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 is the number of calories needed to cover your basic functions, such as 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. 

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.

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

1. It can lower your metabolism
2. It can cause fatigue and nutrient deficiencies 
3. It may reduce fertility
4. It can weaken your bones
5. It may lower your immunity  

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