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A Breakdown of Body Mass Index (BMI)

Written by
Team Nutrisense
Reviewed by
Carlee Hayes
RDN, CD
a BMI calculator

Body mass index (BMI) is a term you may have seen tossed around quite a bit in the health world. You might know that it’s a way to measure body fat, but with so many new tests for measuring overall fitness and health, is BMI still reliable?

Read on to learn more about what exactly BMI is, how it’s calculated, and what it can be helpful for.

What is BMI?

BMI is a measurement of body fat calculated using an individual’s height and weight. While BMI does not measure body fat directly, research shows that it's often somewhat correlated with more direct measures of body fat, such as skinfold thickness measurements, underwater weighing, and dual-energy X-ray absorptiometry (DXA). 

A person's BMI can be calculated using a BMI chart or formula and is considered a reliable indicator of body fatness for most people. But while this measurement can still be helpful, studies have shown that BMI may overestimate body fat percentage in athletes and other people who have a muscular build. It may underestimate body fat in older adults and other people who have lost muscle mass.

So, while BMI is generally a safe ballpark measurement for most people, it’s important to remember that it does have some limitations.

How is BMI Calculated, and What is it Used For?

Body mass index, or a measurement of body fat, is calculated by comparing one's weight and height, with values falling into the following categories: underweight (BMI less than 18.5), normal weight range (BMI between 18.5 and 24.9), overweight (BMI between 25 and 29.9), and obese (BMI of 30 or greater).

Though BMI calculators are often criticized for being overly simplistic when considering an individual's overall health, this number can help assess overall levels of fatness in the general population. Your body mass index can also help you monitor your weight and predict preventative health problems that can arise due to obesity.

While it should not be the only factor in determining overall health status, BMI measurements can provide valuable insight into higher risk factors for certain diseases related to excess body fat, such as heart disease and diabetes.

Is BMI Determined Differently For Children and Adults?

BMI can be an indicator of healthy weight for most adults, but unfortunately, the height-weight formula used to measure adult BMI is not as accurate for children who are still growing and developing. As a result, healthcare professionals use different BMI charts for children of different ages and genders.

BMI is not the only metric to determine whether a person’s weight is ideal. However, it can be a valuable tool for adolescents and adults as long as the correct calculation is used and other factors of body composition are taken into consideration. By taking BMI into account along with other body fat and general health tests, doctors may be able to assess whether a child is at healthy body weight.

What Are Other Methods of Determining Obesity Besides BMI?

A person squeezing their stomach

As body mass index is a simplified measurement, it does not consider factors like body composition, muscle mass, and fat distribution. As a result, BMI can sometimes underestimate or overestimate the amount of body fat a person has.

More accurate obesity measurements such as skinfold measurements, hydrostatic weighing, and waist circumference can shed more light on true body fat percentages in some people.

  • Skinfold measurements involve pinching the skin at different points on the body to estimate the amount of subcutaneous fat (fat beneath the skin).
  • Hydrostatic weighing is one of the most accurate ways to measure total body fat percentage. This test uses a special water tank that submerges your body underwater and measures body density to calculate body fat.
  • Waist circumference measures the distance around the waist and can be used to estimate abdominal fat.

These methods are more accurate than BMI, but they require specialized equipment and training to administer appropriately and are often less accessible.

Is BMI a Reliable Way to Determine Weight Issues?

A person looking frustrated on the scale

Despite its obvious faults, BMI remains popular as it is easy to calculate, provides a reasonable estimate of body fat for most people, and can be used to compare individuals of different heights and weights, which is helpful when population-level data is analyzed.

However, it is crucial to remember that BMI has some limitations. For example, BMI does not account for muscle mass or bone density, meaning that someone with a high muscle mass may have a high BMI without being overweight. On the other hand, it’s also possible for someone with excessive visceral body fat to have a normal BMI, as this measurement does not distinguish between fat and muscle.

Nowadays, BMI is mainly used as a screening tool to identify possible weight problems in adults. It’s important to note that BMI is not a diagnostic tool and that a health care provider would need to perform further assessments to determine if someone has a weight problem. 

Nevertheless, BMI remains a valuable tool for assessing obesity risk and can help to inform public health policy and interventions. That being said, BMI is only one piece of the puzzle, and should not be the only tool used to assess someone’s health status.

Health Conditions Associated with Obesity

Obesity is a complex condition that can affect a person's physical and mental health in various ways. Being overweight is also linked to an increased risk of certain types of cancer, including breast cancer, endometrial cancer, and colon cancer. In addition, obese individuals are more likely to suffer from joint problems, sleep apnea, and respiratory issues.

One of the most common health conditions associated with obesity is type 2 diabetes. This condition is characterized by high levels of sugar in the blood. It can lead to serious complications such as heart disease, osteoarthritis, coronary heart disease, high cholesterol, high blood pressure, cardiovascular diseases, stroke, and kidney failure.

While there is no easy solution to obesity and weight loss, it is vital to be aware of the potential health risks associated with this condition. By understanding the risks involved, people can be more motivated to make lifestyle changes that may help to improve their health.

Avoid Weight Gain and Prevent Obesity

Maintaining a healthy weight is important for both physical and mental health. However, many people struggle to keep their weight in check and achieve their goal waist size. By implementing a few simple lifestyle changes, you can avoid weight gain.

  • First, avoid grazing patterns of eating, and focus on protein at regular intervals throughout the day. While the ideal eating frequency can differ from person to person, it's important to eat in a pattern that works for your body. This can help you avoid mindless eating and keep cravings at bay.
  • Second, choose nutrient-dense, whole foods and limit processed carbohydrates. Meals that contain quality protein, non-starchy veggies and fruits, and whole fats can help keep your body satisfied and nourished. 
  • Finally, move your body regularly. In addition to regular exercise, avoid sitting for long bouts of time to keep your metabolism moving and your body strong.

If you’re trying to manage or lose weight to protect your overall health, following safe weight loss practices is essential to avoid unhealthy side effects.

These simple changes make it possible to maintain a healthy weight or even improve your weight loss goals.

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