Does insulin from carbs make you store fat?
Short answerNot the way you think
Not the way people say. Insulin does help store fuel after a meal. But when scientists fed people exact diets on a hospital ward, cutting carbs didn’t take off more fat than cutting fat with the same calories. Cutting fat took off slightly more.
What we’d do
Choose low-carb or low-fat by which one you can keep eating, because they work about the same. Cut refined carbs and sweet drinks first, since they’re easy to overeat. Keep beans, fruit and whole grains. If you have diabetes or take insulin, plan any big change in carbs with your doctor.
If you remember three things
- Insulin steers fuel in the short term. The calorie gap decides fat over weeks.
- On a hospital ward, cutting carbs took off less fat than cutting fat at the same calories.
- In a year of real life, low-carb and low-fat lost the same, and insulin levels didn’t predict who did better.
Why the theory is appealing
The story has a clean chain. Carbs raise blood sugar. Blood sugar raises insulin. Insulin tells fat cells to store fat and not release it. So cut carbs, keep insulin low, and the fat comes out. Each link has some truth, and the first week of low-carb seems to prove it, because the scale drops fast.
That first drop is mostly water. Carbs are stored with water, and 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.
Insulin is like a parking attendant. It decides where incoming cars park for now. It doesn’t decide how many cars come in.
What the careful studies found
Cut carbs or cut fat, same calories
19 adults with obesity living on a metabolic ward. Hall and colleagues, NIH, 2015
Each person cut 30% of their calories twice, once from carbs and once from fat. Cutting carbs did make them burn more fat, as the theory predicts. But they were eating more fat too, so they lost less of it: 53 grams a day, against 89 when they cut fat.
Six days per diet. The authors’ model predicts the gap shrinks over longer diets, so it’s too small to matter in daily life.
Very low-carb, same calories
17 men with overweight or obesity, 4 weeks on each diet. Hall and colleagues, 2016
Switching to a ketogenic diet raised the calories burned a little, about 57 a day in a sealed room, and bodyfat loss slowed down rather than sped up.
A second method found a bigger rise, about 151 calories a day, which low-carb supporters point to.
A year of real life
609 adults, healthy low-fat or healthy low-carb. DIETFITS, JAMA, 2018
They lost about 12 and 13 pounds (5.3 kg and 6.0 kg), about the same. How much insulin people made in response to sugar didn’t predict which diet worked better for them.
People chose their own amounts, so it tests the diets as people live them.
The best case for the theory
Supporters argue that insulin’s effect plays out over months, longer than ward studies run, and that low-carb diets make some people less hungry. The second point may be real for some people. If cutting carbs makes you eat less without trying, that’s a fine reason to do it. It works through eating less, which is the point. Sugar shows the same: when trials swapped sugar for other carbs at the same calories, weight didn’t change.
More detail: what insulin does
After a meal, insulin rises and helps move sugar into muscle and liver, and it slows the release of fat from fat cells. Between meals and overnight it falls, and fat is released again. Over a day, how much fat you end up storing or burning depends on the balance of energy coming in and going out. That’s why the ward studies, which fix the calories, show only small differences between diets with very different insulin levels.
5 sources
- Hall et al., Calorie for calorie, dietary fat restriction results in more bodyfat loss than carbohydrate restriction in people with obesity, Cell Metabolism, 2015pmc.ncbi.nlm.nih.gov
- Hall et al., Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men, American Journal of Clinical Nutrition, 2016pubmed.ncbi.nlm.nih.gov
- Gardner et al., Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion (DIETFITS), JAMA, 2018pubmed.ncbi.nlm.nih.gov
- Te Morenga, Mallard and Mann, Dietary sugars and body weight: systematic review and meta-analyses of randomised controlled trials and cohort studies, BMJ, 2013pubmed.ncbi.nlm.nih.gov
- Yang and Van Itallie, Composition of weight lost during short-term weight reduction, Journal of Clinical Investigation, 1976pubmed.ncbi.nlm.nih.gov
