The amount of energy supplied by a nutrient is measured in calories. Technically, one calorie is the amount of energy required to raise the temperature of 1 gram of water 1 degree Centigrade (from 14.5 to 15.5). The 'calorie' measure used commonly to discuss the energy content of food is actually a kilocalorie (KCal) or 1000 real calories; this is the amount of energy required to raise one kilogram of water (about 2.2 pounds)
Different foods can be used by the body to produce different amounts of energy. 60 to 65 percent of your calories are spent just keeping you alive and keeping your heart beating, your kidneys filtering waste, and maintaining temperature near 98 degrees. Another 25 percent goes for pure movement. The remaining 10 percent of calories is spent processing food. Various national and international Committees have recommended allowances for different nutrients and the total calorie needs for different age groups with different activity levels.
At least a minimum amount of 1200 calories of nutritious food should be consumed a day. Fewer than that would lower iron level and slow down metabolism.
Your calorie needs depends on a lot of factors like your age, gender, size, height, weight, activity level and also the climate and environment you live in. The lower body size and the warm climate in tropical countries make the calorie requirements of people living there less than those corresponding age groups living in temperate climates. Likewise, Calorie needs will hit the highest point during your mid - twenties, so at 25, you need 2300 calories/day. And then decline at about 2% per decade, by 35 you may need only 2254 calories. This reduction in calorie needs is due partly to an increase in body fat percentage that comes with age. The gender factor also have its influence on calorie needs. Men because of their higher percentage of lean muscle tissue, need 5-10% more calories than women. But women during pregnancy and lactation will need 300- 500 calories more per day than their usual needs.