How do you know whether you need to lose weight for medical reasons? These evaluations can help provide the answer:

• Body mass index (BMI). The BMI is a formula that uses weight and height to estimate body fat and health risks. If your BMI is between 19 and 24, you're considered in a healthy weight range for your height. If your BMI is between 25 and 29, you're considered overweight. And, if the figure is 30 or greater, you're considered obese and should talk to your doctor about losing weight for your health.

• Waist circumference. If you carry most of your fat around your waist or upper body, you may be referred to as apple shaped. If you carry most of your fat around your hips and thighs or lower body, you may be referred to as pear shaped. Generally, when it comes to your health, it's better to have the shape of a pear than the shape of an apple. If you have an apple shape — a potbelly or spare tire — you carry more fat in and around your abdominal organs. Abdominal fat increases your risk of many of the serious conditions associated with obesity. Women's waist measurements should fall below 35 inches. Men's should be less than 40 inches. If you have a large waist circumference, talk to your doctor about weight loss.

Talking to your doctor openly and honestly about your weight is one of the best things you can do for your health. The more your weight increases, the more medical problems you may face.

Also, talk to your doctor if you have weight-related medical conditions, such as high blood pressure, high blood cholesterol, diabetes and arthritis. These health conditions may improve if you talk to your doctor about weight loss and are able to lose weight.

Your doctor can help you determine whether you need to lose weight and, if so, how much.

In addition to evaluating your BMI and waist circumference, your doctor can review your medical history, which helps reveal how dangerous excess fat is to your health. Do you smoke, drink alcohol or live with a high level of stress? In combination with these behaviors, excess weight can have even greater health implications.

Your doctor can also assess your current health. You may have a health problem that would improve if you lost weight or that requires treatment beyond weight loss.

If you're obese, you're more likely to develop a number of potentially serious health problems. These may include:

• High blood pressure. As you put on weight, you gain mostly fatty tissue. Just like other parts of the body, this tissue relies on oxygen and nutrients in your blood to survive. As demand for oxygen and nutrients increases, the amount of blood circulating through your body also increases. More blood traveling through your arteries means added pressure on your artery walls. Weight gain also typically increases the level of insulin, a blood-sugar-controlling hormone, in the blood. The increase in insulin is associated with retention of sodium and water, which increases blood volume. In addition, excess weight often is associated with an increase in your heart rate and a reduction in the capacity of your blood vessels to transport blood. All of these factors can increase blood pressure.

• Diabetes. Obesity is a leading cause of type 2 diabetes (formerly called adult-onset or noninsulin-dependent diabetes). Excess fat makes your body resistant to insulin, the hormone that helps your body maintain a proper level of a sugar (glucose) in your blood. If your body is resistant to insulin, your blood sugar is high — which isn't good — and your cells can't get the sugar they need for energy.

• Abnormal blood fats. A diet high in saturated fats — red meat and fried foods, for example — can lead to obesity as well as elevated levels of low-density lipoprotein ("bad") cholesterol and reduced levels of high-density lipoprotein ("good") cholesterol. Obesity is also associated with high levels of triglycerides. Triglycerides are the form in which most fat exists in food as well as in your body. Over time, abnormal blood fats can contribute to atherosclerosis — the buildup of fatty deposits in arteries throughout your body. Atherosclerosis puts you at risk of coronary artery disease and stroke.

• Coronary artery disease. This is a form of cardiovascular disease. It results from the buildup of fatty deposits in arteries that feed your heart. Over time these deposits can narrow your heart's arteries, so less blood flows to your heart. Diminished blood flow to your heart can cause chest pain (angina). Complete blockage can lead to a heart attack.

• Stroke. Obesity is associated with atherosclerosis — the buildup of fatty deposits in arteries throughout your body, including arteries in your brain. If a blood clot forms in a narrowed artery in your brain, it can block blood flow to an area of your brain. The result is a stroke. Being obese raises your risk of a stroke.

• Osteoarthritis. This joint disorder most often affects the knees, hips and lower back. Excess weight puts extra pressure on these joints and wears away the cartilage that protects them, resulting in joint pain and stiffness.

• Sleep apnea. This serious condition causes a person to stop breathing for short periods during sleep and to snore heavily. The upper airway is blocked during sleep, which results in frequent awakening at night and subsequent drowsiness during the day. Most people with sleep apnea are overweight, which contributes to a large neck and narrowed airways.

• Cancer. Most types of cancer are associated with being overweight. In women, these include cancers of the breast, uterus, colon and gallbladder. Overweight men have a particularly higher risk of cancers of the colon and the prostate.

Obesity can also contribute to gallstones, solid deposits of cholesterol in the gallbladder, and gout, a joint disorder.

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