The body does not recognize the difference between dieting and starvation.
During prolonged periods of starvation, the body slowly begins to feed itself, using fat stores and then muscle stores to produce energy. If the body continues to burn calories at the same rate, with low energy intake, all energy reserves would be quickly depleted and the person would die.
However, to prevent this, a “response to starvation” has been developed, which enables survival even after long periods without food. This mechanism enabled our ancestors to survive, and today it prevents us from implementing a diet and simply achieving results.
Let’s start from the beginning :
Where does the body spend on calories?
A calorie is defined as the amount of heat energy needed to raise the temperature of one gram of water by 1°C.
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 for 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, in most cases as body fat. Eating fewer calories than your body requires leads to weight loss (2, 3, 4).
This calorie balance is the reason why people who want 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 ways.
Metabolism slows 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).
This slower metabolism can persist long after the calorie-restricted diet is stopped (10).
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).
Muscle mass will be more occur if the low calories diet is also 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).
Low calories diet lead to nutrient deficiency and fatigue
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).
Low calories 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 milk 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 organic 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.
The connection between low calories intake and reduced fertility
More specifically, an increase in estrogen and luteinizing hormone (LH) levels is needed for ovulation to occur (31, 32)
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).
Low-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).
Reducing calories can weaken your bones
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).
Low calories=low immunity?
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 consumed low calories diets 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).
My advice to you
When the weight stops falling during the implementation of low calories diet, don’t further reduce calorie intake, it will just result in lowering and slowing down the metabolism and a total stoppage of the weight loss process.
Although this seems counterproductive to many, in the case of slowing weight loss, it is advisable to increase the calorie intake to 1-2 days to achieve a reset of metabolism, and then return to caloric intake before reset.
This temporary increase in caloric intake when you reach a plateau in weight loss will boost hormone production and speed up metabolism.
The body will receive a signal that there is no more starvation and that it can burn calories at the usual rate.