When most of us think of calories, we think of how fattening a food is. In dietary terms, calories are the amount of energy that a food provides.If we consistently take in more energy than we need, we will gain weight. If we take in too little energy, we will lose weight, fat, and eventually muscle mass.
The definition of a calorie is the amount of energy needed to raise the temperature of 1 gram (g) of water through 1° Celsius.
The type and amount of food we eat determine how many calories we consume. For many people on a weight-loss diet, the number of calories in a food is a deciding factor in choosing whether or not to eat it.
How and when we eat can also make a difference, as the body uses energy differently throughout the day. Our body’s energy use will depend on how active we are, how efficiently our body uses the energy, and our age.
According to the 2015-2020 Dietary Guidelines for Americans, women are likely to need between 1,600 and 2,400 calories a day, and men from 2,000 to 3,000. However, this depends on their age, size, height, lifestyle, overall health, and activity level.
Fast facts on calorie intake and use
*Recommended calorie intake depends on factors such as age, size, height, sex, lifestyle, and overall general health.
*Recommended daily calorie intakes in the US are around 2,500 for men and 2,000 for women.
*Eating a big breakfast could help with weight reduction and maintenance.
*The brain uses around 20 percent of the energy used in the human body.
*Factors affecting ideal calorific intake include age, bone density, and muscle-fat ratio.
*A 500-calorie meal consisting of fruits and vegetables has more health benefits and will keep you feeling full for longer than a 500-calorie snack of pop
The 2015-2020 Dietary Guidelines for Americans recommend a calorie intake that ranges from 1,000 calories a day for an infant of 2 years to 3,200 for an active male aged 16 to 18 years.
As people get older, their metabolic rate slows down.
This reduces their need for energy. From age 19 to 25 years, the recommended intake for women is 2,000 calories a day, but after 51 years, this falls to 1,600.
For the human body to remain alive, it needs energy.
Around 20 percent of the energy we take in is used for brain metabolism. Most of the rest is used in basal metabolism, the energy we need when in a resting state, for functions such as blood circulation, digestion, and breathing.
In a cold environment, we need more energy to maintain a constant body temperature, as our metabolism increases to produce more heat. In a warm environment, we need less energy.
We also need mechanical energy for our skeletal muscles, to maintain posture and move around.
Cellular respiration is the metabolic process by which cells get energy by reacting oxygen with glucose to produce carbon dioxide, water, and energy.
How efficiently energy from respiration converts into physical—or mechanical— power depends on the type of food eaten, the type of physical energy, and whether muscles are used aerobically or anaerobically.
In other words, we need calories to fuel bodily functions, such as breathing and thinking, to maintain our posture, and to move around.