3 Harmful Effects of Calorie Restriction


When people go on a weight loss journey they often revert to restricting the number of calories that they eat.

This method is actually prescribed by nearly every commercial diet program available today, sighting reducing calories, counting points, or watching portion size as the best way to lose weight.

However, restricting your calories, especially too severely, can lead to a variety of metabolic and chronic health problems.

This article will outline three potentially harmful effects of calorie restriction.

What is a calorie?

Scientifically, a calorie is defined as the amount of heat energy needed to raise the temperature of one gram of water by 1°C (1.8°F).

However, most people are more likely to think of calories as the amount of energy your body gets from the foods and beverages you consume.

Your body requires calories to function and uses them to sustain three main important processes:

  1. Basal metabolic rate (BMR): This refers to the number of calories needed to cover your basic functions, including the proper functioning of your brain, kidneys, heart, lungs and nervous system.
  2. Digestion: Your body uses a certain number of calories to digest and metabolize the foods you eat. This is also known as the thermic effect of food (TEF).
  3. Physical activity: This refers to the number of calories needed to fuel your everyday tasks and workouts.

Theoretically, eating more calories than your body needs will cause you to gain weight, while eating fewer calories than your body requires leads to weight loss. (1, 2, 3, 4)

This calorie balance concept, which is pushed on us by the commercial dieting industry and reinforced by the media, is why people who want to lose weight repeatedly try to restrict their calorie intake. (5, 6)

However, science clearly shows us that restricting calories, especially too much, may harm your weight loss potential and overall health.

1. Calorie Restriction and Metabolism

Regularly eating fewer calories than your body needs can cause your metabolism to automatically slow down.

Studies show that low-calorie diets can decrease the number of calories the body burns by as much as 23 percent. And what’s more, this lower metabolism can persist long after the calorie-restricted diet is stopped. (7, 8)

In fact, researchers believe that this lower metabolism may partly explain why more than 80 percent of people regain weight once they go off of their calorie-restricted diets. (9)

One of the ways that calorie-restricted diets slow your metabolism is by causing a loss in muscle tissue. (10, 11)

This loss of muscle mass is especially likely to occur if the calorie-restricted diet is low in protein and not combined with exercise. (12, 13)

To prevent your weight loss diet from affecting your metabolism, make sure that you never eat fewer calories than are required to sustain your BMR. Also, slightly increase your protein consumption and add in some resistance training exercises to your workout routine. (14)

Summary: Severely restricting your calories can decrease your metabolism and cause you to lose muscle mass. This makes it more difficult to lose and maintain your weight loss over the long term.

2. Calorie Restriction, Fatigue, and Nutrient Deficiencies

Regularly eating fewer calories than your body requires makes it quite challenging to meet many important daily nutrient needs. For instance, calorie-restriction may not provide sufficient amounts of iron, folate or vitamin B12 which can lead to anemia and extreme fatigue. (15, 16, 17)

Additionally, calorie-restricted diets may limit other vital nutrients as well, including:

  • Protein: Not eating enough protein-rich foods like meat, fish, dairy, beans, peas, lentils, nuts, and seeds may cause muscle loss, hair thinning, and brittle nails. (18)
  • Calcium: Not eating enough calcium-rich foods like dairy, leafy greens, calcium-set tofu, and fortified milks may reduce bone strength and increase the risk of fractures. (19)
  • Vitamin A: Not eating enough vitamin A-rich foods like organ meat, fish, dairy, leafy greens, or orange-colored fruits and vegetables may weaken your immune system and lead to permanent eye damage (20).
  • Magnesium: An insufficient intake of magnesium-rich whole grains, nuts and leafy greens may cause fatigue, migraines, muscle cramps and abnormal heart rhythms. (21)

To prevent fatigue and nutrient deficiencies, avoid restricting your calories below metabolic stimulation (1000 calories per/day) and make sure you eat a majority of your diet from whole, unprocessed or minimally processed foods.

Summary: Restricting calories too severely can lead to fatigue. Maintaining this calorie restriction for too long can also lead to nutrient deficiencies.

