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5 Ways Restricting Calories Can Be Harmful

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 5 potentially harmful effects of calorie
restriction and helps you determine the calorie deficit that’s
right for you.

Worried Blonde with Measuring Tape Over Her Mouth

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
, 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

Green Apple and Measuring Tape

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% (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% 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
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
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,

  • 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
  • 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

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

Brunette Upset About Eating Only Lettuce

Restricting calories too dramatically can negatively affect

. This is especially true for women, as the
ability to ovulate depends on hormone levels.

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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% 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

Fork and Spoon Tied With a Measuring Tape

Restricting calories may increase your risk of infections and

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
  3. Determine your calorie needs for weight
    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

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.

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