Body mass index is a screening tool that evaluates your health based on body size. A body mass index, also known as a BMI, above 24.9 indicates that you may be overweight; a BMI above 30 indicates that you may be obese. A high BMI, which is a computation derived from your height and weight, can correlate with high blood pressure, type 2 diabetes, coronary heart disease, stroke and some cancers. Losing weight helps you become smaller, which reduces your BMI.

About Body Mass Index

Your body mass index is the result of your weight in kilograms divided by your height in meters squared. To determine your BMI using pounds and inches, divide your weight in pounds by your height in inches squared. Multiply this total by 703 for the final result. The equation is originally performed using kilograms and meters, so the conversion factor of 703 is necessary. Many online calculators are available to do the computations for you.

BMI is not meant to be diagnostic, but it serves as a noninvasive way to give a general sense of your body's amount of fat and the potential health risks associated with a high BMI. If you have a high BMI, your medical provider may perform further evaluation of your health, family history and lifestyle habits.

Reduce Your BMI By Cutting Calories

For many people, reducing a high BMI via a comprehensive weight-loss program by reducing calories can help improve health. Determine the daily calorie requirements you need to maintain your weight, using an online calculator or by consulting with a dietitian. This number depends on your age, size, activity level and gender. From that number, subtract 250 to 500 calories and plan to work off an additional 250 to 500 calories through daily activity. This 500- to 1,000-calorie deficit yields a weight loss of 1 to 2 pounds per week, a healthy and sustainable rate.

How much weight you'll need to lose to reduce your BMI depends on your current size and height, as well as on your BMI goal. For example, for a 5-foot, 9-inch man to have a healthy BMI of between 19 and 24, he needs to weigh between 128 and 168 pounds.

Even small reductions in weight can positively affect your BMI. If this 5-foot, 9-inch man is currently 210 pounds and loses 10 pounds, he'll still reduce his body mass index from an obese 31 to an overweight 29.5.

Physical Activity and Lower Body Mass

Exercise is a critical component in reducing body mass. The calories you burn through cardiovascular exercise contribute to the deficit, while strength training builds lean muscle. Without exercise, you might lose too much muscle, leading to an unhealthy ratio of fat to lean mass. For each pound you lose while being sedentary, one-quarter of that pound will be lean muscle mass.

To lose weight in a healthy way, the American College of Sports Medicine recommends at least 250 minutes each week of moderate-intensity exercise, such as brisk walking 5. To give you an idea of calorie burn rates, a 185-pound person walking at a 4-mph pace burns about 200 calories in 30 minutes; swimming burns about 266 calories in 30 minutes.

In addition, aim for at least two total-body weight-training sessions each week. When you lift weights, you increase your body's muscle mass, which helps boost your metabolism, making weight loss easier. A stronger body has greater stamina and improves your daily function.

Body Fat Matters

If you've been working out faithfully with weights and watching your diet diligently, you may change your body composition considerably. You drop fat and put on several pounds of muscle mass, which makes you much healthier. However, your body mass index may not wholly reflect the positive change in your fat-to-muscle ratio.

Highly muscled, healthy people sometimes register as having a high BMI because an abundance of muscle, rather than fat, makes them weigh more than the average person. A trained medical professional can usually look at a patient's physical appearance and lifestyle to determine that a high BMI is due to a high muscle mass.

If you lose considerable weight to lower your BMI, but do so with a very low-calorie plan that includes no exercise, you might reach a smaller size, but retain an excess of body fat. When you lose weight by eating fewer than 1,200 calories per day or at a rate of more than 3 pounds per week, you'll likely lose a considerable amount of muscle mass along with fat. This means you may end up with a high percentage of fat tissue, which still leaves you vulnerable to the health problems associated with being overweight or obese. Too much body fat is 20 percent for a man and 30 percent for a woman.