Body Mass Index (BMI) Calculator – Risks Involved Being Overweight, Underweight

Introduction 

The BMI (Body Mass Index) is the current metric for defining anthropometric height/weight characteristics in adults and categorising them into groups. 

The most common interpretation of BMI is that it represents an index of an individual's fatness. 

It is also widely used as a risk factor for the development or prevalence of various health issues.
 

What is BMI?

Body Mass Index (BMI) is a measurement of a person's weight in relation to their height. It is an indirect indicator of a person's total body fat than a direct measurement.

BMI Formula 

BMI is a weight-for-height measurement calculated as weight in kilogrammes divided by height in metres squared (Kg/m2).

BMI = Weight (Kg) ÷ [Height (m)]2
 

BMI table for adults 

Adults aged 20 and up can use BMI to analyse their category. BMI is calculated using standard weight status categories that are the same for men and women of all ages.

The following is the BMI categorisation for the Asian-pacific population as suggested by World Health Organisation (WHO).

BMI < 18.5 Kg/m2 

BMI less than 18.5 Kg/m2 indicates a low body fat percentage and a lean body mass. A person with this BMI is regarded as underweight. 

If the BMI and body weight are low, one should consider gaining weight through good diet and exercise habits to increase muscle mass.

BMI between 18.5 - 22.9 Kg/m2

This BMI represents an ideal, healthy amount of body fat. A healthy BMI is associated with the longest life expectancy and the lowest incidence of severe illness. 

Coincidentally, it appears that this ratio is what many people consider to be the most elegant.

BMI between 23 - 24.9 Kg/m2

BMI between 23 - 24.9 Kg/m2 is regarded as an acceptable range and is linked to good health. But these individuals have to maintain or lose their weight to not fall under the obese category, which makes them prone to lifestyle diseases.

BMI 25 - 29.9 Kg/m2 

People under the obesity grade I category should work to lose weight through diet and exercise. At their current weight, they are predisposed to several illnesses.

BMI greater than 30 Kg/m2

This BMI signifies an unhealthy state, and people with this BMI are classified as Obesity grade II. 

Surplus weight places a person at risk for Diabetes, heart disease, high blood pressure and some cancers. Striving to lose weight by changing their diet and exercise helps them become normal.

BMI table for children and teens (age below 20) 

The interpretation of BMI is unique for children and teenagers, depending on their age and gender.

BMI Percentile for Child and Teen provides BMI and the corresponding BMI-for-age percentile based on CDC growth charts for children and teens aged 2 to 19.

BMI is not a diagnostic tool for children and adolescents. It is instead used to screen for potential weight and health issues. 
If a child has a high BMI for their age and gender, a health care provider may conduct additional tests to determine if excess fat is a problem. 

Skinfold thickness measurements, family history, dietary and physical activity evaluations and other appropriate health screenings may be included in these assessments.

Centers for Disease Control and Prevention have come up with Body mass index for age percentiles chart separately for boys and girls aged 2 to 20 years. 

After calculating BMI for children and adolescents, it is expressed as a percentile based on a percentile graph. Based on the percentile of a particular child, their weight category is determined.

The following is the table provided by the CDC that categorises BMI for the age of individuals according to their respective percentiles obtained from the percentile chart.

Waist circumference

The waist circumference is used to calculate central obesity. Abdominal obesity is defined by a waist circumference of more than 102 cm in men and more than 88 cm in women.

Waist-to-hip ratio 

The International Diabetes Federation (IDF) has recommended cutoffs for waist circumference and waist-hip ratio. 

The IDF's waist circumference recommendations are not only gender-specific but also population and geography-specific. 
In the Asian population, the waist-hip ratio of 0.80 in women and 0.95 in men signifies abdominal obesity. A ratio less than that is usually considered ideal.

Waist-to-height ratio 

A person's waist-to-height ratio (WHtR) is defined as the waist circumference divided by the height, both in the same units.

According to NICE (The National Institute for Health and Care Excellence), a healthy waist-to-height ratio is 0.4-0.49, which implies no increased health risks. 

A ratio of 0.5-0.59 reveals an increased risk, and high risk is evident from a ratio greater than 0.6.

Factors affecting BMI

The following factors influence an individual's weight and thereby affect their Body Mass Index (BMI).
 

• Improper diet
• Lack of physical activity
• Sedentary lifestyle
• Inadequate sleep
• Alcohol and smoking
• Medication

Who shouldn't use a BMI calculator? 

BMI is not entirely independent of height. It overestimates obesity in shorter people while underestimates it in taller people. 

As a result, BMI should not be used as a guide for adults who are either shorter than 150 cm or taller than 190 cm.

Risks of being overweight 

Obesity and overweight may increase the risk of certain health problems and emotional and social issues.
 

Health risks of being overweight include
 

• High blood pressure
• Type 2 Diabetes
• Few cancers - Breast cancer
• Fatty liver disease
• Respiratory disease
• Sleep apnea
• Cardiovascular diseases
• Stroke
• Kidney disease
• Osteoarthritis
• Pregnancy problems
• Metabolic syndrome

Risks of being underweight 

Underweight can be due to genetics, increased metabolism or eating disorders like Anorexia nervosa. Being underweight increases the health risks of a person. These include

• Poor stamina
• Nutritional deficiencies
• Infertility
• Weak Immune System
• Anaemia - especially Iron deficiency anaemia
• Hair Loss
• Amenorrhea and Pregnancy complications in Women
• Osteoporosis

BMI limitations

For adults 

• In athletes and people with a muscular build, BMI is more likely to overestimate the body fat content.
• There is also a chance for overestimating body fat in older people and those who have lost muscle mass.

For children

• Factors such as puberty alter the link between BMI and body fat in adolescents.
• While BMI predicts excess body fat in obese children, elevated BMI levels in overweight children can be caused by an increase in either fat or non-fat mass.

Ponderal Index 

Ponderal Index (PI) is also called Rohrer's or corpulence index. 

This index integrates an individual's weight and height to produce a value that can classify a person's body weight as normal or abnormal. 

The ponderal index is considered superior to Body Mass Index (BMI). 

This is based on the grounds that BMI considers the body height to be a two-dimensional square. In contrast, the ponderal index considers the body height to be a three-dimensional cube. 

BMI uses the same formula for adults and infants, whereas PI uses a slightly different formula for infants than adults. 

Ponderal Index Formula

For infants

Ponderal index = (Birth weight (g) × 100) ÷ (Crown heel length (cm))3

For adults 

Ponderal index = Weight (Kg) ÷ (Height (m))3

Ponderal Index range

Infants

Adults

Conclusion

BMI is a valuable indicator for deciding the health of a person. While determining the disease risk, the distribution of fat in the body is more important than the quantity, due to which waist circumference is thought to be a better predictor of a health risk than BMI. 

Otherwise, BMI is reliable to an extent and is an easy way to predict an individual's overall health. We also have the ponderal index to measure the body composition of a person, which is considered more accurate than BMI.