Do some foods cause more weight gain regardless of calories?

A persistent argument against CICO is that certain foods — particularly sugar, refined carbohydrates, or processed foods — cause weight gain through mechanisms beyond just their calorie content. This idea has fuelled low-carb movements, insulin hypothesis theories, and countless diet books. The evidence deserves an honest look.

The thermic effect of food

Different macronutrients require different amounts of energy to digest and absorb. Protein has a thermic effect of around 20 to 30 percent — meaning 20 to 30 percent of protein calories are used just in processing it. Carbohydrates have a thermic effect of around 5 to 10 percent, and fat around 0 to 3 percent. This means a 400 calorie meal high in protein effectively delivers fewer net calories than a 400 calorie meal high in fat. This is a real difference, but not large enough to override the fundamental calorie balance.

The insulin hypothesis

Some researchers and popular diet authors have argued that insulin — driven primarily by carbohydrate intake — is the main driver of fat storage, and that reducing carbohydrates causes weight loss independent of calories. Controlled studies that carefully match calorie intake have consistently failed to support this. When calories and protein are equal, low-carb and low-fat diets produce similar fat loss outcomes. Insulin affects where and how fat is stored, but the overall energy balance determines whether fat is gained or lost.

Ultra-processed foods and overconsumption

There is strong evidence that ultra-processed foods — products engineered for palatability using combinations of sugar, fat, salt, and texture — cause people to eat more calories than they would from whole foods. A randomised controlled trial published in Cell Metabolism in 2019 found that participants given ultra-processed diets consumed around 500 more calories per day than those given minimally processed diets, even when matched for available macronutrients and palatability ratings. The mechanism appears to be the disruption of normal satiety signalling. Ultra-processed foods do not cause weight gain by magic — they cause weight gain by making it easier to eat more.

Sugar and fat distribution

High sugar intake, particularly from fructose, has been linked to increased visceral fat — the metabolically dangerous fat stored around organs. This appears to be partly independent of total calorie intake. However, the effect is modest and becomes clinically significant only at very high intakes. The primary driver of fat gain remains total calorie surplus.

The bottom line

No food causes weight gain by bypassing the laws of thermodynamics. However, food composition affects satiety, hunger hormones, and how easy it is to maintain a calorie target. Prioritising whole foods and adequate protein makes staying in a deficit easier — not because they have magic properties, but because they help you eat less without feeling deprived.