It’s easy to think about losing or putting on weight as a relatively simple strategy of balancing calories consumed against calories used up during exercise. Research presented at the International Conference of Nutrition suggests it’s a lot more complicated. Consider the varying degrees of success experienced by millions of dieters each year.

In a 12-week study of active and relatively inactive adults, researchers found that even though all participants burned the equivalent of 500 calories during five weekly workouts maintaining an average heart rate of 70% and consumed a tightly controlled diet, weight loss ranged from 2.2 to 17.6 pounds. A few subjects even gained weight, although all lost body fat and built up lean mass.

True Strength Moment: It looks like very active people can consume more calories without putting on weight. Unfortunately, the complexities of weight loss aren’t that easily explained. For instance, your caloric needs are reduced when you begin to lose weight. Altering your energy balance triggers numerous changes; some that work in your favor and others that can make continued weight loss more difficult.