5 Things That Happen When You Drastically Cut Calories

Tape measure wrapped around hands over a dinner plate containing an extremely small amount of food holding a knife and fork illustrating that calorie restriction doesn't work


At one time, extreme dieting was a popular way to lose weight. But, with the realization that such an approach is unhealthy, more people are upgrading the quality of their diet rather than crunching the numbers and obsessively tracking every calorie. Improving diet quality makes sense! The type of food you eat impacts hormones that control your appetite and metabolism, so WHAT you put into your body, matters too.

What’s so bad about extreme calorie restriction? For one, the weight that you lose through extreme dieting will be temporary. Your body quickly adapts to such severe changes by conserving energy in hopes of preserving homeostasis. Therefore, you end up fighting a slowing metabolism and a ravenous appetite. That’s a bad combination! Let’s look at some of the changes that happen when you aggressively cut back your calorie intake and how these changes can impact your weight.

Stress Hormones Rise

Your adrenal glands, two small glands located above your kidneys, produce cortisol, a stress hormone. Cortisol has a variety of functions. It helps regulate immune function, blood sugar, and controls pathways involved in inflammation and stress. In other words, you need cortisol, but you don’t want your level to be too high or too low. Unfortunately, calorie restriction can upset this delicate equilibrium, leading to a rise in cortisol. That’s because your body perceives inadequate calories and nutrients as stressful and pumps out more of this hormone.

Cortisol has a negative impact on your body composition as well. When cortisol is high, it signals your liver to produce glucose from amino acids to supply your body with more energy. Some of these amino acids come from the breakdown of muscle tissue. So, cortisol has a catabolic effect on your hard-earned muscle. Plus, when your cortisol level is high, where you store fat shifts to the middle of your body. After a long period of calorie restriction, you might notice that you’re carrying more fat around the waist and tummy. Cortisol also contributes to bone breakdown and bone loss, a risk factor for osteoporosis. So, extreme calorie restriction negatively affects body composition over the long term.

What is extreme calorie restriction? In one study, cutting calories back to 1200 daily was enough to boost the production of cortisol. Of course, this will vary from person to person, but avoid making drastic changes to your calorie intake. Instead, eat more foods that are nutrient dense but not calorie dense.

Decreased Thyroid Function

Your thyroid gland is the master regulator of your metabolism. The active form of thyroid hormone is called T3 and it sets the rate at which your body burns energy. When you reduce calories too low, T3 drops and this leads to a slowing of your metabolic rate. Trimming carbs too much can also cause a drop in T3. In addition, during extreme calorie restriction, production of an inactive form of thyroid hormone called reverse T3 rises, and this further blocks T3 activity. One reason people tend to regain weight after a calorie restricted diet is related to the drop in thyroid function.

Changes in Estrogen and Testosterone

The portions of your brain, called the hypothalamic-pituitary axis, that control the production of estrogen and testosterone are also impacted by calorie restriction. In fact, these portions of your brain are exquisitely sensitive to a drop in energy intake. If you combine calorie restriction with excessive exercise, it leads to a further energy deficit and a reduction in estrogen and progesterone. What are the consequences? Low estrogen and testosterone contribute to bone loss and reduced fertility. You’ve probably heard of the female athlete triad, common among female athletes. Athletes restrict calories and exercise excessively to the point that their periods stop and they develop bone loss, often at a young age.

Changes in Immune Function

The rise in cortisol that comes from the stress of calorie restriction also impacts immune function.  T-lymphocytes are immune cells that protect you against foreign invaders, including viruses and tumor cells. Cortisol stifles their activity and places you at higher risk of infection. If you get more colds and upper respiratory infections when you’re “on a diet” that’s one of the reasons why. Studies show that stress, including the stress of calorie restriction, increases the susceptibility to viral infections, diminishes the response to vaccinations, so that you’re less affected, and increases the time it takes to heal from injuries.

Mood Changes

Eating a very low-calorie diet isn’t lots of fun, to begin with, but over time it can negatively impact your mood in a number of ways. For one, watching every calorie that you take in creates anxiety. In one of the best-known studies on calorie restriction, researchers monitored 36 healthy men who drastically cut their calories as a form of conscientious objection to World War II. The men cut their calories over 3 months to the point that they lost 25% of their body weight. During the study, the men developed an obsessive preoccupation with food and experienced other symptoms, including mood changes, problems concentrating, dizziness, hair loss, balance problems, and intolerance to cold. Two of the men developed psychiatric problems and had to be hospitalized, one for suicidal thoughts and the other for self-mutilation. Your body simply wasn’t designed to be deprived of calories, especially for a long period of time.

The Bottom Line

As you can see, excessive calorie restriction has some unpleasant side effects and, for most people, this approach isn’t sustainable anyway. Any diet plan built around severe calorie restriction is a bad diet. A better approach is to upgrade the quality of your diet. Choose whole, unprocessed foods and eliminate empty calories from sugar. Then, practice the basic tenets of mindful eating. Doing this will help you build a healthy relationship with food rather than a detrimental one, as extreme calorie restriction does. The take-home message? Focus less on calories and more on eating nourishing foods. Combine it with regular strength and aerobic exercise, adequate sleep, and stress management and you’ll have the ultimate recipe for healthy weight control.



Psychosom Med. 2010 May; 72(4): 357–364.
Am J Clin Nutr. 2001 Jan;73(1):7-12.
J Clin Endocrinol Metab. 1976 Jan;42(1):197-200.
Medscape.com. “Female Athlete Triad”
Dartmouth Undergraduate Journal of Science. “The Physiology of Stress: Cortisol and the Hypothalamic-Pituitary-Adrenal Axis”
Psychology Today. “Dieting Can Make You Lose Your Mind”


Related Articles:

Is Crash Dieting Harmful to Your Heart?

Why It’s Time to Take a Break from Dieting

Metabolic Adaptations – 3 Ways Your Body Tries To Keep You From Losing Weight

What is Metabolic Compensation & How Does It Keep You From Losing Weight?


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