Ten people had the same complaint. Each had been eating under 1,200 calories a day, by their own records, and none of them could lose weight. They put it down to their genes.
So what they burned and what they ate were measured for fourteen days.
Their bodies burned what bodies of their size should, to within 5%. The records were the problem. They were eating 47% more than they had written down, and they had overstated their exercise by 51%.
This needs no liars. Counting calories is that hard, and it is harder than they knew: the numbers they were writing down were wrong before the pencil touched the paper.
"Calories in, calories out" has been the foundation of most diet advice for decades. At a surface level it makes sense, and as physics it is correct. The trouble is the bookkeeping. The sum is right. The entries are not.
Where the calorie came from
The first man to keep this ledger did it without calories. For thirty years Santorio Santorio of Padua sat in a chair that was also a balance, weighing himself, everything he ate and drank, and everything that left him. For every eight pounds that went in, three came out. He wrote it up in 1614. The other five pounds had gone somewhere he could not see.
Why food is measured in heat was settled with a guinea pig. In 1780 Antoine Lavoisier used the warmth of the animal's breathing to melt snow, and showed that respiration is a slow form of burning. He named his apparatus the calorimeter.
The calorie itself began as a unit of heat. The word is from the Latin calor, and Nicolas Clément introduced it in lectures between 1819 and 1824. Food came later.
It came in 1887, when Wilbur Atwater took the unit up in an article on the energy in what people eat. His method gave the figures still printed on packets: burn the food to see what heat it holds, subtract what the body passes out unused, and round off. Four calories a gram for protein and carbohydrate, nine for fat. Why Diet and Exercise Are Non-Negotiable for Fat Loss, linked below, has the copper chamber he shut people in.
Then, in 1918, Lulu Hunt Peters turned her newspaper column into a book, Diet and Health: With Key to the Calories. "Hereafter you are going to eat calories of food," she told her readers. "Instead of saying one slice of bread, or a piece of pie, you will say 100 calories of bread, 350 calories of pie." It sold two million copies and topped the non-fiction list in 1924 and 1925.
So the number on the packet is an average, worked out more than a century ago, of what an average body takes from a class of food. Every reason below is a way in which you, or your food, are not the average.
1. The label is an estimate
Eighteen adults ate almonds as part of a controlled diet, and everything that came out of them was collected and measured. The almonds delivered 129 calories a serving. The standard method says 168 to 170. That is a 32% overstatement.
Restaurant menus fail dish by dish. When 269 items from 42 restaurants were burned in a laboratory, the menus were right on average. But 19% of the items held at least 100 calories more than stated. In a smaller test of dishes sold as lower in calories, some held twice the stated figure.
Without a figure to copy, the eye does worse. Outside 89 fast-food restaurants, 1,877 adults were asked how many calories were in the meal they had just bought. The meals averaged 836. The guesses came in 175 short, and the bigger the meal, the bigger the miss.
An average is no use to you. You eat the dish.
2. Your body charges for digestion
The body does not treat 100 calories of sweets the same as 100 calories of chicken. Digesting food costs energy, and the charge depends on the food: 0 to 3% of the calories for fat, 5 to 10% for carbohydrate, 20 to 30% for protein.
Processing changes the bill as well. Seventeen people ate two cheese sandwiches on different days, with the same calories in each. One was multi-grain bread and cheddar. The other was white bread and a processed cheese product. Digesting the first cost 137 calories. The second cost 73.
Cooking pushes the same way. Mice fed cooked meat gained more weight than mice fed the same meat raw, though they ate no more and moved no less. That is mice. The authors' conclusion applies to any label, though: the standard figures overstate what you get from food that has had little done to it.
No label shows any of this.
3. Your gut takes its own cut
What you absorb depends partly on what lives in your gut. Twelve lean and nine obese volunteers lived in a clinic while their diets were switched between 2,400 and 3,400 calories a day, and the calories left in their stools were measured.
Their gut bacteria shifted quickly with the change of diet. In the lean volunteers, a 20% swing between two large groups of bacteria went with about 150 more calories absorbed.
Twenty-one people is a small test. It points the same way as the rest: two people can eat the same plate and bank different amounts.
4. The watch is guessing
That is the "in" side. The "out" side is worse.
It starts with the number your app begins from: what your body burns at rest. That comes from an equation. When the four equations in common use were checked against people's measured figures, the best of them was the one most likely to land within 10%, "but noteworthy errors and limitations exist when it is applied to individuals". An equation describes a crowd. You are one person.
Then comes the movement. Sixty volunteers wore seven popular wrist devices while their real energy use was measured as they sat, walked, ran and cycled. Most of the devices read heart rate well. None got calories burned to within 20%.
So the 400 calories your wrist says you have earned is a guess. And exercise moves the day's total less than the display suggests: Why Diet and Exercise Are Non-Negotiable for Fat Loss, linked below, has why.
5. "Out" moves when "in" moves
The body is designed for survival. When calories are cut, it spends less.
Eighteen obese people and 23 who had never been obese were held at a weight 10% or more below their usual one. They burned 6 to 8 fewer calories a day for every kilo of lean tissue, meaning everything in the body that is not fat, than they had before. For someone carrying 50 kilos of lean tissue, that is 300 to 400 calories a day gone from the "out" column, with no change in what they do.
It runs both ways. Held at 10% above their usual weight, the same people burned more. The body defends the weight it knows.
This is what breaks the best-known rule in dieting: 3,500 calories equal a pound of fat, so 500 fewer a day takes off a pound a week. Set against seven experiments in which intake was controlled or measured, the rule "grossly overestimates actual weight loss". Weight answers a change in eating slowly, with about half the eventual change arriving in the first year.
