What happens when you eat too few calories

We’re told that reducing calories is the key to weight loss, and very low-calorie diets are normalized as a “healthy” approach. While it may cause short-term changes on the scale, eating too few calories can have serious consequences for your metabolism, hormones, and overall well-being.

1. Slowed metabolism

When you reduce your calories to lose weight, your metabolism slows down, which means you burn fewer calories at rest. Surprisingly, these adaptations appear to persist long after the period of calorie restriction, and even if the weight is regained.

2. Fatigue

Calories are your body’s source of energy. When you don’t eat enough, you might feel more tired. This can be especially prevalent among athletes.

3. Hormonal imbalances

Your body needs energy and fat to produce healthy hormones. Studies of athletes who do not consume enough calories show, among other things, lower levels of testosterone, estrogen, luteinizing hormone and follicle-stimulating hormone. Low energy intake is also associated with higher levels of cortisol, a stress hormone that can increase inflammation and the risk of other negative health consequences.

4. Muscle loss

When you cut calories to lose weight, you will lose both fat and muscle mass. Eating enough protein and incorporating strength training can minimize muscle loss, but when you cut calories too much, it may not be possible to compensate for the loss. Losing muscle mass also contributes to a slower metabolism.

5. Nutritional deficiencies

Studies have shown that low-calorie diets can lead to short- and long-term micronutrient deficiencies, which can impact immune health, bone health, metabolism, digestive health, and other aspects of overall well-being. Supplements can help fill some nutritional gaps, but they don’t always provide the same benefits as nutrients from food.

6. Weaker bones

Significant calorie restriction is associated with lower bone mineral density, likely due to reduced energy intake and insufficient intake of vitamins and minerals.

7. Hair loss

Macronutrients, including proteins and fats, as well as micronutrients such as vitamins A, D and E, B vitamins, iron, zinc and selenium, play a crucial role in hair growth. When you don’t get enough calories for your body, you can develop deficiencies in these nutrients, which is why hair breakage and hair loss are side effects of undernutrition.

8. Increased food noise

A landmark study, known as the Minnesota Starvation Study, showed that after a period of calorie restriction, men became preoccupied with food: they thought about it, collected recipes, and read cookbooks.

Since then, many other studies have also shown a link between calorie restriction and food preoccupation. Calorie restriction and dieting have also been linked to increased food-related anxiety.

9. More desires

Eating too few calories can increase cravings, especially for sweets and carbohydrates. Studies also show that people who eat moderately eat as much, if not more, when tempted by “unhealthy” foods.

10. Increased risk of binge episodes

Significantly reducing calories increases the risk of binge eating and overeating. This can happen in the short term, particularly at night or on weekends, when willpower is reduced. Alternatively, this may manifest as alternating more prolonged periods of calorie restriction and binge eating.

11. Possible weight regain and weight cycle

Calorie restriction is associated with short-term weight loss. However, in the long term, this increases the risk of weight regain and weight cycling. Weight cycling is associated with many adverse health effects, including decreased muscle mass, increased fat mass, increased inflammation, and increased risk of diabetes and cardiovascular disease.

12. Constipation

A significant reduction in calories is associated with an increased risk of constipation. Although more research is needed to understand why this happens, it is thought that eating enough and regularly stimulates the movement of food through the colon. The body may also prioritize other, more vital bodily functions when energy intake is too low.

13. Difficulty getting pregnant

When you eat too few calories, your body prioritizes survival and doesn’t prioritize reproduction. When calories are significantly reduced, women may stop menstruating, but even with regular periods, significantly restricting calories can still reduce the chances of getting pregnant.

What counts as too few calories?

Your calorie needs depend on a variety of factors, including genetics, age, gender, activity level and body composition. There is no “safe” calorie deficit that guarantees you won’t experience the above consequences. However, it is helpful to know your approximate basal metabolic rate (BMR) to determine the minimum number of calories your body needs to function before exercising and daily activities.