“Carbs are what make you gain fat.”
Carbs are what make you gain fat.
Energy balance drives fat gain; carbohydrate percentage barely moves the needle.
609 adults were randomised to a healthy low-carb or healthy low-fat diet for twelve months with no calorie target. Average weight loss was 6.0 kg vs 5.3 kg — a difference small enough to be noise, with huge variation inside each group.
Where the myth comes from
Carbohydrate raises insulin, and insulin promotes fat storage. That chain is real biochemistry, which makes the conclusion feel obvious. What it skips is that the body also releases fat continuously, and over a full 24 hours net fat balance is set by how much energy came in versus went out.
What the research actually shows
DIETFITS is the cleanest test to date: a year long, well-supported, and deliberately designed to find a difference. Both arms cut refined starches and added sugar and ate more vegetables. Neither arm counted calories. Weight loss was near-identical, and — notably — neither insulin secretion status nor a low-carb/low-fat 'genotype pattern' predicted who did better on which diet.
Tightly controlled feeding agrees
Metabolic-ward studies, where every calorie is measured, find no meaningful fat-loss advantage for cutting carbs when protein and total energy are matched. Short-term scale drops on low-carb diets are largely glycogen and its associated water — roughly 3 g of water per gram of glycogen — which returns when carbs do.
What actually matters
Adherence, protein intake and food quality. Carbs that arrive as whole grains, fruit, legumes and potatoes come with fibre, volume and satiety; carbs that arrive as liquid sugar and ultra-processed snacks are easy to overeat. That's a food-choice problem, not a macronutrient one.
Pick the carb level you can stick to, keep protein high, and let total intake do the work. There is no fat-loss penalty for eating carbs.