How fat loss works

You lose fat when you burn more energy than you eat for weeks, so your body draws on what it has stored. Most of that fat leaves as carbon dioxide you breathe out. Hormones change how hungry you are, not the arithmetic.

Where does fat go when you lose it?

Ask people where lost fat goes and most will guess it turns into heat or energy. When two scientists at UNSW Sydney asked 150 doctors, dietitians and personal trainers, more than half said exactly that. In 2014 the two, the physicist Ruben Meerman and the biochemist Andrew Brown, worked it out atom by atom and published it in the BMJ. To burn 10 kg of fat, your body takes in 29 kg of oxygen and gives off 28 kg of carbon dioxide and 11 kg of water.

Counted by the fat’s own atoms, 84% of it leaves as carbon dioxide and 16% as water. If you lose 10 pounds of fat, about 8.4 pounds of it goes out through your lungs. The rest becomes water, which leaves in urine, sweat, breath and tears.

Your lungs are the main exit, but breathing harder won’t speed it up. Meerman and Brown point out that breathing faster without burning more fuel just leads to hyperventilation, which can make you dizzy or even faint. The only way to send more carbon out is to burn more than you take in, and that’s set by what you eat and how much you move. Sweating more doesn’t burn more fat either (more on sweat).

Where your body keeps its energy

Think of your body’s energy as money. Some of it is in your hand, some in your wallet, and most of it in savings.

1. Blood sugar is the cash in your hand. It’s a small amount, always being spent and topped up.
2. Glycogen is your wallet. It’s sugar stored in your liver and muscles, about a day of fuel. Each gram is stored with three to four grams of water, which is why it shows up on the scale.
3. Bodyfat is your savings account. It holds weeks of energy. The old rule of thumb is about 3,500 calories in a pound of bodyfat, so 30 pounds of fat holds around 105,000 calories, almost two months of food for someone who eats 2,000 a day.
4. Muscle isn’t a store. Your body will break some down when food is short, more so when protein is low and you don’t lift. That’s the part of weight loss you want to avoid.

You don’t choose which account you spend from. The mix shifts through the day and with how hard you’re moving, and what decides your fat is the balance across all of them, week after week.

Does it matter if you burn fat or carbs?

Is paying from your wallet different from paying with a credit card? Sort of, but not really. The money comes out of a different place first, and it all comes out of the same account by the end of the month.

Burning fat works the same way. A low carb day burns more fat than a high carb one, and in the NIH ward study below, cutting carbs raised fat burning within days. But your body refills whatever it spent from wherever the next food arrives, so the share burned in one session doesn’t decide where you end up. In one small trial, 20 young women did an hour of cardio three times a week for four weeks, either fasted or after a small meal, on the same diet. Both groups lost weight and fat, and the difference between them was too small to count. There’s more on the fat-burning zone.

Is it calories or hormones?

Both sides have a point. The calories set the result. The food you choose, your sleep and your stress set your hunger, and your hunger sets the calories. That’s why losing fat starts with eating less without being hungry. There’s more on insulin and fat storage.

Why does weight loss slow down?

The 3,500 calorie rule says a deficit of 500 calories a day takes off a pound a week, every week, forever. It’s right about the first few weeks and wrong about the rest.

In 2013 a group of researchers checked the rule against seven weight loss studies where people lived in a research unit and every meal was measured, 103 adults in all. The rule predicted they would lose about 28 pounds. They lost about 20.

The rule misses two things. A lighter body burns less, so the same food becomes a smaller deficit as you go. And the body makes up some of the gap on its own, which is the subject of why your burn drops.

Kevin Hall’s team at the NIH built a model of this from the experiments. Their rule of thumb is that every change of 10 calories a day changes your weight by about a pound in the end. Half of that change comes in about a year, and 95% in about three years. Cut 250 calories a day for good and you’ll end up about 25 pounds lighter, not 26 pounds lighter every year.

That table explains the most common plateau. Your calories haven’t stopped working. You’ve arrived near the weight those calories support, and to keep going you need a new number. Your calories, pace and protein covers how to set and re-set it.

Why is the first week so fast?

In the first week of a diet the scale often drops fast, and people think they’ve found the answer. A lot of that is glycogen and the water stored with it. You eat less, you run your wallet down, and the water leaves with it. It comes back just as fast when you eat more carbs again, which is why one big dinner can put pounds back on the scale overnight.

None of that is fat. Fat changes slowly, and it takes about three or four weeks of trend before the scale shows it. Tracking your food and your weight explains how to see through the swings.

Why lose fat at all?

Most people start for how they look, and the health gains are bigger than the mirror shows.

• Where it sits counts. The NIH’s heart and lung institute says a waist over 40 inches for men or 35 for women raises your risk of heart disease and type 2 diabetes, and that fat around the waist is riskier than fat at the hips.
• A small loss does a lot. In the Diabetes Prevention Program, 3,234 US adults with raised blood sugar were coached to lose 7% of their weight and walk 150 minutes a week. Over about three years they got type 2 diabetes 58% less often than the placebo group.
• It’s the American normal now. In the latest national survey, 2021 to 2023, 40.3% of US adults had obesity and 9.4% had severe obesity.