Fat Loss and Insulin: You Don't Need to Fear Carbs

You've probably heard insulin called the fat storage hormone, or been told that carbs make you fat because they spike it. Maybe you've wondered whether you need to give up bread and rice to lose weight.

You don't. Plenty of my clients lose weight eating carbs every day, and the controlled research explains why. Once you know what insulin does, the food choices get simpler.

Energy balance still decides

Insulin decides where nutrients go after a meal. How much fat you store or lose over a month is decided by calories in and calories out. If you eat fewer calories than you burn over weeks, you lose fat, whatever your insulin did after lunch.

The cleanest test of this is a metabolic ward study, where people live in a lab and every calorie is measured. In one, 17 men ate a high-carb diet for four weeks, then switched to a ketogenic diet with the same calories and protein. Insulin dropped as expected. Body fat loss didn't speed up. If anything it slowed during the first weeks of the switch.

A review pooled 32 of these controlled feeding studies. At equal calories, the lower-fat, higher-carb diets produced slightly more fat loss and slightly higher energy expenditure than the lower-carb diets. The differences were small, about 16 grams of fat a day, and they ran in the opposite direction from the insulin theory.

What insulin does with a meal

Insulin moves the glucose from a meal out of your blood and into your cells, and the fat-storing step people worry about only happens when there's a surplus to store. How carbs, protein and fat each affect it gets its own article.

Eat a 400 calorie meal of rice and chicken inside a day where you're 300 calories under maintenance, and insulin handles the meal, then drops, and your body pulls from fat stores for the rest of the day. The brief insulin rise didn't prevent that.

Why low-carb diets seem to prove the point

Low-carb diets work for many people. That's true, and it's still a bad argument for insulin. They work for ordinary reasons.

• Cutting carbs cuts out most calorie-dense processed food, which lowers calories.
• Protein and fat keep many people fuller per calorie than refined carbs.
• Water weight drops in the first week as glycogen empties, which feels like fast progress.

In the largest head-to-head trial, 609 adults were randomly assigned to a healthy low-carb or healthy low-fat diet for a year. Weight loss was about the same, 13 pounds versus 12. Baseline insulin secretion didn't predict who did better on which diet. Pick the eating style you can stick to, and the deficit does the work.

Insulin resistance is a medical question

Some people develop insulin resistance, where cells respond less to insulin and the pancreas makes more to compensate. Fat loss is still possible with a deficit, and losing weight usually improves insulin sensitivity. Food choices and meal structure may need more attention. If you have symptoms like energy crashes after meals or a family history of type 2 diabetes, ask your doctor for the blood tests. Guessing from a video is how people end up cutting food groups they didn't need to cut.

What to do with carbs while dieting

Carbs are a tool for making a deficit livable, and these habits keep them working for you.

• Set the deficit first. Use the maintenance calories calculator and eat about 300 to 500 below it.
• Put protein at 0.6 to 1 gram per pound of bodyweight. It protects muscle and it fills you up.
• Get most of your carbs from potatoes, rice, oats, fruit, beans and whole grains. They bring fiber, which slows digestion and keeps you full for longer.
• Eat carbs with protein and some fat. Mixed meals digest more slowly.
• Move. Exercise improves insulin sensitivity, so you need less insulin to handle the same meal, and that effect begins with the first workout.
• Sleep seven hours. In controlled studies, a short night added roughly 385 calories of eating the next day.

Three myths you can drop

No carbs after 6 pm. Total intake across the day decides fat loss, whatever time the carbs land.

Fruit is bad because it spikes insulin. Fruit is low in calories for its volume and full of fiber and water, which makes it hard to overeat.

Fasted cardio burns more fat. You burn more fat as fuel during the session, and that doesn't turn into more fat loss. In a four-week study on a calorie deficit, women doing fasted cardio and women doing fed cardio lost the same amount of fat.

Insulin will rise after your next bowl of oatmeal, which is what it's for. With the deficit and the protein in place, the oatmeal is fine.

Sources

1. Hall et al., American Journal of Clinical Nutrition, 2016. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. Metabolic ward study. Switching to a ketogenic diet at the same calories lowered insulin but did not speed up body fat loss.
2. Hall & Guo, Gastroenterology, 2017. Obesity energetics: body weight regulation and the effects of diet composition. Meta-analysis of 32 controlled feeding studies: at equal calories, lower-fat diets produced slightly more fat loss than lower-carb diets.
3. Gardner et al., JAMA, 2018. Effect of low-fat vs low-carbohydrate diet on 12-month weight loss (DIETFITS). 609 adults, 12 months: no meaningful difference in weight loss, and baseline insulin secretion did not predict which diet worked better.
4. Schoenfeld et al., Journal of the ISSN, 2014. Body composition changes associated with fasted versus non-fasted aerobic exercise. Fasted and fed cardio produced the same fat loss on the same diet.
5. Bird & Hawley, BMJ Open Sport & Exercise Medicine, 2017. Update on the effects of physical activity on insulin sensitivity in humans
6. Al Khatib et al., European Journal of Clinical Nutrition, 2017. The effects of partial sleep deprivation on energy balance: a systematic review and meta-analysis. Short sleep raised next-day intake by about 385 calories with no change in energy burned.