Body mass index (BMI) is a number we get when we divide a person’s weight by the square of their height. This helps us estimate how much body fat a person has.
Health experts use BMI to check if a person is underweight, normal weight, overweight or obese, as these categories can give us a clue about a person’s health risks like diabetes, heart problems, and some cancers.
However, BMI doesn’t tell us everything about a person’s body composition, like how much muscle or bone they have, or how these factors may be influenced by their age, sex, and ethnicity.
Your genetics play a big role in deciding BMI levels. The genetic factors can impact how our body stores and burns fat, controls our hunger and metabolism, and how we react to factors like diet and physical activity.
By understanding the genetics of BMI, we can identify people who have a higher risk of obesity and its complications, discover new biological pathways that regulate body weight, and develop personalized prevention and treatment strategies.
In this easy-to-understand article, we will give you a full picture of what BMI is, how to calculate BMI, the different BMI categories, and the role of genetics in BMI.
Understanding Body Mass Index (BMI)
The BMI meaning is simple – it’s a calculation that tells us about the amount of fat in our body.
How to calculate BMI?
You need to divide your weight in kilograms by the square of your height in meters.
For example, if a person weighs 70 kg and is 1.75 m tall, their BMI is 70 / (1.75 x 1.75) = 22.9 kg/m^2.
Put in your weight and height to check your BMI.
What is considered a high BMI?
The World Health Organization (WHO) has defined four BMI categories:
- Underweight: BMI less than 18.5 kg/m^2
- Normal weight: BMI between 18.5 and 24.9 kg/m^2
- Overweight: BMI between 25 and 29.9 kg/m^2
- Obesity: BMI 30 kg/m^2 or higher
Obesity is further divided into three classes:
- Class I: BMI between 30 and 34.9 kg/m^2
- Class II: BMI between 35 and 39.9 kg/m^2
- Class III: BMI 40 kg/m^2 or higher
These categories help us understand the link between BMI and health risks. However, they may not be accurate for everyone, as they don’t consider factors like age, sex, ethnicity, muscle mass, bone density, or existing health conditions.
Effects of Low and High BMI
Low and high BMI can both have negative effects on health.
A low BMI might mean a person is undernourished or has a disease that causes weight loss, like cancer or an eating disorder. It can increase the risk of infections, fragile bones, anemia, fertility issues, mental health issues, and death.
On the other hand, a high BMI can be a sign of obesity, which is when a person has too much body fat that it negatively affects their health.
What is BMI obese? BMI and obesity are linked, with a high BMI often indicating obesity. High BMI can increase the risk of diabetes, heart disease, high blood pressure, fatty liver disease, sleep apnea, and certain types of cancer.
BMI in Children
BMI calculation in children is a bit different than in adults.
While the formula to calculate BMI remains the same, it is interpreted differently for children because their body composition changes as they grow.
Thus, children’s BMI is compared with charts that provide age- and sex-specific percentiles for BMI.
Children’s BMI categories are:
- Underweight: BMI below the 5th percentile
- Normal weight: BMI between the 5th and 85th percentiles
- Overweight: BMI between the 85th and 95th percentiles
- Obesity: BMI above the 95th percentile
However, these categories might not apply to all children, as they don’t consider factors like puberty, body fat distribution, or existing health conditions.
High or low BMI in children can have serious health implications.
High BMI in children can lead to various health problems, such as insulin resistance, type 2 diabetes, fatty liver disease, high blood pressure, sleep apnea, and reduced quality of life. It can also increase the risk of obesity-related diseases later in life.
On the other hand, a low BMI in children might mean they are undernourished, which can affect growth and development, immune function, cognitive function, and academic performance.
It can also increase the risk of infections, anemia, osteoporosis, fractures, depression, and death.
The Link Between Your Genes and BMI
Just like how our genes decide the color of our eyes, they also play a role in determining our body mass index (BMI), which is a way of measuring if we’re at a healthy weight.
Genes (part of DNA) are the instructions that decide how our bodies work, and they can affect how we store and use up fat, how hungry we feel, and how our bodies respond to what we eat and how active we are.
In this way, our genes can interact with other things to influence our BMI levels.
Understanding the Role of Genes in BMI
There are many different genes that are related to BMI and how much body fat we have. These genes come in two types: monogenic genes and polygenic genes.
Monogenic genes are rare ones that can lead to severe forms of obesity which usually show up when we’re very young. These genes affect a pathway in our brain that controls how much energy we take in and use up.
Some examples of these genes are LEP, LEPR, POMC, MC4R, BDNF, NTRK2, SH2B1, PCSK1, SIM1, ALMS1, CEP19, KSR2, MRAP2, TUB, MKKS, BBS4, and others.
