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
This article describes 5 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):
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.
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).
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).
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.
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).
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.
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.
In addition, the number of
carbs you eat may play a role in fatigue.
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
Restricting calories too dramatically can negatively affect
fertility. This is especially true for women, as the
ability to ovulate depends on hormone levels.
Accordingly, studies show that reproductive function is
suppressed in women who eat 22–42% fewer calories than are
needed to maintain their weight (33).
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).
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
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
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
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:
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.
Estimate your daily requirement: Use
this online calculator to estimate the
number of calories you need to maintain your current body
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
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
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.