We have been told that losing weight is simply a matter of calories in and calories out. Eat less. Exercise more. Have more willpower. And certainly, calories matter. We cannot completely ignore the laws of energy balance. But after working in nutrition for 50 years, I can tell you that the human body is much more complicated than a calorie calculator.

Your body is a living, breathing, adapting biological system. And the question isn't simply, “How many calories did I eat today?” We need to ask: What kind of food did those calories come from? What did that food do to my hunger? What did it do to my blood sugar and insulin? Did it nourish my muscles? Did it leave me satisfied—or searching the cupboard two hours later or wanting to finish the meal with something sweet?