What Is Body Mass Index?
Body mass index or (BMI) is an indicator of your body fat, which is calculated based on your height and weight.
This number is used to classify individuals into different groups – optimum weight, underweight, overweight, or obese.
BMI can be used as a screening test rather than a diagnostic test.
Several factors, such as age, sex, disease, genetics, and lifestyle, affect BMI measurements, and thus, normative standards must be applied for specific groups and individuals.
Both high and low BMI can cause health issues. BMI has proven to be a very useful tool to screen for weight problems in both adults and children. However, it does come with a few caveats:
1. BMI does not furnish information such as the mass of fat in different regions of the body.
2. BMI tends to overestimate the amount of body fat in people who are very muscular - that is, it does not differentiate between lean body mass and body fat mass.
3. BMI may also underestimate the amount of body fat in older adults and other people who have lost muscle mass.
Calculating Your BMI
To calculate the BMI using the metric system, you need to divide your weight (in kilograms) by the square of your height (in meters):
WEIGHT(Kg) / [HEIGHT(meters)]^2
Since height is usually measured in centimeters, the formula can be written as:
[WEIGHT(Kg) / HEIGHT (cm)/ HEIGHT(cm)] x 10,000
To calculate the BMI in the English system, the formula is:
WEIGHT(lb) / [HEIGHT(in)]^2 x 703
Before the BMI calculation, the weight needs to be converted into decimal values in case it is given in terms of ounces.
BMI Categorization
In order to calculate an individual’s BMI, his/her weight(in kgs) must be divided by the square of his/her height(in meters).
Based on the above-mentioned calculation, individuals are categorized as:
- <18.5: Underweight
- 18.5 to 25: Normal
- 25 to 30: Overweight
- 30 or higher: Obese
Based on BMI, obese individuals are further classified as:
- 30 to 35: Mild obesity
- 35 to 40: Moderate obesity
- 40 or higher: Extreme or severe obesity
BMI in Children
Unlike in the case of adults, BMI measurements during childhood and adolescence take age and sex into consideration. The BMI is calculated the same way by measuring height and weight. This is then plotted on a sex and age-specific chart. This will indicate whether the child’s weight is within a healthy range.
- Below the 5th percentile: Underweight
- 5th percentile to less than the 85th percentile: Healthy weight
- 85th to less than the 95th percentile: Overweight
- Equal to or greater than the 95th percentile: Obesity
How Does Genetics Influence Your BMI?
FTO Gene and BMI
FTO or Fat mass and obesity-associated gene, as the name suggests, is linked to body weight. It contains instructions for producing a protein known as alpha-ketoglutarate-dependent dioxygenase FTO.
The FTO gene is one of the most researched genes for obesity.
rs9939609
rs9939609 is an SNP in the FTO gene. It has been linked to an increase in total body fat levels. According to a study conducted, the presence of the AA allele in this SNP has been shown to contribute to obesity and increased BMI, irrespective of how the adipose (fat) tissue distribution is.
Factors influencing an individual’s BMI, like insulin sensitivity and plasma cholesterol levels, are also associated with the SNP rs9939609.
BDNF Gene and BMI
The BDNF gene contains instructions to produce the protein by Brain-Derived Neurotrophic Factor. This protein is found in the brain and spinal cord. It is especially found in the regions of the brain that control eating, drinking, and body weight. Hence, this protein influences all of these functions.
rs6252
rs6265, also known as Val66Met, is a Single Nucleotide Polymorphism (SNP) in the BDNF gene. A study carried out a detailed examination of eating behavior in persons with different Val66Met types (Val-Val or GG, Val-Met or AG, and Met-Met or AA). It was discovered that people who have the Met-Met (AA) type had a lower BMI than those with the Val-Met (AG) or the Val-Val (GG) genotype.
Non-genetic Factors That Influences Your BMI
Certain factors can predispose you to a higher BMI. The good news is that most of these factors are modifiable and can be worked around to achieve the ideal BMI.
Age
Adults who have a normal BMI often start to gain weight in young adulthood and continue to gain weight until they are ages 60 to 65. In addition, children who have obesity are more likely to have obesity as adults.
Sex
Women are likely to accumulate fat near their hips and buttock areas. Men build up fat around their abdomen (belly) region.
Women tend to build up fat in their hips and buttocks. Extra fat, particularly if it is around the abdomen, may put people at risk of health problems even if they have a normal weight.
Ethnicity
In American adults, the prevalence of obesity is the highest in African Americans, followed by Hispanics/Latinos, then Caucasians. This is true for men and women.
Diet
Needless to say, dietary habits influence your body weight. High-calorie and high-sugar foods increase your risk for overweight and obesity.
Other factors that influence your BMI include your levels of physical activity, your work environment, and your family habits and culture.
Effects of High BMI
High BMI and obesity can increase the risk of many chronic health conditions, including:
- High blood pressure
- Type 2 diabetes
- Liver disease
- Osteoarthritis
- Cardiovascular disease
- Musculoskeletal problems
Effects of Low BMI
Being underweight and have insufficient fat in your body can also lead to health complications like:
- Anemia
- Malnutrition
- Bone loss and osteoporosis
- Decreased immune function
BMI May Not Be The Best Indicator Of Obesity?
One of the prime reasons for this is that BMI doesn’t differentiate between muscle and fat.
It may not be accurate, especially if you are in one of the following groups:
Athletes: Athletes tend to have higher bone mass and lean muscle mass. As a result, they may have higher BMI.
But this increased lean muscle mass can actually be healthy as it helps boost metabolism and prevent heart diseases and diabetes.
Pregnant or breastfeeding women: Most of the weight gain during pregnancy is to provide nourishment for the growing fetus and is usually not an indication of bad health or obesity.
Older people: In people who are 65 or older, a BMI of less than 23 is associated with health risks. The ideal BMI for this age group is considered to be 27.
Alternative Ways To Measure Body Fat
- Waist circumference
- Waist to height ratio
- Body fat percentage
- Waist to hip ratio
Summary
- Body Mass Index or BMI is a value that is derived by measuring a person’s height and weight. It could be an indicator of whether a person’s weight is healthy.
- A BMI reading of >30 could indicate overweight, and <18.5 could indicate underweight.
- BMI in children is also measured using height and weight. This is plotted on a chart specific for age and sex and then analyzed.
- Your genes influence your BMI by modulating several weight-related factors. FTO is one such gene, which has been widely studied for obesity.
- Other factors like age (early adulthood to 60 years), sex (women are at higher risk), and ethnicity (African Americans have a higher risk) also increase obesity risk.
- Both higher and lower BMI can be harmful to health. High BMI has been associated with health conditions like obesity and type 2 diabetes. Lower BMI can increase your risk for bone loss and anemia.
- BMI need not always be the best indicator of obesity. This is because it doesn’t differentiate between lean muscle mass and fat. So, BMI may not be a useful tool for athletes, pregnant women, and older people.
- Maintaining your calorie intake and calorie expended can help you maintain a healthy weight.
References
https://pubmed.ncbi.nlm.nih.gov/29466028/
https://medlineplus.gov/genetics/gene/bdnf/
https://pubmed.ncbi.nlm.nih.gov/16707914/
https://pubmed.ncbi.nlm.nih.gov/27272580/
https://pubmed.ncbi.nlm.nih.gov/22805182/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3661116/
https://pdfs.semanticscholar.org/449a/cc928759c8dab3abbed7d227dfbb4836ef43.pdf
