Crash Dieting


How does crash dieting affect your genes? Can it actually make it harder to lose weight, or regain weight? Epigenetics may have the answers.

Background Obesity Statistics

Why is the obesity rate in the USA 42.4% (“age-adjusted prevalence of obesity in adults”, Centers for Disease Control and Prevention: 2018)?

Why has that climbed from 30.5% in the year 2000? Here’s a chart showing the increase in Obesity and Severe Obesity over the past 20 years:

Trends in age-adjusted obesity and severe obesity
CDC data, public domain.

Surely those people don’t want to be obese? There’s certainly enough messages saying to eat less and move more and that being overweight causes all kinds of maladies, possibly including mental health conditions.

There’s also an unending supply of diet books, weight loss programs, pills and intervention programs, with seemingly very little effect. So what could be the underlying problem?

Whatever it is, the problem must be serious and widespread, with so many people being affected and seeming unable to overcome it.

Epigenetics and Crash Dieting

Epigenetics And Crash Dieting
Epigenetics And Crash Dieting

Obesity is a multi-faceted problem. While it’s obvious that people eat too many calories and do too little exercise, simply saying to eat less and move more isn’t working.

What concerns me is that people start off at a normal weight, overeat and become overweight or obese as a consequence, then can’t sustain any weight losses. People who regain weight seem to forever struggle with their weight thereafter.

"Roughly 90 percent of people who lose a lot of weight eventually regain just about all of it." (University of Michigan Health Blog)

My theory is that people try to lose too much weight too quickly and as a result they change their gene expression profile, their epigenetics.

When you try to lose weight too quickly, the body thinks it’s undergoing a period of starvation. Various genes are activated to conserve energy which means the body becomes more efficient at using fuel.

Your metabolism changes after a crash diet

  • After a crash diet, your body uses calories more efficiently.
  • You can therefore put weight on, even if you only eating what would’ve been “maintenance” calories.
  • If you return to your pre-weight loss diet, you’ll put on weight faster than before!

The person who sees the weight reappearing feels like they failed. Psychologically that causes them distress and results in them returning to old patterns of eating. Unfortunately, as their body is now more efficient at using calories they will more readily store excess weight as fat .

This cycle can be repeated multiple times which we refer to as yo-yo dieting

  • The person becomes more aggressive at trying to lose weight, maybe by restricting calories and exercising at the same time.
  • This only emphasizes the starvation mode that the body enters as a reaction again this resulting in a need for fewer calories at a basal metabolic rate
  • The person loses fewer pounds of fat than expected (again)
  • They get discouraged and return to old eating habits
  • The ultra efficient use of calories by their body means that even if they eat fewer calories than they had done previously they will still put on weight.
  • The vicious circle perpetuates

This could explain why why so many people are obese despite there being  huge numbers of people dieting at any given time.

It’s inherent in our psychology to try to lose weight rapidly (when did you last read a bestselling diet book, “How to lose weight sensibly over time”! or watch the TV show, “the biggest chap to lose weight sensibly!”)

Unfortunately crash dieting results in epigenetic changes which mean the attempt to lose weight will almost certainly be futile.

An interesting question would be to understand how long these epigenetic changes last. The body is quite understandably changing to deal with the stresses of a starvation diet, but how long for?

Is it days, weeks, months, years or even passed down to future generations?

During the Second World War there was an event in The Netherlands called the Hongerwinter during which people existed on starvation rations or died.

Epigenetic metabolic changes happened during the Dutch Hunger Winter.
By Menno Huizinga – from Wikimedia.

In fact 20,000 people died. Epigenetic changes to the people’s gene expression profiles as a result of starvation seem to have been passed on to their offspring who had various health and metabolic issues:

"The offspring of F1 women who were exposed to famine in utero also had poor health 1.8 (95% CI 1.1-2.7) times more frequently in later life (due to miscellaneous causes) than that of F1 unexposed women".

"Our findings may imply that the increase in chronic disease after famine exposure in utero is not limited to the F1 generation but persists in the F2 generation". (

(translated, that means that the mothers who were pregnant during the time of the famine (known as F0) had babies (known as F1) who had “increases in chronic disease” which then persisted in their offspring (F2). This implies that epigenetic changes caused by the famine in pregnant women were present in their grandchildren).

If the epigenetic changes last a long time in an individual then that individual is going to struggle with their weight for the rest of their lives.

Interestingly, people who had gastric bypass surgery in this study saw their Resting Energy Expenditure (REE) stay the same after accounting for body mass changes, indicating that surgery may not trigger the same epigenetic changes that rapid, non-surgical weight loss does.

The “Minnesota Starvation Experiment” is one of the most well known experiments looking at changes in Basal Metabolic Rate (BMR) after rapid weight loss. In that experiment, men who were conscientious war objectors were subjected to caloric restriction and exercise to lose 25% of their body weight over 6 months. The idea was to work out how best to “re-feed” the victims of concentration camps and famine at the end of the second world war.

After three months, the men’s BMR had reduced by roughly 20%, and by six months by 25%, after adjusting for their weight loss. The change in BMR was put down to changes in thermogenesis, so the men’s bodies were producing less heat than before. This is a metabolic adaptation to the reduction in calories and the “starvation mode” the men were enduring. Their bodies were trying to conserve calories.

I would argue that a 25% reduction was a massive decrease in the caloric output of the body, and showed how hard the bodies of the men were working to conserve energy by supressing thermogenesis.

Furthermore, after three months of re-feeding, the average metabolic adaptation was still a reduction of roughly 10%.

While this was an extreme example, it’s possible that someone undergoing crash dieting would restrict their calories severely too. During the lead-up to the experiment, the men averaged 3,200 calories per day, then 1,560 during starvation. I could easily see someone who is crash dieting replicating that.

Unfortunately, during the re-feeding phase, or after stopping the diet, people would return to normal levels of eating, but they’d have an adapted BMR 20% less than before the crash diet (if they dieted for 3 months). Even three months after re-feeding, the BMR was still 10% less.

It’s not known how long it took to return to normal, if it ever did.

This would result in weight gain beyond that expected by returning to a normal level of calories.

Neil Shearing, Ph.D.
Neil Shearing, Ph.D.