Changes in these genes can mess up the production or work of hormones or receptors that control our hunger and energy balance, leading to a lot of hunger, eating, and weight gain.
On the other hand, polygenic genes are common ones that contribute to the differences in BMI and body fatness in everyone.
These genes typically have small effects on their own, but can have big effects when all put together.
They can affect many biological processes that influence our body weight and fat distribution, such as how fat cells are formed and broken down, how much heat our bodies produce, inflammation, how sensitive we are to insulin, the microbes in our gut, and our body’s internal clock.
Some examples of these genes are FTO, MC4R, TMEM18, GNPDA2, KCTD15, SH2B1, MTCH2, NEGR1, SEC16B, TFAP2B, BDNF, ETV5, IRX3, IRX5, ADCY3, RBJ, POMC, PCSK1, TCF7L2, PPARG, ADIPOQ, LEP, LEPR, and others.
Changes in these genes can change the work or amount of proteins that control fat storage and metabolism, hunger and fullness signals, energy use and temperature control, inflammation and immune responses, insulin action and blood sugar control, gut flora composition and function, and sleep patterns.
Evidence from Studies on Twins and Families
Studies on twins and families give strong proof for the role of genes in BMI and body fatness.
Twin studies compare how similar the BMI is between identical twins who have the exact same genes and fraternal twins who share half of their genes on average.
If BMI is influenced by genes, identical twins should have more similar BMI than fraternal twins.
Family studies compare how similar the BMI is between relatives who share different amounts of their genes, like parents and children, siblings, cousins, etc. If BMI is influenced by genes, relatives should have more similar BMI than people who are not related.
Many twin and family studies have figured out the “heritability” of BMI, which is how much of the differences in BMI can be explained by genes in certain people.
The heritability of BMI is between 40% to 70%, depending on the way the study was done, how many people were in the study, how old they were, their ethnicity, and their environment.
This means that genes are responsible for a big part of the differences in BMI and body fatness.
These studies also showed that genes affect not just the overall level of BMI but also how it changes over time.
For instance, studies that tracked twins over a long time showed that genes contribute to how stable or changeable BMI is from childhood to adulthood.
Genes also affect when and how much BMI changes during important growth and development stages, like puberty and pregnancy.
Epigenetics and Your BMI: A Complex Relationship
Epigenetics is like a set of switches that turn your genes on or off. Epigenetics can change how your genes act in response to things in your environment, like stress, diet, or toxin.
This includes your diet when you were a baby, the experiences you had when you were young, and how old you are.
There are different types of epigenetic changes. One type is when a small molecule, called a methyl group, attaches to your DNA. This usually turns the gene off. Another type is when the proteins, called histones, that DNA wraps around are changed. This can turn a gene on or off too.
What you ate when you were a baby can affect your BMI when you grow up. This is because it can change how your genes act and this can affect your metabolism and risk of obesity later in life.
For example, if your mom didn’t eat enough or ate too much when she was pregnant with you, it could change how your genes work. This could make you more likely to put on weight and increase your BMI levels when you grow up.
The things you go through like stress or food habits when you’re young can also affect your BMI.
As you get older, your BMI can change too. This is because your genes can change as you age and this can affect how your body works.
These epigenetic changes can affect how your body stores fat and uses energy. This could make you more likely to put on weight and increase your BMI levels.
The Interplay between Non-genetic and Genetic Factors in BMI
Your genes play a big part in determining your Body Mass Index (BMI). But your genes are not the only players in this game.
Non-genetic factors, like the food you eat and how active you are, also have a say in your BMI levels.
Non-Genetic Factors That Affect Your BMI
Your BMI can change based on the choices you make every day. In some cases, the effect of a gene and an environmental factor on BMI can add up together.
The interaction can make the effect of a gene or an environmental factor on BMI bigger or smaller.
These factors can include:
Things You Eat
Things like smoking or drinking alcohol can play a role in your BMI. Smoking might lower your BMI because it can make you eat less and burn calories faster.
But, it’s not healthy and can make you very sick. Drinking alcohol, on the other hand, might make your BMI go up because it gives your body extra calories and can make you want to eat more.
But a little bit of alcohol could be good for your heart.
The food and drinks you have every day can also change your BMI. If you eat more calories than you use up, it can make your BMI go up. But not all calories are the same.
Some foods, like those high in fiber, can help you feel full and keep your BMI in check. If you eat a lot of sugary or fatty foods, it can make your BMI go up.
