contents, glycogen content and so on," said Levitsky. "Over time, however, the loss of body tissue would accumulate and result in significant weight loss," he said.

The Division of Nutritional Sciences funded the study, which was co-authored by Lisa DeRosimo, M.S. '92, president of the Weight & Wellness Center in Jupiter, Fla. The article is available online at

Ted Boscia is assistant director of communications for the College of Human Ecology.


What is the reason most diets fail? ›

Many diets involve eliminating certain foods or even whole food groups. This is not only unhealthy but also unrealistic for the long term. Diets are temporary. Once they have reached their goal, most people go back to "normal" eating, so the weight comes back.

Why fasting diets don t work? ›

Why Fasting for Weight Loss Can Backfire. When you eat less than you need and you lose weight, your body goes into a starvation mode. To save energy, your metabolism slows down. When you're done fasting and you go back to your usual diet, you may regain the weight you lost, and then some.

Why do fasting diets fail to achieve a loss of fat? ›

"Since you weigh less after fasting or dieting, your metabolism is slower, and so you regain the weight with normal eating," said Levitsky, a Stephen H. Weiss presidential fellow.

Why do most diets fail quizlet? ›

- Most diets fail because they don't incorporate lifetime changes in food selection and an overall increase in physical activity. - It is recommended that person should lose no more than 0.5 to 2 pounds per week in order to maximize fat loss and minimize lean tissue loss.

Do diets work why or why not? ›

Diets are not associated with long-term weight loss.

Moreover, diets create a restrictive, controlling relationship with food. Those who attempt to manage their weight through restriction and deprivation often discover that these tactics may worsen a weight problem. Ditch dieting for an intuitive eating approach.

Why do so many weight loss diets fail to work? ›

Some diets fail because of the misunderstanding of changes in the body: For every kilogram (2.2 lbs) of weight lost: Calories spent by the body decreases by 20 kcal/day, but appetite increases by 100 kcal/day. Calorie intake can fluctuate by 20-30% on a diet, sending mixed signals to the brain about appetite.

Do fasting diets work? ›

On a 'fast' day, you would typically consume between 500 and 600 calories. There is some evidence to suggest intermittent fasting helps with weight loss, and some studies have linked it to lower rates of coronary heart disease and diabetes, but more research is needed before this can be proven.

How effective is the fasting diet? ›

The Research So Far. A systematic review of 40 studies found that intermittent fasting was effective for weight loss, with a typical loss of 7-11 pounds over 10 weeks. [2] There was much variability in the studies, ranging in size from 4 to 334 subjects, and followed from 2 to 104 weeks.

Why fasting is the fastest way to lose weight? ›

Your body's insulin levels increase when you eat and decrease when you fast. Lower insulin levels resulting from fasting can prevent additional fat storage and help your body mobilize stored fat.

How often do diets fail? ›

Research suggests that roughly 80%of people who shed a significant portion of their body fat will not maintain that degree of weight loss for 12 months; and, according to one meta-analysis of intervention studies, dieters regain, on average, more than half of what they lose within two years.

What is the biggest problem with diets? ›

Most diets fail most of the time. Repeated diet failure is a negative predictor for successful long term weight loss. Chronic dieters consistently report guilt and self-blame, irritability, anxiety and depression, difficulty concentrating and fatigue.

What is the biggest problem with dieting for weight loss? ›

Studies have shown that food restriction is a poor mechanism for weight loss and may instead contribute to weight gain. Dieting may also lead to the development of eating disorders, which can be much more deleterious to health than obesity.

Do diets have a 95% failure rate? ›

Experts think as many as 80 to 95% of dieters gain back the weight they've worked so hard to lose. Why? (WHY?!?) Dr. Griebeler says the culprit is your “weight set point”: the weight your body is programmed to be.

What are the main issues with dieting? ›

While it is important to maintain healthy eating behaviours, dieting can lead to physical illness and depression, especially if your weight goes up and down after dieting. It is well documented that even 'moderate' dieting can increase the risk of developing an eating disorder.

Are most diets doomed to fail? ›

If you've ever gone on a diet only to regain your lost weight soon after its completion, you're not alone — research shows the vast majority of diets are doomed to fail.

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