Perfecting the 'calories in, calories out' strategy to get fit

Forty-five percent of the world's population is trying to shed a few pounds, according to a 2020 Ipsos poll. And they prefer to do it by exercising more and eating better, not by dieting. That's essentially the well-known "calories in, calories out" method, where you try to burn more calories than you take in. There is just one problem: Nailing down your calorie burn is harder than you may think.

The number of calories a person burns doing a particular exercise or movement depends on various factors, including height and weight, sex, and muscle mass. But even your health and the weather can affect the number of calories you burn performing a certain exercise.

"You work harder running 6 miles (9.7 kilometers) in 80-degree (27 C) heat and high humidity than when it's 50 degrees (10 C) and dry," said Dr. Keith Anderson, a family and sports medicine doctor with Novant Health in Charlotte, North Carolina. "Your body also works harder if you're fighting off a cold or if you didn't sleep well the night before. There's so much combined into metabolism."

In addition, calorie-tracking devices such as fitness watches aren't always accurate. While seven wrist-worn devices adequately measured heart rate in subjects who were walking, running and cycling, none accurately measured energy expenditure, according to one study published in the Journal of Personalized Medicine. The most accurate device was off by nearly 27%, while the least accurate was off by 93%.

By Melanie Radzicki McManus, CNN