What is basal metabolic rate (BMR)?
Basal metabolic rate (BMR) is the number of calories the body requires to sustain essential physiological functions -- breathing, circulation, cell production, temperature regulation and organ function -- while at complete rest in a thermoneutral environment after an overnight fast. BMR accounts for approximately 60-75% of total daily energy expenditure in sedentary individuals, making it the largest single component of calorie needs. BMR is influenced by body size, lean muscle mass, age, and sex.
BMR is distinct from resting metabolic rate (RMR), which is measured under less strict conditions and is typically slightly higher than true BMR. In everyday usage and in most online calculators, the terms are often used interchangeably, though technically BMR requires strict measurement conditions. Both BMR and RMR represent the minimum energy the body needs in the absence of any physical activity or digestive effort.
Mifflin-St Jeor vs Harris-Benedict: which formula is more accurate?
The Harris-Benedict equation, originally published in 1919 and revised by Roza and Shizgal in 1984, was the most widely used BMR formula for most of the 20th century. The revised Harris-Benedict equations use weight, height and age with sex-specific constants to predict BMR. For men: BMR = 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) - (5.677 x age in years). For women: BMR = 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) - (4.330 x age in years).
In 1990, MD Mifflin and ST St Jeor published revised predictive equations based on a study of 498 healthy adults. The Mifflin-St Jeor equations are: men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5; women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161. A systematic review by Frankenfield and colleagues published in the Journal of the American Dietetic Association (2005) found the Mifflin-St Jeor equation to be the most accurate for estimating measured RMR in non-obese and obese adults, predicting measured RMR within 10% in approximately 70% of subjects. The Academy of Nutrition and Dietetics endorsed the Mifflin-St Jeor equation as the preferred formula for most clinical applications.
Total daily energy expenditure (TDEE) and activity multipliers
Total Daily Energy Expenditure (TDEE) represents the total number of calories a person burns in a day, encompassing BMR, the thermic effect of food (approximately 10% of calorie intake), and physical activity energy expenditure. TDEE is estimated by multiplying BMR by an activity factor, also called a physical activity level (PAL) multiplier. The table below shows commonly used activity multipliers, which are broadly consistent with values used in energy balance research and clinical practice.
| Activity Level | Description | Multiplier |
|---|---|---|
| Sedentary | Little or no exercise, desk job | 1.2 |
| Lightly active | Light exercise 1-3 days/week | 1.375 |
| Moderately active | Moderate exercise 3-5 days/week | 1.55 |
| Active | Hard exercise 6-7 days/week | 1.725 |
| Very active | Very hard exercise, physical job, or twice-daily training | 1.9 |
Why very low calorie intakes require medical supervision
Very low calorie diets (VLCDs) are generally defined as providing fewer than 800 kcal per day, while low calorie diets typically provide 800-1,500 kcal per day. Health organisations including the NHS and the Academy of Nutrition and Dietetics caution that intakes below approximately 1,200 kcal/day for women and 1,500 kcal/day for men are likely to be insufficient to meet micronutrient requirements without careful medical oversight and supplementation. Very low intakes risk deficiencies in protein, vitamins and minerals essential for health.
Sustained very low calorie intake can also trigger adaptive thermogenesis -- a reduction in metabolic rate beyond what is explained by changes in body composition alone. Research has documented that individuals who have lost significant weight through caloric restriction can have measurably lower resting energy expenditure than weight-matched controls who have not dieted, a phenomenon sometimes described as 'metabolic adaptation'. The NHS and NICE guidelines recommend that calorie deficits for weight loss should generally not exceed 500-600 kcal/day below TDEE, and that VLCDs should only be undertaken with medical supervision.
Factors that affect individual calorie needs
Beyond the variables captured in BMR formulas (weight, height, age, sex), calorie needs are influenced by body composition, hormonal factors, medications and health conditions. People with a higher proportion of lean muscle mass have higher BMRs than those with more fat mass at the same body weight, because muscle tissue is metabolically more active than fat tissue at rest. Conditions such as hypothyroidism can reduce BMR, while hyperthyroidism can increase it.
Pregnancy and breastfeeding substantially increase calorie requirements. Dietary guidelines from major health organisations including the NHS recommend additional energy intake during pregnancy and breastfeeding, with the amount varying by stage and individual circumstances. Age is also relevant: BMR tends to decline with age partly due to decreasing lean mass, meaning calorie needs in older adults are generally somewhat lower than in younger adults at the same activity level.
How to use calorie estimates practically
Formula-based TDEE estimates are starting points, not exact measurements. The most practical approach is to use a formula estimate to establish an initial calorie target, then monitor body weight over two to four weeks and adjust intake based on actual response. Body weight fluctuates daily due to hydration, glycogen storage and digestive contents, so weekly averages are more informative than single daily measurements.
