Where Does the Fat You Burn Go? 6 Exercise Myths
Many ideas about exercise get repeated without any scientific backing. We look at six common beliefs and explain what really happens in the body, including where the fat you lose actually goes.
Muscle soreness isn't caused by lactic acid crystals
A popular claim online is that muscle soreness is caused by lactic acid crystals. The timing doesn't add up. Lactate returns to resting levels within one or two hours. Soreness, on the other hand, peaks between 24 and 72 hours after exercise.
The most widely accepted explanation is microscopic damage to muscle fibers. It happens when a muscle brakes while being stretched under load, such as when walking down hills or stairs. Think of a car on a mountain pass: it isn't the engine that takes the strain, it's the brakes. What hurts is the inflammation that repairs those fibers.
What's more, lactate isn't a waste product. In the 1980s, physiologist George Brooks showed that it is a fuel. The heart, for example, uses it.
A side stitch isn't caused by a lack of oxygen
In one survey, more than half of runners had experienced a side stitch in the previous year. For decades, the diaphragm was blamed, supposedly due to a lack of oxygen. But one fact doesn't fit: horse riders get stitches too, and they barely strain their breathing.
The most likely explanation is irritation of the peritoneum, the membrane that lines the abdomen. This membrane rubs with every jolt. On a full stomach, the friction increases. That's why it's best to avoid large meals, juices and very sugary drinks in the two hours beforehand.
Sometimes the pain shows up at the tip of the shoulder. This is because the upper part of the peritoneum shares a nerve root with that area.
Stretching before running doesn't prevent injuries
In 2000, a study of around 1,500 Australian army recruits compared those who stretched before exercise with those who didn't. Injury rates were virtually the same in both groups.
In addition, holding a stretch for more than a minute slightly reduces strength for a while. The approach with the strongest evidence is a dynamic warm-up: five to ten minutes of easy jogging, followed by strides and high knees, getting progressively faster.
Ab exercises don't get rid of belly fat
In 2011, a study had a group of people do ab exercises five days a week for six weeks. They gained endurance, but their abdominal fat didn't drop significantly.
The reason is that the body draws on its fat reserves as a whole. It doesn't burn fat from the area being exercised.
Running doesn't wreck your knees
A 2017 meta-analysis of more than 100,000 people found hip or knee osteoarthritis in 3.5% of recreational runners. Among sedentary people, the figure was 10.2%. Only competitive runners, who log very high mileage, exceeded both groups, at 13.3%.
This is an association, not a proven cause. But it makes sense: cartilage has no blood vessels and gets its nourishment by being compressed and released, like a sponge.
Sweating doesn't burn fat: here's how it really leaves your body
Sweating more doesn't mean burning more fat. If you run in a rain jacket in August, the scale may show a kilo less. After you drink a liter of water, that weight comes back.
In 2014, physicist Ruben Meerman and biochemist Andrew Brown asked 150 doctors, dietitians and personal trainers where fat goes. The most common answer was that it turns into energy or heat. But matter doesn't just disappear.
The researchers traced every atom and published their calculation in the journal BMJ. Of every ten kilos of fat, 8.4 leave the body as carbon dioxide through the lungs. The remaining 1.6 leave as water: urine, sweat, tears and breath. In other words, most of the fat you lose, you breathe out.
Frequently asked questions
What really causes muscle soreness?
The most widely accepted explanation is microscopic damage to muscle fibers when a muscle brakes while being stretched under load. The pain comes from the inflammation that repairs those fibers.
How can you avoid a side stitch?
It's best to avoid large meals, juices and very sugary drinks in the two hours before exercise, since a full stomach increases friction on the peritoneum.
Is it better to stretch or warm up before running?
The approach with the strongest evidence is a dynamic warm-up: five to ten minutes of easy jogging, followed by strides and high knees, getting progressively faster.
Where does the fat you lose go?
Of every ten kilos, 8.4 leave as carbon dioxide through the lungs. The other 1.6 leave as water through urine, sweat, tears and breath.
Informative article written with the help of AI tools from the verified script of the video. The video is in Spanish.