The all new
CBS News App for Android® for iPad® for iPhone®
Fully redesigned. Featuring CBSN, 24/7 live news. Get the App

No Getting Around It: Calories Count

Dr. Emily Senay and Harry Smith
CBS/The Early Show
No matter how you look at it, there's no avoiding it: If you're trying to trim your waistline, you have to count your calories, and take in fewer than you use.

In the first of a three-part Early Show series on nutrition Thursday, medical correspondent Dr. Emily Senay observed that while there's all sorts of advice available about the foods to eat to lose or avoid gaining weight, the bottom line remains the same: Calories do matter.

She explained that a calorie is a unit of energy. There's a complicated mathematical formula that defines exactly what constitutes a calorie. The number of calories in a food represents how much energy the food supplies to our bodies. That energy fuels our muscles, our nervous systems, our digestive systems, and our senses and brains.

If we have any calories left after all those functions are taken care of, the body stores the excess as fat. And that, in simple terms, is how we gain weight.

Then how do we know which diet is best to take pounds and keep pounds off?

If you want to lose weight, Senay stressed, you have to limit your caloric intake. You need to consume fewer calories than your body burns up.

What's more, weight isn't the only important issue. Of course, nutritious foods are better for long-term health and reducing the risk of serious disease than foods that lack nutrients or are loaded with unhealthy fats. So, a diet based on the most nutritious foods is a big plus.

But if weight loss is your major goal, then calories are key. Even if your meals are extremely nutritious, you can only eat so much.

The number of calories someone should take in depends on the person. Among adults, it can be as little as 1,600 calories a day, for a woman over 50 who gets very little exercise. Or it can be as many as 3,000 a day for a man under 50 who is very physically active.

Most adults fall somewhere between those two figures, and the Department of Agriculture has a chart where you can find an estimate of your ideal intake. To see that chart, click here. Or, your can ask your doctor or a dietitian. Once you have that calorie goal, put together a diet that will keep you from exceeding that goal.

Senay warned that you can "absolutely" take in too few calories: Your body needs fuel and nutrition from food to function and maintain good health. Going too low can threaten your health. The National Institutes of Health suggests that, unless their diets are strictly supervised by a health professional, women shouldn't go below 1,200 calories per day, and men shouldn't get below 1,500.

To lose a pound a week, the rule of thumb is that you need to reduce your daily food intake by 500 calories. Double that to lose two pounds per week. And that's really all you want to do.

You don't want weight loss to be sudden. Losing weight gradually will help instill better eating habits that you can sustain permanently. Gradual weight loss is also less of a shock to your body.