Counting on calories

How many daily calories do you really need? It depends on many factors.

When it comes to nutrition, calories embody the ultimate love-hate relationship. Consume too little, and you don't have the energy to function at your best. Eat too much, and you risk weight gain. It's no surprise, then, that counting calories can be such an obsession.

But how much do you really need every day? Unfortunately, there is not a single, simple answer. Men ages 50 and older require 2,000 to 2,800 calories per day, or an average of about 2,500, says Dr. Qi Sun, associate professor in the Department of Nutrition at the Harvard T.H. Chan School of Public Health.