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Can Eating Too Few Calories Lower Your Metabolism?

By Alina Petre

People trying to lose weight often restrict the number of calories they eat.

However, restricting calories too severely can lead to a variety of health problems, including reduced fertility and weaker bones.

This article describes five potentially harmful effects of calorie restriction and helps you determine the calorie deficit that's right for you.

Your Calorie Needs, Explained

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, you're more likely to think of calories as the unit of measurement for 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 processes (1):

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.

Generally speaking, eating more calories than your body needs will cause you to gain weight, mostly in the form of body fat. Eating fewer calories than your body requires leads to weight loss (2, 3, 4).

This calorie balance concept, which is supported by strong scientific research, is why people wanting to lose weight often try to restrict their calorie intake (5, 6, 7).

However, restricting calories too much may harm your health in the following 5 ways.

1. It Can Lower Your Metabolism

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

Several studies show that low-calorie diets can decrease the number of calories the body burns by as much as 23 percent (8, 9, 10).

What's more, this lower metabolism can persist long after the calorie-restricted diet is stopped (10).

In fact, researchers believe that this lower metabolism may partly explain why more than 80 percent of people regain weight once they go off their calorie-restricted diets (10).

One of the ways that calorie-restricted diets slow your metabolism is by causing muscle loss (11, 12, 13).

This loss of muscle mass is especially likely to occur if the calorie-restricted diet is low in protein and not combined with exercise (14, 15).

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.

Slightly increasing your protein intake and adding resistance exercises to your workout routine may also help (14, 15).

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

2. It Can Cause Fatigue and Nutrient Deficiencies

Regularly eating fewer calories than your body requires can cause fatigue and make it more challenging for you to meet your daily nutrient needs.

For instance, calorie-restricted diets may not provide sufficient amounts of iron, folate or vitamin B12. This can lead to anemia and extreme fatigue (16, 17, 18).

In addition, the number of carbs you eat may play a role in fatigue.

Some studies suggest that calorie-restricted diets with low amounts of carbs may cause feelings of fatigue in some individuals (19, 20, 21, 22).

However, other studies find that low-carb diets reduce fatigue. Therefore, this effect may depend on the individual (23, 24).

Calorie-restricted diets may limit other nutrients too, 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 (25).
  • 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 (26).
  • Biotin and thiamine: A low intake of whole grains, legumes, eggs, dairy, nuts and seeds may limit your intake of these two B vitamins, potentially resulting in muscle weakness, hair loss and scaly skin (27, 28).
  • 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 (29).
  • Magnesium: An insufficient intake of magnesium-rich whole grains, nuts and leafy greens may cause fatigue, migraines, muscle cramps and abnormal heart rhythms (30).

To prevent fatigue and nutrient deficiencies, avoid overly restricting your calories and ensure you eat a variety of whole, 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. It May Reduce Fertility

Restricting calories too dramatically can negatively affect fertility. This is especially true for women, as the ability to ovulate depends on hormone levels.

More specifically, an increase in estrogen and luteinizing hormone (LH) levels is needed in order for ovulation to occur (31, 32).

Interestingly, research has shown that LH levels partly depend on the number of calories available in a woman's diet (31, 32).

Accordingly, studies show that reproductive function is suppressed in women who eat 22–42 percent fewer calories than are needed to maintain their weight (33).

An insufficient calorie intake may also reduce estrogen levels, which is thought to have lasting negative effects on bone and heart health (34, 35, 36).

Signs of reduced fertility may include irregular menstrual cycles or a lack of them. However, subtle menstrual disturbances may not have any symptoms, so they may require a more thorough medical examination to be diagnosed (37, 38).

Researchers believe that severe calorie restriction may also affect men's reproductive function, but few studies exist on the topic (39).

Summary: Overly restricting calories may potentially reduce fertility, especially in women. More studies are needed to determine the effects of calorie restriction in men.

4. It Can Weaken Your Bones

Consuming too few calories can weaken your bones.

That's because calorie restriction can reduce estrogen and testosterone levels. Low levels of these two reproductive hormones are thought to reduce bone formation and increase bone breakdown, resulting in weaker bones (40, 41, 42, 43).

In addition, calorie restriction — especially when combined with physical exercise — can increase stress hormone levels. This may also lead to bone loss (44).

Bone loss is especially troublesome because it is often irreversible and increases the risk of fractures (45, 46).

Summary: Restricting calories may disturb hormone levels, which may result in weaker bones and an increased risk of fractures.

5. It May Lower Your Immunity

Restricting calories may increase your risk of infections and illness.

This applies to viruses like the common cold and appears to be especially true when it's combined with a high level of physical activity (47, 48).

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 (47).

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 (48).

The effects of calorie restriction in non-exercising individuals are less clear and more research is needed before strong conclusions can be made (49).

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

How to Eat the Right Number of Calories

Calorie needs vary from person to person because they depend on factors such as age, sex, height, current weight 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. The easiest method consists of three simple steps:

1. Determine your BMR: Use this online calculator to estimate the minimum number of calories your body requires per day. Aim to never consume fewer calories than this.

2. Estimate your daily requirement: Use this online calculator to estimate the number of calories you need to maintain your current body weight.

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 like Cronometer, 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 your diet covers your nutrient needs.

The Bottom Line

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

Instead, opt for diets that are focused on diet quality and encourage you to make sustainable lifestyle changes.

Reposted with permission from our media associate Authority Nutrition.

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