Nutritional quality matters alongside total calorie quantity. Meeting calorie targets with foods that are nutrient-dense -- providing adequate protein, vitamins, minerals and fibre -- supports health outcomes beyond weight management. Dietary guidelines from the WHO and national health agencies recommend basing calorie intake on a varied diet of vegetables, fruits, whole grains, lean proteins and healthy fats, rather than focusing on calorie counting alone. Anyone considering a significant change in calorie intake for health or weight management purposes is advised to consult a registered dietitian or healthcare provider.
Questions fréquentes
How many calories do I need per day?
Daily calorie needs vary significantly by body size, age, sex, and activity level. They are estimated by calculating BMR (basal metabolic rate) using a validated formula such as Mifflin-St Jeor, then multiplying by an activity factor to obtain TDEE (total daily energy expenditure). For reference, many adult women have TDEEs in the range of 1,600-2,400 kcal/day and many adult men 2,000-3,000 kcal/day, but individual variation is substantial. A calorie calculator using personal measurements provides a more accurate estimate than generic population averages.
What is the difference between BMR and TDEE?
BMR (basal metabolic rate) is the number of calories the body needs to maintain basic life functions at complete rest -- breathing, circulation, cell repair and temperature regulation. TDEE (total daily energy expenditure) is the total calories burned in a day, including BMR plus the energy used for physical activity and digesting food. TDEE is calculated by multiplying BMR by an activity factor ranging from approximately 1.2 (sedentary) to 1.9 (very active). TDEE represents the calorie intake at which body weight remains stable.
Which BMR formula is most accurate: Mifflin-St Jeor or Harris-Benedict?
The Mifflin-St Jeor equation (published 1990) is currently considered the most accurate formula for estimating BMR in most adults. A systematic review by Frankenfield and colleagues published in the Journal of the American Dietetic Association (2005) found it predicted measured resting metabolic rate within 10% in approximately 70% of subjects, outperforming the Harris-Benedict equation. Both formulas provide estimates with individual error margins; no prediction formula is a substitute for direct measurement in a metabolic laboratory.
How many calories should I eat to lose weight?
Weight loss requires a sustained calorie deficit -- consuming fewer calories than are expended (TDEE). A deficit of 500 kcal/day is commonly recommended as a moderate starting point, broadly associated with approximately 0.5 kg of weight loss per week. Health organisations including the NHS recommend that intake for women should generally not fall below approximately 1,200 kcal/day and for men below approximately 1,500 kcal/day without medical supervision. Very low calorie diets risk nutritional deficiencies and metabolic adaptation and should only be undertaken under clinical oversight.
Does metabolism slow down with age?
BMR tends to decline with age. This is partly explained by the loss of lean muscle mass (sarcopenia) that typically occurs with ageing, as muscle tissue has higher metabolic activity than fat tissue at rest. Maintaining muscle mass through resistance exercise and consuming adequate dietary protein can help partially offset age-related reductions in BMR. Hormonal changes associated with ageing may also influence energy metabolism.
Are online calorie calculators accurate?
Online calorie calculators based on validated formulas such as Mifflin-St Jeor provide useful estimates, but they have inherent error margins because they use population-derived equations rather than individual measurements. The Mifflin-St Jeor equation predicts measured RMR within 10% in approximately 70% of people, meaning some individuals will see notable differences between their formula estimate and their actual metabolic rate. Using the estimate as a starting point and adjusting based on real-world weight response over two to four weeks is a practical approach.
Références
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. "A new predictive equation for resting energy expenditure in healthy individuals." American Journal of Clinical Nutrition, 1990;51(2):241-247.
- Roza AM, Shizgal HM. "The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass." American Journal of Clinical Nutrition, 1984;40(1):168-182.
- Frankenfield D, Roth-Yousey L, Compher C. "Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review." Journal of the American Dietetic Association, 2005;105(5):775-789.
- NHS. "Understanding calories." NHS, 2022. www.nhs.uk
- National Institute for Health and Care Excellence (NICE). "Obesity: identification, assessment and management." NICE guideline CG189, 2014.
- Hall KD, Heymsfield SB, Kemnitz JW, Klein S, Schoeller DA, Speakman JR. "Energy balance and its components: implications for body weight regulation." American Journal of Clinical Nutrition, 2012;95(4):989-994.
- Levine JA. "Non-exercise activity thermogenesis (NEAT)." Best Practice and Research Clinical Endocrinology and Metabolism, 2002;16(4):679-702.