The American Heart Association says to limit your consumption of fats and cholesterol. The American Cancer Society says to eat more fiber. The National Research Council says to watch out for fats, sugar, and salt. The American Diabetes Association says to eat regular meals so your blood sugar stays even.

The Food Police say if it tastes good, forget it! The U.S. Departments of Agriculture and Health and Human Services have incorporated virtually all but the “tastes good, forget it” rule into the Dietary Guidelines for Americans 2005 and even added some advisories of their own.

The Dietary Guidelines for Americans is a collection of sensible suggestions first published by the Departments of Agriculture and Health and Human Services (USDA/HHS) in 1980, with five revised editions since then (1985, 1990, 1995, 2000, 2005).

The Guidelines lay out food and lifestyle choices that promote good health, provide the energy for an active life, and may reduce the risk or severity of chronic illnesses, such as diabetes and heart disease.

During the past two decades, as the number of overweight Americans has bounced upward like a rubber ball, the incidence of obesity-related conditions such as Type 2 diabetes, high blood pressure, and heart disease also has risen. The challenge (as always) is to set, reach, and hold a healthful weight.

Some foods provide lots of nutrients per calorie. Some don’t. The former are called “nutrient-dense foods.” The latter aren’t. As you may expect, the Guidelines recommend choosing foods from the first group to meet your calorie needs each day, while limiting the amount of:

• Foods high in saturated fat
• Foods high in trans fats
• Foods high in cholesterol
• Foods with added sugar
• Foods with added salt
• Alcohol beverages

In other words, stick to a balanced diet. No surprise there.

To reach and keep a healthful weight, follow a few realistic rules:

• Evaluate your weight. The best test of who’s actually overweight is the Body Mass Index (BMI), a measure of body fat versus body lean mass (in other words, muscle) that can be used to predict health outcomes.

• If you need to lose weight, do so gradually. Forget the “lose 30 pounds in 30 days” jazz. Depending on how much weight you have to lose, your long-term goal needs to be losing about 10 percent of your total weight over a 6-month period. Losing ½ to 2 pounds a week is a safe and practical way of doing so.

• Encourage healthy weight in children. One unhappy fact is that overweight kids become overweight adults. Helping children stick to a healthy weight pays large dividends down the road of life.

• Check with your doctor before starting a weight loss diet. This advice is most important for women who are pregnant or nursing, for children, and for anyone — young or old — who has a chronic disease and/or is on medication.

When you take in more calories from food than you use up running your body systems (heart, lungs, brain, and so forth) and doing a day’s physical work, you end up storing the extra calories as body fat. In other words, you gain weight. The reverse also is true.

When you spend more energy in a day than you take in as food, you pull the extra energy you need out of stored fat and you lose weight. I’m no mathematician, but I can reduce this principle to two simple equations in which E stands for energy (in calories), > stands for greater than, <>

If E_in > E_out: E_total = +W
If E_in<>out: E_total = –W

It ain’t Einstein’s theory of relativity, but you get the picture!

For real-life examples of how the energy-in, energy-out theory works, stick your bookmark in this page and go to Table 1 in "What Calories Mean To You" to find out how to calculate the number of calories a person can consume each day without pushing up the poundage.

Even being mildly active increases the number of calories you can wolf down without gaining weight. The more strenuous the activity, the more plentiful the calorie allowance.

Suppose that you’re a 25-year-old man who weighs 140 pounds. The formula in Table 1 shows that you require 1,652 calories a day to run your body systems. Clearly, you need more calories for doing your daily physical work, simply moving around, or exercising.

After you decide to start moving, the Guidelines say, do it every day. How much should you do? Per the Guidelines:

• Most people will benefit from 30 minutes of moderate physical activity — such as a brisk walk — per day.

• To manage body weight and/or prevent gradual weight gain, make it 60 minutes of moderate-to-vigorous-intensity activity several days a week.

• To keep weight off, try 60 to 90 minutes of daily moderate physical activity.

• To reach true physical fitness, your regimen should include cardiovascular conditioning, stretching exercises for flexibility, and resistance exercises or calisthenics for muscle strength and endurance.

Not everybody can — or should — run right out and start chopping down trees or throwing touchdown passes to control his or her weight.

In fact, if you have gained a lot of weight recently, have been overweight for a long time, haven’t exercised in a while, or have a chronic medical condition, you need to check with your doctor before starting any new regimen. (Caution: Check out of any health club that puts you right on the floor without first checking your vital signs — heartbeat, respiration, and so forth.)