Keeping your body fat low reduces your risk of breast cancer as well as your risk of developing many other diseases. Women who are obese (that is, have a BMI over 30) have a much higher risk of postmenopausal breast cancer: 50 to 250 percent higher. About 20 to 30 percent of all postmenopausal breast cancers are thought to be caused primarily by obesity. If you gain weight as an adult, your risk of breast cancer is higher than if you’ve been over-weight all your life. In addition, studies show that obese women with breast cancer are more likely to have advanced breast cancer at the time of their diagnosis and to die from the disease.

One big reason why obesity is associated with an increased risk of breast cancer is because fat cells produce estrogen. Estrogen isn’t created just by the ovaries. It's also made by fat cells. After menopause, fat becomes the primary site where estrogen is manufactured in your body. So the more fat you have, the more estrogen your body will produce.

Obesity is also associated with higher levels of insulin and insulin-like growth factor–1 (IGF-1) both significantly increase your risk of breast cancer, and if you have the disease, they make your cancer grow faster.

There are many other serious reasons why you should avoid gaining too much weight. An estimated 300,000 adults die in the United States each year from obesity-related causes, such as heart disease, high blood pressure, and diabetes, and that number grows every year.

Defining Obesity—Body Mass Index (BMI)
Researchers use very specific measures to define a body as being overweight or obese. The measures include the BMI or body mass index and the percentage of body fat (percent BF). Your BMI is traditionally calculated by dividing your weight in kilograms by your height in meters squared. Then, that number is divided by your height in inches again, and the result is multiplied by 703. The National Heart, Lung, and Blood Institute has a BMI calculator on its website (nhlbisupport.com/bmi/bmicalc.htm), so you don’t have to do the math yourself. If you don't know how to convert your weight and height to metric measurements, that's okay; this website can calculate your BMI using standard American measurements. Ideally, your BMI should be in the range of 18.5 to 24.9. If your BMI is greater than 25, you’re overweight. If it’s greater than 30, you’re considered obese.

BMI, however, is not the best measure to determine if you’re overweight or obese because it doesn't take body composition into account. Muscle weighs more than fat. For example, bodybuilders may weigh a lot for their height, but their above-normal weight is usually due to their large muscle mass, not excess fat. These toned athletes may have a BMI greater than 30, but they are certainly not obese.

Percent Body Fat
A better way to determine whether you are overweight, obese, or just "solid" is to measure your percent body fat (percent BF). This measurement is an assessment of your body composition. It evaluates how much of your weight is lean body mass (muscle, bones, and so on) and how much of it is actually fat. There are several different ways to get this measurement. The most accurate way involves completely submerging your body into a tank of water. This fairly expensive test measures the amount of water you displace in the tank and compares it to your height and weight. Fat is lighter than muscle. So pound for pound, fat takes up much more space than muscle. The more water you displace for your height and weight, the higher your percent of body fat.

Body fat can also be calculated by the method known as "bioelectrical impedance." This test is performed by passing a small, low-amp electrical current through your body and measuring the speed at which the current flows through you. Fat doesn’t conduct electricity very well, but muscle does. So, the more fat you have, the slower the current travels.

The simplest and least expensive way to measure percent body fat is to use a series of skin-fold measurements. However, calculating body fat using this technique has some limitations and is a lot less accurate than the other methods. The accuracy of this approach very much depends on the skill of the person doing the evaluation. Also, skin-fold measurements are unreliable for estimating the amount of body fat on people who are either extremely thin or very obese. To calculate percent body fat using this technique, a caliper is used to measure the thickness of skin folds in several very specific areas of the body. The skin and the underlying fat are pinched into the caliper—a device that looks and feels a lot like a vice. Yes, sometimes it hurts a little. The thickness of each skin fold is read from the numbers on the caliper. After all the measurements are taken, they are added up and divided by the person’s body weight. That number is then multiplied by a conversion factor to obtain the estimated percent body fat. Certified personal trainers are taught how to take these measurements as part of their certification training. Most gyms and fitness clubs have a personal trainer who can do these measurements for you.

Studies have shown that your BMI and percent body fat (BF) are associated with your risk of breast cancer. A study from Sweden published in January 2003 in the International Journal of Cancer found that your percent BF has a higher association with your risk of breast cancer than your BMI does. The normal overall range for percent BF in non-athletic women is 16 to 32 percent; the desirable range is 18 to 28 percent.

Healthy Diet and Lifestyle=Healthy Weight and Breast Cancer Protection
Many of the diet and lifestyle choices that protect against breast cancer will help you to lose and maintain a healthy weigh too. For example, eating fresh organic fruits, vegetables, and whole grains; avoiding red meat, processed foods, and sugar; taking flax oil or eating flaxseeds; and exercising every day.

If you find that you have frequent desires to snack, try this: Eat six small meals instead of three large meals, and plan what you’re going to eat on a particular day the night before. Every three hours or so, eat a small portion of protein with a serving of vegetables. Include a serving of fresh fruit and whole grains in two of your meals. Eating planned, small, frequent meals will keep you from getting hungry, overeating, and having the compulsion to eat the wrong things. If three meals a day works well for you, remember to eat your main meal at noon because that’s when your digestion is strongest. In the evening when your digestion is much weaker, eat lightly.

If you have a serious weight problem, joining a weight-loss program such as Weight Watchers can be very helpful. However, consult your doctor before starting a weight-loss program.