3. Calorie Restriction and Immunity

Restricting calories may increase your risk of infection and illness. This especially applies to viruses like the common cold and appears to be especially true when it’s combined with a high level of physical activity. (22, 23)

For instance, one study compared athletes in disciplines that put a strong emphasis on body leanness, such as boxing, gymnastics or diving, to those in disciplines less focused on body weight.

The researchers reported that athletes in disciplines that required leanness made more frequent attempts to lose weight and were almost twice as likely to have been sick in the previous three months. (24)

In another study, taekwondo athletes who were dieting to reduce their body weight in the week before a competition experienced reduced immunity and an increased risk of infection. (25)

The effects of calorie restriction in non-exercising individuals are less clear, and more research is needed before strong conclusions can be made. (26)

Summary: Calorie restriction, especially when combined with strenuous physical activity, may lower your immune defenses.

How to Eat the Right Number of Calories

Caloric intake varies from person to person due to factors such as age, sex, height, weight, body composition, and physical activity level.

Determining the number of calories that’s right for you will reduce your likelihood of developing the negative health consequences outlined above.

There are various ways to estimate your own calorie needs. However, the easiest method consists of three simple steps:

  1. Determine your BMR: Use this BMR calculator to estimate the minimum number of calories your body requires per day. Aim to never consume fewer calories than this. (27)
  2. Estimate your daily requirement: Use Calorie Calculator to estimate the number of calories you need to maintain your current body weight. (28)
  3. Determine your calorie needs for weight loss: If weight loss is your goal, aim for a daily calorie intake falling between the amount required to sustain your BMR and the amount needed to maintain your current body weight.

In addition, make sure you record what you eat in an online food journal, at least in the beginning of your weight loss process. Tracking your diet will help you ensure that you continue to reach your daily recommended nutrient intakes.

Summary: Use the method above to estimate the daily calorie intake that’s right for you, in addition to an online diet journal to ensure that your diet covers your nutrient needs.

The Bottom Line

When it comes to long-term weight loss, patience is the key. It’s best to stay clear of diets that require you to severely restrict your calories.

Instead, opt for a diet that is focused on quality whole, unprocessed or minimally processed foods that encourage you to make sustainable lifestyle changes.


Stop Dieting & Start Eating 

Did you ever sit back and wonder why even though more people are dieting and exercising today than at any other time in history we’re actually getting fatter and sicker as a result and not leaner and healthier?




“Our body is biology not math. That’s why when I hear things like a “calorie is a calorie” or it’s all about “calories in vs. calories out” I shake my head knowing that the human body is much more complex than a simple math equation”. — Coach Tony

Tony’s introduction into the world of health and fitness began in the summer of 1969 at the very early age of 8. He says “I can remember (like it was yesterday) picking up my first copy of Joe Weider’s Muscle Builder magazine, which is now known as Muscle & Fitness” and being completely enthralled by muscles. “I would literally fantasize about having muscles like the men in that magazine”. As he journeyed through life, those thoughts never left his mind. This was Tony’s beginning of what would become a lifelong journey into the world of fitness and nutrition.

Over the following four decades Tony has spent his time in the health and fitness industry as a competitive athlete, trainer, nutrition coach, educator, speaker, publisher and author.

In his competitive years, Tony became fascinated, biologically, with how nutrition and the foods we consumed had a profound effect on performance, body composition and over-all blood chemistry, as well as, state of mind. “These laws governed by the food /body relationship were completely different from what I had been taught about conventional mainstream nutrition”.

Over the past 30 years Tony has devoted his time to educating, coaching and mentoring people to take charge of their own health’s destiny.

He says “As we continue to listen to outdated nutritional advice, it’s clearly evident that our approach has completely missed the mark proven by the fact that society is getting bigger and sicker as a result, not leaner and healthier”.

Tony has been a dedicated writer, blogger and educator, and has spent time writing for several publications and national websites as a health advocate on topics including nutrition, fitness, chronic disease prevention and personal 

Coach Tony – CNS, CMT

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