Calorie needs: estimating what your body uses in a day
Your daily calorie needs can be estimated in two steps. First, calculate your resting energy expenditure with the Mifflin-St Jeor equation: ten times your weight in kilograms, plus 6.25 times your height in centimetres, minus five times your age, plus 5 for a man or minus 161 for a woman. Then multiply the result by an activity factor: 1.2 for a desk job with no exercise, about 1.375 with light exercise, 1.55 with training three to five days a week, 1.725 with hard daily training. An 80-kilogram man of 45, 180 centimetres tall, training four times a week, arrives at about 2,640 kilocalories. The figure is an estimate, accurate to within roughly 10% for most people. The reliable method is to eat at that level for three weeks and watch what your weight does.
Estimate your daily energy needs
The Mifflin-St Jeor equation gives your resting expenditure; the activity factor scales it to your week.
- Resting energy expenditure (Mifflin-St Jeor)
- 1,705 kcal per day
- Estimated daily needs
- 2,643 kcal per day
An estimate accurate to about 10%. Not suitable during pregnancy or with a metabolic condition.
Where the energy goes
The energy you use in a day has three components.
- Resting expenditure: what the body uses to stay alive, at rest. It accounts for 60 to 70% of the total in most adults and depends mainly on body size, lean mass, age and sex.
- Activity: everything from formal training to walking between meetings. This is the part that varies most between people and from day to day.
- Digestion: the energy cost of processing food, about a tenth of what you eat.
Training is often a smaller share than people assume. An hour of hard exercise might use 500 to 700 kilocalories. Being on your feet and moving through the day, as opposed to sitting through it, can account for as much. See sitting all day.
The equation, and why this one
Several equations predict resting expenditure. The one published by Mifflin and St Jeor in 1990, developed on 498 healthy adults, has proved the most accurate for people of normal weight and for those who are overweight, and it is the one most dietitians use.
It needs four inputs: weight, height, age and sex. The calculator above applies it and multiplies by the activity factor you choose.
Its limits are worth knowing.
- It predicts the average for people like you. Individuals differ from the prediction by 10% or so in either direction.
- It does not see body composition. A muscular person uses more than the equation says; someone with little muscle uses less.
- The activity factor is a coarse guess. Most people overestimate it.
Choosing the activity factor honestly
- 1.2: seated work, car to desk to car, no structured exercise.
- 1.375: seated work with one to three light sessions a week, or a job that keeps you on your feet part of the day.
- 1.55: three to five proper training sessions a week.
- 1.725: hard training on six or seven days, or physical work.
A senior executive who trains four times a week and sits for eleven hours a day is somewhere between the second and third. When in doubt, choose the lower.
Turning the estimate into a decision
The estimate is where you start. Your body gives the answer.
- Eat at the estimated level for three weeks.
- Weigh yourself on waking, on at least four mornings a week, and compare weekly averages, not single days.
- If the average is stable, you have found your maintenance intake.
- To lose fat, reduce by 300 to 500 kilocalories a day. To gain, add 200 to 300.
A deficit of that size produces a loss of roughly a quarter to half a kilogram a week, a rate at which muscle can be kept if protein intake is adequate and you continue to lift. See protein needs and strength training for executives.
Why crash diets fail people with demanding jobs
Large deficits work on paper and collapse in practice, for reasons that are particularly sharp when your days are long.
- Concentration suffers. Decision-making on too little food is noticeably worse by late afternoon.
- Sleep deteriorates, and short sleep in turn increases appetite the next day. See sleep for executives.
- Muscle is lost along with fat, which lowers resting expenditure and makes the weight easier to regain.
- Business meals undo a rigid plan in an evening. A moderate plan absorbs them.
Counting or not counting
Tracking every meal for two or three weeks is instructive: most people discover that their intake is 20 to 30% higher than they believed, usually because of oils, drinks and what is eaten standing up. After that, few need to continue. A handful of fixed habits does the work.
- A protein source and vegetables at every meal.
- Starch scaled to the day's training.
- Alcohol counted for what it is: seven kilocalories per gram, and a reliable way to eat more afterwards. See alcohol and training.
- Nothing eaten in the car or at the desk without a decision.
When to seek individual advice
An equation is not appropriate if you are pregnant, have diabetes or a thyroid condition, have a history of disordered eating, or are over 70. A physician or registered dietitian can measure resting expenditure directly by indirect calorimetry, which removes the guesswork.
Body weight is in any case an incomplete measure. Two people of the same weight can carry very different amounts of abdominal fat: see waist-to-height ratio.
The figure from the calculator is a starting estimate, not a prescription. Adjust it to what your weight does over three weeks.
Questions
How many calories should I eat per day?
It depends on your size, age, sex and activity. Estimate your resting expenditure with the Mifflin-St Jeor equation and multiply it by an activity factor between 1.2 and 1.725. Most adults fall between 1,800 and 3,000 kilocalories. Treat the result as a starting point and adjust it according to what your weight does over three weeks.
How accurate are calorie calculators?
For most people, within about 10% of their true expenditure. The equations predict an average for someone of your size and age, and cannot see how much muscle you carry or how much you move outside training. The activity factor is the main source of error, because people tend to rate themselves as more active than they are.
How large should a calorie deficit be to lose fat?
A deficit of 300 to 500 kilocalories a day is a sustainable range for most adults. It gives a loss of roughly a quarter to half a kilogram a week, slow enough to preserve muscle when protein intake is adequate and strength training continues. Larger deficits bring faster loss, at the cost of hunger, poor sleep and lost muscle.
Sources
- Mifflin et al.: a new predictive equation for resting energy expenditure (Am J Clin Nutr, 1990)
- Academy of Nutrition and Dietetics, Dietitians of Canada and ACSM: nutrition and athletic performance (2016)
- World Health Organization: healthy diet, key facts
- World Health Organization: obesity and overweight, key facts