Content warning: This article contains mentions of eating disorders.

The calorie emerged as a unit of heat — specifically, the amount of heat required to increase the temperature of one kilogram of water by one degree Celsius — in 1824, as defined by French physicist and chemist Nicolas Clément-Desormes. In modern usage, the calorie as a measure of energy within food appears on food labels as the kilocalorie.

However, measurements on food labels are not exact. Estimates of food energy are derived from the Atwater general factor system, which was developed by American chemist and physiologist Wilbur Olin Atwater through a series of experiments in the nineteenth century. In Atwater’s general factor system, each main group of nutrients — protein, fat, and carbohydrates — carries a single energy value. Protein and carbohydrates have an energy value of four kilocalories per gram, while fat has an energy value of nine kilocalories per gram. Atwater also calculated an energy value for alcohol — seven kilocalories per gram. For instance, a snack containing 12 grams of protein, 15 grams of carbohydrates, and nine grams of fat would be labelled as containing 189 calories.

Calorie counting problems

The conventional wisdom on weight loss states that you should create a calorie deficit: expend more calories — through daily activities and exercise — than you consume. But this wisdom is riddled with inaccuracy.

Usually 50 to 70 per cent of calorie expenditure results from an individual’s resting metabolic rate, which is the amount of energy that the body needs to maintain basic functions. However, resting metabolic rate may vary, based on several factors, including body composition, age, ethnicity, illness — such as thyroid disease or diabetes — and being on medication.

Resting metabolic rate equations account for weight, height, age, and sex, expressing resting metabolic rate in kilocalories per day. Most online calorie calculators use this equation. The most common equation for resting metabolic rate, the Mifflin-St Jeor equation, has an error rate of 20 per cent: a 2005 systematic review found that the equation could underestimate resting metabolic rate by up to 20 per cent and overestimate by up to 15 per cent.

Other issues arise when people use resting metabolic rate to determine their ideal calorie intake. Metabolism fluctuates and varies for everyone. A 2021 comprehensive study on metabolism found that metabolism speeds up in the first year of life and stays high throughout childhood, before tapering off to adult levels at age 20. Then, metabolism remains relatively stable throughout adulthood — from age 20 to 60 — before steadily declining in old age. The researchers also found considerable variation between individuals.

Calorie (in)equalities: A calorie is not a calorie

Researchers from the Department of Family and Social Medicine at the Albert Einstein College of Medicine have critiqued the simplistic assumption implicit in the reliance on calorie deficits for weight loss: that a ‘calorie is a calorie’ so the quality of food consumed doesn’t matter. In fact, all calories are not equal. Different types of food have different effects on the body, depending on how they affect various hormones that regulate appetite, satiety, body composition, and blood sugar. For instance, 100 calories of salmon affect the body differently than 100 calories of white rice.

The behavioural angle: Calorie counting and eating disorders

Calorie counting may trigger, worsen, or maintain disordered eating. A 2017 study conducted on an undergraduate sample found that students who used calorie counting and fitness tracking apps exhibited dietary restraint and higher levels of anxiety about their eating compared to students who did not use those apps. Another 2017 study, conducted on a sample of 105 individuals with eating disorders, found that using a calorie counting app had contributed to 73 per cent of participants’ eating disorders.

Considering the inaccuracy of food labels, complexities of metabolism, and behavioural considerations, calorie counting is not as straightforward as it initially appears.