The Biology of Calorie Restriction

By Jason Fung, MD

Calorie restriction for weight loss does not work.

  • The calorie restriction method of weight loss seems like a reasonable strategy but almost always fails.
  • Calorie restriction usually decreases the metabolic rate, which reduces weight loss efforts.
  • Calorie restriction can often cause harmful psychological effects including hunger and obsession with food.
  • If the diet is relaxed, this lowered metabolic rate results in weight regain – the rebound effect.
By Jason Fung, M.D., Co-founder of The Fasting Method

In the conventional view of obesity, eating too much makes you fat and eating fewer calories causes weight loss. However, things are not quite so simple, as virtually all long-term scientific studies to date show that the caloric reduction as primary strategy fails. One of the main reasons is, eating fewer calories almost always causes you to burn fewer calories too.

In the 1950s, researcher Ancel Keys, who would later play a crucial role in shaping current nutritional orthodoxies around dietary fat, studied the effects of caloric reduction in the famous Minnesota Starvation Experiment. With World War II raging, many millions of people were on the verge of starvation and this experiment was an attempt to understand the effects of calorie restriction on human physiology.

Ancel Keys estimated the 36 study subjects were eating roughly 3,200 calories per day at baseline. They were given a ‘semi-starvation’ diet of 1,560 calories per day with foods similar to those available in war-torn Europe at the time – potatoes, turnips, bread, and macaroni – all foods very high in carbohydrates and relatively lower in fat and protein, which were hard to get in post-war Europe. This diet, while called a ‘semi-starvation’ diet, is actually quite similar in macronutrients to most low-fat, low-calorie weight loss diets we would prescribe today. So, this is not quite a ‘starvation study’ as advertised but more of a ‘calorie restriction’ study.

What happened to these subjects? We can divide the effects into physical and psychological effects.

Physical Effects of Calorie Restriction

The main symptoms subjects noted after changing diets were coldness, incessant hunger, weakness, exhaustion, dizziness, muscle wasting, and hair loss. Men complained they could not stay warm even in mid-summer with an ‘abundance of clothes’. They also noted an inability to concentrate. Further physiologic testing revealed:

  • Heart volume shrank by 20%
  • Heart rate slowed
  • Body temperature dropped
  • Blood pressure drops
  • Resting metabolic rate dropped by 40%

In other words, their body was shutting down. Think about it from the body’s point of view. It’s accustomed to 3,200 calories per day and now it only receives 1,560 calories from food. Since their weight had previously been stable, we know their body was burning 3,200 calories per day. With only 1,560 calories coming in and 3,200 calories going out, the body must implement across the board reductions in energy expenditure to fix the deficit. Here’s how it plays out:

  • The heart gets less energy – heart rate slows and heart volume shrinks. Blood pressure drops – subjects feels weak and exhausted
  • Body heat generation (very energy intensive) is turned down – subject feels cold
  • Muscles get less energy – physical exhaustion and muscle wasting
  • Hair and nails get less energy – hair loss and brittle nails
  • The brain gets less energy – poor concentration ability

This physiologic response is perfectly rational because it ensures survival of the individual under a time of extreme stress. Yeah, you might feel lousy, but you’ll live to tell the tale. Your body is not so stupid as to keep burning energy it does not have.

Consider the alternative. The body is burning 3,200 calories per day but only gets 1,560. Things feel normal. Three months later, you are dead, because you’ve run out of energy. Nice. It is inconceivable that the body does not react to caloric reduction by reducing caloric expenditure. The body is smart and does not want to die right now. Why would we assume Mother Nature is so stupid?

Consider the often repeated ‘scientific’ statement that if you reduce 500 calories per day, you will lose one pound in one week. If you weigh 180 pounds, does that mean in 180 weeks you will weigh zero pounds? It’s clear at some point, the body must, must, must reduce caloric expenditure. It turns out the adaptation happens almost immediately and persists in the long term. The human body responded to caloric restriction by reducing caloric expenditure by approximately 40%.

Psychological Effects of Calorie Restriction

In addition to the physical effects of calorie restriction, Keys observed a number of psychological effects. There was evidence of:

  • Obsessive thoughts of food
  • Bingeing behavior
  • Extreme depression
  • Severe emotional distress
  • Irritability
  • Loss of libido
  • Interest in everything other than food vanished
  • Social withdrawal and isolation

Grown men would flip through cookbooks like teenagers poring over a Playboy magazine. They became fascinated with everything food related, like cooking utensils, which previously were uninteresting to them. Their body is desperately signalling to increase food intake – resulting in hunger, food obsession, and bingeing. Again, this is a rational response to make sure you are doing your utmost to get more to eat. Deliberate denial of that urge becomes harder and harder over time.

Consider the last time you tried to diet by reducing calories and portion size. Does any of this sound familiar? Yeah, thought so. Almost everybody who has done calorie-restricted, low-fat diets has experienced all of these phenomena.

After Effects of Calorie Restriction

During the calorie restriction phase, people did lose weight, which is often what dieters want to do. But what happened after the dieting period? We can get some clues from the study.

During the 24 weeks of calorie restriction (0 – S24), both body weight and body fat dropped. All good. As subjects went back to a more typical diet, they very quickly regained the lost weight. By about 12 or so weeks, their weight returned to baseline. However, it did not stop there.

Their body weight continues to increase until it is actually higher than it was before the experiment started. And just look at that body fat! It goes soaring above baseline. The dirty little secret of most dietary studies is that as weight is lost, both fat and lean weight are decreased. But when weight is regained, most of it is fat.

Sound familiar? Thought so.

Think about it in dietary terms. Suppose you normally eat 2000 calories/ day and your weight is stable, so you are burning 2000 calories/day. You start a calorie restricted (1,560 calories/ day), high carbohydrate, low fat diet – just like your doctor or dietician might tell you to do in order to lose weight. You feel lousy, tired, cold, hungry, irritable, and depressed. It’s not just because you are dieting, there are real, physiologic reasons why you feel so crappy. Metabolic rate drops, hormones make you hungry, body temperature drops, and there are a multitude of psychological effects.

As you suffer these myriad physical and psychological effects of your diet, it becomes harder and harder to stick with it. Your weight initially went down, but now the weight loss is starting to plateau. Finally, fed up with feeling so crummy, you decide to relax your diet just a bit. You eat about 1,800 calories which is more than the 1,560 calories, but less than the 2,000 calories you started with. Since your diet reduced your caloric expenditure, let’s estimate it goes down to about 1500 calories/ day. So, even on 1,800 calories per day, you will regain your lost weight, even though you are eating less than when you started.

Said another way – reducing Calories In reduces Calories Out. Reducing caloric intake inevitably leads to reduced caloric expenditure. Eating less calories for a prolonged period makes you tired and hungry. And worst of all… you regain all the weight you lost. Sound familiar to anybody? Sounds familiar to everybody. In our heart of hearts, we knew it already. We have chosen to forget this inconvenient fact because our doctors, our dieticians, our government, our scientists, our politicians, and our media have all been screaming at us for decades it’s all about ‘Calories in vs. Calories Out’. No, instead, what’s important is to understand how our body decides how much body fat to carry. The secret lies in our hormones.

For a complete discussion, see The Obesity Code.

Learn more about Pique Fasting Tea.

For fasting education, support and community, go to The Fasting Method.

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Jason Fung, MD

By Jason Fung, MD

Jason Fung, M.D., is a Toronto-based nephrologist (kidney specialist) and a world leading expert in intermittent fasting and low-carb diets.

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