The other part of the equation is physical activity. If you move around a lot, it helps your body burn more energy and can lower your BMI.
But the right amount and type of physical activity can depend on things like your age, sex, how fit you are, and your genes.
Your sleep habits are also important. Sleeping well helps control the hormones that manage your appetite, which can help keep your BMI at a healthy level.
How you handle stress can affect your BMI too. If you’re always stressed, your body might store more fat and change how it uses energy, which can increase your BMI.
Socio-economic factors, like your income and education, can also have a big impact on your BMI. If you don’t make a lot of money, it can be hard to find and afford healthy food or places to exercise.
On the other hand, if you make a lot of money, you might have more chances to eat high-calorie foods and less time to be active, which can increase your BMI.
Is BMI the Right Tool to Measure Obesity?
While BMI is a useful initial screening tool for assessing a population’s general health status concerning weight, it is not a diagnostic tool.
It’s not a perfect measure of obesity. Here’s why:
- It doesn’t account for muscle mass: BMI does not distinguish between weight from muscle and weight from fat. Because muscle is denser than fat, a highly muscular person might have a high BMI, even though they’re not obese and may actually be quite fit.
- It doesn’t consider fat distribution: Where your body stores fat is significant for health. People who carry extra fat mainly around their abdomen (apple-shaped) are at higher risk for heart disease, diabetes, and certain types of cancer than people who carry it mainly in their hips and thighs (pear-shaped), regardless of BMI.
- It does not account for age, sex, or ethnicity: These factors can all influence body composition and health risks associated with overweight and obesity. For instance, women generally have more body fat than men at the same BMI, and older people, on average, tend to have more body fat than younger adults at the same BMI. Furthermore, some ethnic groups seem prone to health risks at different BMIs compared to others.
A healthcare professional will need to perform further assessments, such as skinfold thickness measurements, evaluations of diet and physical activity, family history, and other appropriate health screenings to diagnose obesity accurately.
The concept of health is multi-faceted, and there’s more to it than just numbers on a scale or a BMI reading. It’s always best to consult a healthcare professional to comprehensively evaluate your health.
Implications and Applications of Understanding Genetics and BMI
In the realm of health and wellness, understanding the relationship between genetics and BMI (Body Mass Index) holds considerable significance.
As we unravel the complex interplay between genetics and BMI, we encounter the scope for promising applications in obesity prevention and treatment.
Genetics influence BMI by affecting how our bodies store fat, regulate appetite, and process food. Every individual carries a unique genetic makeup, making it easier or harder to gain or lose weight, which in turn impacts their BMI.
This understanding of the genetics of BMI also extends to the idea of ‘obesity genes’ or specific genes associated with an increased risk of obesity.
Implications of Understanding Genetics and BMI:
- Personalized Obesity Prevention and Treatment: With an understanding of an individual’s genetic predisposition to a high BMI, it’s possible to develop personalized diet and exercise plans to help prevent obesity. This could involve customized nutritional advice and physical activity recommendations.
- Screening and Risk Assessment: Genetic screening can be used to identify individuals at high risk of obesity, even before they display symptoms. This could allow for earlier intervention and management, reducing the risk of associated health complications.
- Development of New Therapies: Insights from the genetics of BMI can potentially inform the development of new drug therapies for obesity. For example, by identifying genetic targets for obesity, pharmaceutical researchers can design drugs to influence these specific genes or the biological pathways they influence.
Genes play a big part in how different people’s BMI and body fat turn out, but they aren’t the only thing that matters.
Things that aren’t related to your genes, like how you live your life, how much you move around, what you eat, and how much money you make, can also change your BMI and body fat.
They do this by changing how much food you take in and how much energy you use, or by changing how your body keeps its weight and fat levels in check.
What’s more, these non-genetic factors can mix with your genes to change how they affect your BMI and body fat.
What genes you inherit is not in your hands, but what lifestyle you choose to live is in your control and you can most of the times counter the affect of obesity genes by better lifestyle.
If we can understand how genes, non-gene factors, and epigenetics work together to affect BMI and body fat, we can better understand why people become obese and get diseases related to obesity.
This could also help us come up with ways to prevent and treat obesity.
Dr. Sumeet is a seasoned geneticist turned wellness educator and successful financial blogger. GenesWellness.com, leverages his rich academic background and passion for sharing knowledge online to demystify the role of genetics in wellness. His work is globally published and he is quoted on top health platforms like Medical News Today, Healthline, MDLinx, Verywell Mind, NCOA, and more. Using his unique mix of genetics expertise and digital fluency, Dr. Sumeet inspires readers toward healthier, more informed lifestyles.