A calorie is a unit of measurement for energy. The unit's name is French and derives from the Latin calor (heat). In most fields, it has been replaced by the joule, the SI unit of energy. However, it remains in common use for the amount of energy obtained from food.
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Many different definitions for the calorie emerged during the 19th and 20th centuries. They fall into two classes:
The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 g of water by 1 °C. This is about 4.185 J.
The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.185 kJ, and exactly 1000 small calories.
In scientific contexts, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal. SI prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal.
Colloquially, and in nutrition and food labelling, the term "calorie" almost always refers to the kilogram calorie. This applies only to English text; if an energy measurement is given using a unit symbol then the scientific practice prevails there. A convention of capitalising "Calorie" to refer to the kilogram calorie, with uncapitalised "calorie" referring to the gram calorie, is sometimes proposed, but neither recognized in any official standards, nor commonly followed.
The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).
Once you know the number of calories needed to maintain your weight, you can easily calculate the number of calories you need to eat in order to gain or lose weight:
Calorie needs to gain weight
If you want to gain body weight, you need to consume more calories than you burn. One pound of body weight is roughly equivalent to 3500 calories, so eating an extra 500 calories per day will cause you to gain one pound a week.
For optimum health, if you increase your calories to gain weight then (health permitting) gradually increase your level of physical exercise in order to maintain or increase your lean body mass. The benefits of exercise on physical and mental health are well documented and shouldn't be ignored.
Calorie needs to lose weight
There are approximately 3500 calories in a pound of stored body fat. So, if you create a 3500-calorie deficit through diet, exercise or a combination of both, you will lose one pound of body weight. (On average 75% of this is fat, 25% lean tissue) If you create a 7000 calorie deficit you will lose two pounds and so on. The calorie deficit can be achieved either by calorie-restriction alone, or by a combination of fewer calories in (diet) and more calories out (exercise). This combination of diet and exercise is best for lasting weight loss. Indeed, sustained weight loss is difficult or impossible without increased regular exercise.
If you want to lose fat, a useful guideline for lowering your calorie intake is to reduce your calories by at least 500, but not more than 1000 below your maintenance level. For people with only a small amount of weight to lose, 1000 calories will be too much of a deficit. As a guide to minimum calorie intake, the American College of Sports Medicine (ACSM) recommends that calorie levels never drop below 1200 calories per day for women or 1800 calories per day for men. Even these calorie levels are quite low.
An alternative way of calculating a safe minimum calorie-intake level is by reference to your body weight or current body weight. Reducing calories by 15-20% below your daily calorie maintenance needs is a useful start. You may increase this depending on your weight loss goals.