FatCalc

Calorie Calculator for Weight Management

This calorie calculator is a great tool to help you reach your weight loss or weight gain goal within a realistic time frame and maintain it. It calculates the calories and macros you need to intake daily to reach your goal weight and displays the projected changes in your body's composition over time. The calculator is accurate for individuals 18 years and over who are not pregnant or breastfeeding.

Calorie Calculator

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Enter your age, sex, height, current weight, and goal weight. Click the Physical Activity field to find your physical activity level. Physical activity levels can range from 1.4 (little activity) to 2.3 (very active). The default is 1.4. If you are unsure of your physical activity level, start with the default - the worst-case scenario. Enter a time frame for your goal weight (start date and target date).

The calculator will only produce results if it determines the required food energy intake to reach your goal weight by the target date is more than 1,000 calories per day. Otherwise, it will automatically modify your target date for a minimum daily intake of 1,200 calories for women and 1,500 for men. Nutrition therapists will tell you that, below those levels, you will not meet food group targets and nutrient recommendations. You can change your target date later to increase or decrease your required calorie intake. Consult a doctor for guidance and support if you consider consuming a diet of fewer than 1,000 kcal or 4,200 kJ per day.

Body Dynamics of Weight Change

Your body requires energy to power its essential functions to maintain life and perform physical work. Glucose (blood sugar) is the body's principal energy source from food. When your body gets more glucose than it needs for energy, the surplus first gets sent to the liver and muscles and stored as glycogen for later use. The liver and muscles have a limited storage capacity for glycogen, so once the glycogen stores are full, the excess glucose is converted to triglycerides and stored in fat cells.

To lose weight, you need to consume fewer calories than your body burns, putting your body in an energy deficit state. In that state, your body will compensate for the difference by extracting the extra energy from its energy stores to meet its daily calorie needs, resulting in weight loss from the breakdown of fat and lean body mass tissues.

A popular rule-of-thumb for weight loss says that to lose 1 pound of body weight a week, eat 500 fewer calories daily (3,500 fewer calories weekly). This rule dates back to when there was limited understanding of fundamental metabolic processes. It takes the simplistic view that the energy content of weight loss is 3,500 calories per pound because fat tissue contains approximately 3,500 calories per pound, so burning that equivalent number of calories will result in a 1-pound weight loss. Unfortunately, the body does not work that way. Eating 3,500 fewer calories does not mean your body automatically burns 3,500 calories worth of fat.

The reality is that when you lose weight, you lose fat along with lean tissue in muscle - not just fat. Lean mass consists of everything that's not fat, including muscle, bones, organs, ligaments, tendons, other tissues, and water. The rule also ignores other body dynamics of weight loss, such as the reduction in basal metabolic rate and decreased energy cost of physical activity. Your total daily energy expenditure (TDEE) changes with weight loss. Consequently, followers of diet programs based on this simple rule-of-thumb usually fail to reach their weight loss goal in the long run.

Calculation of Calorie Requirements

This calculator uses the mathematical body model developed by Kevin Dennis Hall, Ph. D., and his team of researchers at the National Institute of Diabetes and Digestive Kidney Diseases of the National Institute of Health. It has been shown that his mathematical model can accurately determine an individual's energy requirements for weight management. It challenges the popular 3,500 calories per pound rule by accounting for the body dynamics discussed earlier, including body adaptations that oppose weight change and the weight change associated with changes to glycogen and extracellular fluid levels.

Calculation of Macronutrients

The calculator will calculate your macronutrients (carbs, protein, and fat) amounts in grams based on the required caloric intake to reach your goal weight with a balanced, low fat, low carbs, and high protein diet. According to the Food and Nutrition Board of the Institutes of Medicine (IOM) dietary guidelines, an acceptable macronutrient distribution range is 45%-65% of calories for carbohydrates, 10%-35% for protein, and 20%-35% for fat. The calculated amounts put each macro's percentage within the IOM acceptable range for the different diet types.

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References:

  1. Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. doi:10.1016/S0140-6736(11)60812-X
  2. Jackson, A., Stanforth, P., Gagnon, J. et al. The effect of sex, age and race on estimating percentage body fat from body mass index: The Heritage Family Study. Int J Obes 26, 789–796 (2002).
  3. M D Mifflin, S T St Jeor, L A Hill, B J Scott, S A Daugherty, Y O Koh, A new predictive equation for resting energy expenditure in healthy individuals, The American Journal of Clinical Nutrition, Volume 51, Issue 2, February 1990, Pages 241–247.
  4. Manore MM. Exercise and the Institute of Medicine recommendations for nutrition. Curr Sports Med Rep. 2005;4(4):193-198. doi:10.1097/01.csmr.0000306206.72186.00
  5. Hall KD. Body fat and fat-free mass inter-relationships: Forbes's theory revisited. Br J Nutr. 2007;97(6):1059-1063. doi:10.1017/S0007114507691946