If you pick up any health or fitness magazine, you’ll likely see an array of calorie recommendations. Some say 1200 per day. Another may say 1500 per day. So, how much is enough? Calories come from all the different foods and beverages we consume and provide energy for our bodies. The energy in food is measured in kilocalories (Kcal), also known as Calories ©. However, it is common practice in non-scientific writing to use the term “calorie,” with a lowercase c, when discussing the energy value of food. When you see the term “calorie” in relation to diet and nutrition, it refers to the kilocalorie. Both calorie and kilocalorie are units of energy. To be precise:
1 calorie = the amount of energy (heat) needed to raise 1 gram of water 1 degree Celsius.
1 kilocalorie (Calorie) = the amount of energy (heat) needed to raise 1 kilogram of water 1 degree Celsius.
The fat in the foods we eat provides 9 calories per gram, alcohol provides 7 calories per gram and carbohydrate and protein each provide 4 calories per gram. This is why many health professionals recommend cutting back on fat to aid in weight loss. Reducing fat intake may also be beneficial in reducing risk of heart disease, diabetes and certain cancers.
The calories (or energy) we consume from food are needed for breathing, metabolizing and absorbing nutrients, maintaining body temperature and other physiologic functions, including exercise. To maintain body weight, calories you consume must equal calories you burn (through normal bodily functions or exercise). When excess calories are eaten and not burned up by the body, the extra calories are stored as fat and result in weight gain. When fewer calories are eaten than are burned, weight loss is the result.
You can estimate calorie requirements through a variety of formulas that are based on a person’s:
- Activity level
Therefore, everyone’s calorie requirements are different. In general, most individuals need a minimum of 1200 calories per day to maintain normal physiological functions. If you consume less than this, your body may feel as though it is ‘starving’ and will lower your metabolism. You will also likely not consume enough calories to meet the RDAs for most nutrients. Very low calorie diets are not recommended for weight reduction.
How to Calculate Energy Needs
Basal Metabolic Rate (BMR)
A common formula used to estimate calorie needs is called the Harris-Benedict Equation (named after the scientists that developed it). It is used to determine a person’s basal metabolic rate (BMR, also called the Basal Energy Expenditure, or BEE). This represents the amount of energy (calories) the person uses for basic bodily processes, such as breathing and maintaining blood pressure. The BMR does not take into account a person’s physical activity.
Total Energy Expenditure (TEE)
Your TEE, or total energy expenditure, is your BMR multiplied by an activity factor. This is the amount of energy you use during a normal day, including calories for physical activity. To put this in context, your BMR are the calories you need to carry out normal bodily functions such as breathing and maintaining body temperature. Your TEE takes into account how active you are. Are you a couch potato, or a marathon runner? The TEE takes into account the extra calories your body needs to keep up with your daily activity.
Decreasing your daily activity level, or eating more calories than you need to support your daily TEE will lead to weight gain. Increasing your activity level, or eating fewer calories than your body needs, will lead to weight loss.
The BMR should be multiplied by a factor that approximates an individual’s physical activity level (PAL) to estimate that persons TEE. Following table gives an approximation of an individual’s daily TEE based on example lifestyles.
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Sources and References:
Harris JA, Benedict FG (1918). “A Biometric Study of Human Basal Metabolism”. Proceedings of the National Academy of Sciences of the United States of America. 4 (12): 370–3.
Roza AM, Shizgal HM (1984). “The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass”. The American Journal of Clinical Nutrition. 40 (1): 168–82.
Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990). “A new predictive equation for resting energy expenditure in healthy individuals”. The American Journal of Clinical Nutrition. 51 (2): 241–7.
Shetty PS, Henry CJ, Black AE, Prentice AM. Energy requirements of adults: an update on basal metabolic rates (BMRs) and physical activity levels (PALs) Eur J Clin Nutr. 1996;50(Suppl 1):S11–S23.