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How Do I Know If I'm Underweight or Overweight?

author image Andrea Cespedes
Andrea Cespedes is a professionally trained chef who has focused studies in nutrition. With more than 20 years of experience in the fitness industry, she coaches cycling and running and teaches Pilates and yoga. She is an American Council on Exercise-certified personal trainer, RYT-200 and has degrees from Princeton and Columbia University.
How Do I Know If I'm Underweight or Overweight?
Knowing whether you're under or overweight by your weight can be challenging. Photo Credit: © Elizabeth Fernandez G./Moment/Getty Images

Assessing your weight helps you determine whether you're at risk for chronic disease or if you have compromised immunity. Too much fat, even on a person of normal weight, can have deleterious health effects. Sometimes, you might be able to tell if you're too thin or thick simply by looking in the mirror, but it can be difficult to know solely by appearance whether you're at a healthy weight. A scale can give you a gross weight, but it won't tell you the type of tissue you carry. A number of alternative methods can help you determine if you're at a healthy weight.

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Body Mass Index

The body mass index, or BMI, is calculated using the relationship of your height to your weight; it can provide a good estimation of body fatness for much of the population. A BMI of 18.5 or less indicates you're underweight, but a BMI of 25 or more indicates you're overweight. A BMI of 30 or more is a sign of obesity. You can figure out your own BMI, using an online calculator or your doctor's guidance.

BMI isn't a perfect measure, however. It can qualify muscular people, such as body builders, as overweight when they actually have low body fat levels and are actually very healthy. BMI also misses some people who are of normal weight but who carry too much fat tissue. This is especially true of older adults who are mostly sedentary and who have lost some bone density and muscle mass.

Body Fat Measurements

For a man, carrying more than 20 percent body fat or more than 30 percent for a woman, greatly increases your risk of chronic disease and other complications common to obese people. If your doctor uses BMI as the sole measurement to determine body fat, he might miss a condition known as normal weight obesity. A body fat composition test, using an electrical impedance scale, body fat calipers or a more invasive body scan, such as a DEXA bone density test, will help you get a more accurate measurement for the amount of fat you carry and, by extension, for your health status.

Having a too-low body fat count can also be a health risk. Almost every bodily system needs you to have essential fat levels. Without enough fat, you compromise heart health, immunity, reproduction and the nervous system. For a man, essential fat averages about 3 percent; for a woman, it's about 13 percent. Having less than 8 percent for a man and 14 percent for a woman doesn't provide any notable health benefits.

Getting a measurement from a professional at a clinic will provide a better look at your fat levels than using an at-home body fat scale, which can give you different readings from day to day, due to changes in hydration.

Waist Check for Overweight Status

An easy way to know if you have too much fat is by using a tape measure to measure your waist circumference. A man whose waist measures 40 inches or greater, or a woman whose waist is 35 inches is at greater risk for health complications associated with being overweight or obese. A large waist shows that you have a lot of belly fat, also known as visceral fat. This type of fat is especially inflammatory and may increase your risk of chronic disease more than carrying excess weight in the buttocks, hips and thighs.

A waist measurement isn't as useful for pinpointing an underweight status and may not be able to tell you if you're too thin.

Other Indications of Underweight

Since being underweight is a less common condition than being overweight, BMI is the standard by which you're usually assessed. You can physically observe if you are underweight by assessing your skin, hair, energy and understanding your immunity. Brittle, thinning hair, dry skin that breaks out regularly, poor memory and focus, digestive disturbances and poor sleep are all indications that you aren't taking in or absorbing all the nutrients you need for optimal health. If you are lacking the energy to do activities you enjoy, can visually see your ribs and people tell you that you "look thin," you may be underweight. Ultimately, your doctor will help you determine if and how to gain healthy weight.

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