What is CICO? What calories in, calories out gets right

Key takeaways

• CICO, short for calories in, calories out, is the idea that body weight changes with the gap between the energy you eat and the energy you burn. Controlled feeding studies back it up.¹²
• At the same calories, swapping carbs for fat changed little. Across 32 controlled feeding studies, lower-fat diets came out ahead by just 26 calories a day of burn and 16 grams a day of fat loss.¹
• Food choice changes the calories in side. In a 2021 ward study, people eating freely took in about 689 fewer calories a day on a low-fat, plant-based menu than on a ketogenic one.³
• Calories out isn't fixed. In 332 people measured with doubly labeled water, daily burn rose with activity at lower levels and then leveled off.⁵
• Weight responds slowly. In Hall's model, about half the eventual change from a lasting shift in intake takes about a year.⁶

What is CICO?

CICO stands for calories in, calories out. It's the idea that body weight changes when the energy you eat and drink doesn't match the energy your body burns, and it holds up in tightly controlled studies.¹²

What CICO doesn't say is that both sides are easy to measure, or that they move on their own. Hall and Guo describe the parts of energy balance as linked to each other, with the body working against weight loss as it happens.¹

Does calories in, calories out work?

Yes, where it can be tested cleanly. In a 2015 metabolic ward study, 19 people with obesity lived on a research unit and ate diets that cut 810 calories a day from a 2,740 calorie baseline, about 30 percent, by removing either carbs or fat. Each person did both diets in random order.²

Both cuts led to body fat loss, measured in a metabolic chamber. People lost 89 grams of body fat a day on the fat cut and 53 grams on the carb cut, even though only the carb cut lowered daily insulin secretion, by about 22 percent.²

The team's model predicted that the body works to shrink fat loss differences between diets over longer periods, when calories and protein are held the same.²

Is a calorie a calorie?

Mostly, for body weight. Hall and Guo pooled 32 controlled feeding studies that swapped carbs for fat at the same calories. Burn was 26 calories a day higher and fat loss 16 grams a day greater on the lower-fat diets. Both gaps were statistically clear and small, and both run opposite to the claim that cutting carbs speeds fat loss at equal calories.¹

Where food type matters more is how much people end up eating when nobody sets the portions. That's the calories in side, covered next.³⁴

Why does CICO feel like it doesn't work?

Calories in depends on the menu. In a 2021 study, 20 people lived at the NIH Clinical Center and ate as much as they wanted from a plant-based, low-fat menu for 2 weeks and an animal-based, ketogenic menu for 2 weeks. They took in 689 fewer calories a day on the low-fat menu over the 2 weeks, and 544 fewer in the final week.³

In an earlier study at the same center, menus matched for calories offered, sugar, fat, fiber and other nutrients still led to 508 more calories a day eaten on the ultra-processed menu than on the unprocessed one. Weight went up 0.9 kilograms on the first and down 0.9 on the second over 2 weeks each.⁴

Calories out adjusts too. Pontzer's team measured 332 people in five populations and found that daily burn rose with activity at lower levels but plateaued at higher levels. Extra activity didn't add calories out one for one.⁵

Weight also moves slowly. Hall's 2011 model puts the half time of the weight response at about a year, and people with more body fat take longer to settle at a new weight. The 3,500 calorie rule leaves out these changes in burn as weight changes.⁶

What is the CICO diet?

CICO isn't a menu. It's an accounting rule. The studies above show that at fixed calories the mix of carbs and fat barely moves fat loss, while the menu people choose from can move how much they eat by hundreds of calories a day.¹³⁴

How to use CICO with a food log

Of the two sides, intake is the one you can write down meal by meal. Your weight trend then tells you how intake compares with what you burn.

• Log everything you eat and drink, including cooking oil, sauces and drinks, so the calories in side is complete.
• Weigh yourself on a regular schedule and read the weekly average rather than single days.
• After a few weeks, set your average logged intake next to the weight trend. The TDEE guide shows how to turn that into your own burn number.
• Treat a watch's calorie number as a rough guide for calories out, not an exact figure.
• In TrueCal, log calories by hand for free, or type a meal in plain words with Pro to get calories and macros.

Common myths

• Myth, cutting carbs burns more fat at the same calories. In the 2015 ward study, people lost 53 grams of body fat a day on the carb cut and 89 on the fat cut.²
• Myth, CICO means food choice doesn't matter. Same-calorie swaps changed little, but freely eaten menus changed intake by hundreds of calories a day.¹³
• Myth, every extra workout adds the same calories out. Daily burn leveled off at higher activity levels.⁵
• Myth, a 3,500 calorie gap always means a pound. That rule ignores how burn falls as weight falls.⁶

A note on medical advice

This article is for general information and isn't medical advice. Talk with your clinician about your own situation.

Worked example

Say intake changes by a steady 100 calories a day and stays that way. Hall's rule of thumb is 10 calories a day per pound, so that's about 10 pounds of eventual weight change, with about half in the first year and 95 percent by about year 3. The 3,500 calorie rule would instead multiply 100 by 365 and divide by 3,500, about 10 pounds every year with no end, because it assumes burn never adjusts.⁶

A closer look

Across 32 controlled feeding studies that swapped carbs for fat at equal calories, lower-fat diets came out slightly ahead, with 26 calories a day more burned and 16 grams a day more body fat lost. That runs against the claim that cutting carbs adds a metabolic edge, and both gaps are small.¹

Questions and answers

Related guides

• How to track calories
• What is TDEE?
• Is starvation mode real?
• How many calories should I eat to lose weight?
• Do foods that boost metabolism work?
• How accurate is Apple Watch?
• TDEE calculator

Sources

1. Gastroenterology. Hall, Guo. 2017.
2. Cell Metabolism. Hall. 2015.
3. Nature Medicine. Hall. 2021.
4. Cell Metabolism. Hall. 2019.
5. Current Biology. Pontzer. 2016.
6. Lancet. Hall. 2011.