Do carbs make you fat?

No. In a year-long trial of 609 adults, people on a healthy low-carb diet lost about 13 pounds and people on a healthy low-fat diet lost about 12. The gap was too small to matter. What you can keep eating decides more than which one you cut.

Why low-carb feels faster

The first week on low-carb, the scale drops fast, and that feels like proof. Most of it is water. Your body stores carbs in muscle and liver with at least 3 grams of water for every gram. Cut carbs and that water goes. In a 1970s study on an 800 calorie diet, 61% of the weight lost in 10 days on a very low-carb diet was water, against 37% on a mixed diet.

Cutting carbs or cutting fat is like cutting rent or cutting groceries. Either one saves money if the total goes down.

What the trials found

• A year, real life. In DIETFITS (2018), 609 adults ate a healthy low-fat or healthy low-carb diet for 12 months and lost about 12 and 13 pounds (5.3 kg and 6.0 kg). Neither their genes nor how much insulin they made predicted which diet worked better for them.
• Calorie for calorie, in a lab. In a 2015 NIH study, 19 adults lived on a metabolic ward and cut 30% of their calories, once by cutting carbs and once by cutting fat. Cutting fat took off slightly more bodyfat, 89 grams a day against 53. That’s the opposite of what the insulin theory predicts, and the gap is too small to matter in daily life.

What to do

• Choose the way of eating you’d still follow in six months.
• Cut refined carbs first, like white bread, pastries and sweet drinks, since they’re easy to overeat. Keep beans, fruit and whole grains, which fill you up.
• Judge it after a month, not after the first week of water.