Why People Fail at Weight Loss (And How to Succeed), linked below, has how long the slowdown lasts.
6. You are not the average
Two people eating the same number of calories can have completely different results.
Twelve pairs of identical twins, all young men, were each fed 1,000 calories a day more than they needed, six days a week, until every one of them had eaten the same surplus: 84,000 calories. They gained 8.1 kg on average. The smallest gain was 4.3 kg and the largest 13.3 kg.
The gains ran in pairs. Each man gained much as his twin did, with about three times more variation between pairs than within them. Where the fat settled ran in pairs more strongly still.
The same surplus, a threefold spread, and a good part of it inherited.
The hormone argument, tested
There is a popular way out of all this. Forget calories, it says: carbohydrate raises insulin, insulin tells the body to store fat, so cut the carbohydrate and the fat comes off whatever the count.
It is a serious idea, set out at length in , and it has been tested where the count cannot drift: in wards where every mouthful is measured.
In one, 19 obese adults had the same number of calories cut from their diet twice, once from carbohydrate and once from fat, for six days each. Cutting carbohydrate took off 53 grams of body fat a day. Cutting fat took off 89.
In another, 17 overweight men spent four weeks on a high-carbohydrate diet and four on a ketogenic one with the same calories. On keto they burned slightly more, by an amount near the limit of what the instruments could detect, and their loss of body fat slowed.
A third points the other way. In it, 164 adults who had just lost 12% of their weight were kept at the new weight for 20 weeks, on diets high, moderate or low in carbohydrate. Those on the low-carbohydrate diet burned 209 calories a day more than those on the high.
Put together, with calories held equal, cutting carbohydrate does not strip extra fat in the short run, and it may help the body burn more while holding a loss. The reports draw opposite conclusions from one another, and the question is not settled.
Where low-carb, keto and carnivore eating have the firmer case is appetite, the half of the ledger you can feel. In that third test, the low-carbohydrate group also had lower levels of ghrelin, the hunger hormone. The first article linked below has the diets compared, and what each can promise.
The other hormone blamed for a stubborn belly is cortisol, and The Invisible War Inside Your Body (And How to Win It) has what holds there.
Does counting work at all?
As measurement, no. Start with a whole country. Across 39 years of the United States' national nutrition survey, 28,993 men and 34,369 women reported eating 281 and 365 calories a day less than bodies of their size needed. For 67.3% of the women and 58.7% of the men, the figure they gave was "not physiologically plausible".
Training helps less than you would hope. Ten women were taught to weigh and record everything they ate for a week, and were told the record would be checked against what their bodies burned. They still came up 429 calories a day short. Ten dietitians, who are practised at recording food, did better.
Coaching closes only part of the gap. In one two-year test, 218 adults who were not obese were coached to eat 25% less. They managed 11.7%. It was still enough to take off a tenth of their weight and keep it off.
As a habit, somewhat, and for a reason that has little to do with arithmetic. Across 22 reports on people keeping records of their food, their exercise or their weight, the record-keepers consistently lost more. The reviewers called the evidence weak, because so much of it rested on the people's own word.
What a record does is make you look, and that is worth having.
So count if it helps you look. Do not mistake the total for a measurement.
And stop if it starts to count you. Among 493 students, those who used calorie trackers scored higher on worry about eating and on restricting food. That is a link, and it cannot say which came first. The authors' own conclusion was that "for some individuals, these devices might do more harm than good". If the numbers begin to run your day, put them down.
If you count anyway
If you like a number, a rough one beats none. Four habits make it less wrong, each taken from a finding above.
Weigh the food. The eye came in 175 calories short on a single meal.
Add something for anything you did not cook. One restaurant dish in five ran at least 100 calories over its menu.
Give the watch no credit. Do not eat back what it says you burned.
Let the scale correct you. If the trend over three or four weeks is flat, you are eating what you burn, whatever the app says. Change the plate, and leave the arithmetic alone.
What to count instead
Things you can count without a laboratory.
Meals. Two or three, and nothing between them. Why Snacking and Small Meals Can Sabotage Your Weight Loss has what the bites between meals do.
Protein. Is there a real portion on the plate: meat, fish or eggs? It costs the most to digest, and the first article linked below has what it does to hunger.
How processed it is. Whole, unprocessed foods regulate appetite better, leading to less overeating without needing to track calories. The same article has the test: the same calories on offer, and 500 more eaten a day from the processed menu.
What you drink. Count the sweet ones as food. The Hidden Cost of Your Daily Sugary Drink has why they slip past your appetite.
Your nights. The first article linked below has the test in which the same diet took off less fat on short sleep.
Your waist. A sees fat that the scale misses.
The trend. Weigh on the same terms each time and read the line over weeks. A keeps the line for you. One day's number is noise, for every reason above.
The sum and the entries
The ten people at the start had done what they were told. They were eating nearly half as much again as they had counted, the labels they copied from were averages, and their bodies were free to move the target.
Calories matter, but so does biology. The law holds. The ledger never balanced, because so many of its entries were guesses. Santorio weighed every mouthful for thirty years and still could not see where most of it went.
For one week, stop counting calories and count two things instead: your meals, and how many of them had a real portion of protein.
Want the full plan for losing weight? Why People Fail at Weight Loss (And How to Succeed) — why the body fights back, real food and protein first, and what the long-term keepers do.
Wondering what exercise adds, if it burns so little? Why Diet and Exercise Are Non-Negotiable for Fat Loss — what training does that the scale never shows: muscle, belly fat and years.
Know someone who logs every bite and cannot work out why the scale will not move? Send them this, and point them to the ten people at the top.
Disclaimer: This article is general information. For advice about your own health, talk to a qualified health professional.
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