Can you be obese with a ‘normal’ BMI? The definition of obese is changing as doctors and health researchers learn more about how our body uses its fat stores.
Most of us have heard a story about a skinny uncle who died from heart disease, or a very overweight friend of a friend who ran a marathon and lived to 100. The question is, how common are these exceptions? Can we find better ways to predict cardiometabolic risk than blunt tools like BMI?
Body Mass Index Vs Abdominal Circumference
Once upon a time, we all thought that obesity was as simple as excess adiposity (i.e. extra fat). Now medics worry that using BMI to predict metabolic disorders misses a significant number of folk who are at risk. A significant number of us could be obese despite our seemingly normal BMI.
In clinical terms, obese doesn’t mean ‘very fat’, it means that excess fat in your body is disrupting the function of your organs to the point it is damaging your health. In other words, ‘obesity’ is the state where a person is at significant danger of developing cardiometabolic illnesses. Experts are now campaigning to change how we diagnose obesity, emphasizing the importance of belly fat in predicting your future well-being rather than relying on BMI.
So can you be obese with a so-called normal BMI? It turns out, yes. According to a study published earlier this year in JAMA Network Open, as many as one in five adults with a ‘normal BMI’ of between 18.5 and 24.9 could be unknowingly at high risk of cardiometabolic disorders such as diabetes and hypertension.
Figuring Out Cardiometabolic Risks at Normal BMI
Australian epidemiologists investigated World Health Organization survey results from nearly half a million people worldwide to find out whether using a BMI of 30 or above to identify folk at risk of cardiometabolic disorders does the job.
The WHO collected information from 91 countries over 20 years (2000–2020) to find out how lifestyle affects our risk of cancer, cardiovascular and metabolic diseases, amongst others using the Stepwise Approach to Surveillance of Noncommunicable Disease Risk Factors (STEPs) survey.
The Charles Sturt University, New South Wales based team report that of the 471, 228 survey participants, just over 20% with a BMI between 18.5 and 24.5, had an obese waist measurement. This means that solely applying BMI to predict risk to cardiovascular and metabolic health misidentifies a large segment of people as low risk.
What’s more, the survey also highlighted the correlation between having a substantial amount of belly fat despite a normal BMI and a person’s chances of having hypertension, diabetes, high cholesterol levels or high triglycerides. These are all measures that tell doctors about our cardiovascular health.
Apples and Pears
Before we take the plunge into the data, we need to get familiar with what the researchers were looking for and why they made these comparisons.
For some time epidemiologists and doctors have noticed that some people with a higher BMI don’t have the same risk of cardiometabolic diseases as others with the same weight to height ratio. It is also true that in Asian populations the chances of a person developing high blood pressure, heart disease or type 2 diabetes are higher at lower BMIs than in European populations.
These observations, amongst others, led researchers to ask, when it comes to cardiometabolic disease, is the amount of fat what matters or where the fat is deposited that matters more? Researchers are discovering mounting evidence that excess belly fat, or visceral fat, has very different effects on your health than excess subcutaneous fat, that’s the fat under your skin.
Everybody deposits fat inside their abdominal cavity around their organs and under their skin with concentrations on the hips, thighs, chest and buttocks. There are people, however, whose body is more prone to packing the fat in their belly, and people whose fat tends to accumulate outside the belly, most often under the skin on their lower body. Healthcare professionals often refer to these two sets of people, as apple shaped – fat goes to the belly, and pear-shaped – fat gores to the hips.
People with an apple shape, epidemiologists say, are at higher risk of cardiometabolic diseases than people at the same BMI who have a pear shape. In this study the researchers looked at how apple or pear-shaped fat distributions changed an individual’s risk profile and how it relates to BMI.
What is abdominal obesity?
Most of us are familiar with BMI – or Body Mass Index; it’s basically the ratio between your weight and your height. It gives a rough measure of our overall mass that makes it easier to compare. For example, two different people who weigh the same but are vastly different heights will have different BMI.
Your BMI, however, doesn’t tell us anything about what is making you heavier or where your body is storing fat. It’s a quick and easy measure that can be very useful as a first pass, but these days we know that the location of your fat is as important as the quantity.
Epidemiologists have figured out that anthropometric measures and body fat percentages give a better read out of obesity and potential risk to health than BMI alone. In this study they call obesity detected by anthropometric measures ‘abdominal obesity’ and a BMI over 30, ‘obese’.
The new criteria for obesity are based on waist measurements – waist is over 90 cm for Asian men, 102 cm, all other men, 80 cm for Asian women and 88 cm for all other females.
In this study the researchers examined how BMI, obese BMI and abdominal obesity at different BMIs affect a person’s risk of cardiometabolic disease.
A normal BMI is defined as BMI 18.5-24.9 kg/m2; overweight BMI is 25–29.9 kg/m2; obese BMI is 30 + kg/m2.
Feast of Data
The researchers collected data from the WHO’s STEPS datasets. This is a global survey that captures information about various aspects of people’s lives, for example, weight, BMI and smoking status, to see how they connect to noncommunicable diseases like diabetes, heart disease and cancer.
The team managed to get information from 91 countries across all continents. They found that the prevalence of obese BMI and abdominal obesity was different across continents, and that obese BMI and abdominal obesity were both linked to a higher risk of various metabolic diseases.
The researchers used the lower cut-offs for an obese waist measurement to make sure that Asian populations would not be uncategorized.
The team found that if you have an obese waist measurement (AKA abdominal obesity) but a normal BMI, your odds of having hypertension were 1.3 times higher than a person with a slim waist. Likewise, abdominal obesity and a normal BMI almost doubled the odds of having type 2 diabetes. Everywhere but the Americas, cholesterol and triglyceride levels were more likely to be high in abdominally obese people with normal BMI (1.4 times more, 1.6 times more).
Pear Shape Risk Factors
If having an obese waist measurement can increase your risk of cardiometabolic warning signs, is the opposite true? Can a slim waist reduce your risk even with an obese BMI?
Disappointingly, no. A low waist measurement paired with an obese or over weight BMI still increases your risk of developing signs of metabolic disease compared to a person with a normal BMI and a non-obese waist measurement.
Everywhere except Africa and the Western Pacific, a low waist circumference and an over-weight BMI still increased your risk of hypertension and elevated cholesterol levels. People of every continent showed increased odds of diabetes and high triglycerides at an over wight or obese BMI regardless of their waist circumference. The good news is that people with a lower waist measurement at a high BMI were slightly less likely than people with a high waist measurement and high BMI to have cardiometabolic signs of obesity.
What Does it Mean if I Am Obese With a Normal BMI?
People who have a normal BMI but a waist circumference over 80 cm for women (non-pregnant) or 90 cm for men have a higher risk of developing metabolic syndrome and diabetes than people with a lower waist measurement. If you have been flying under the radar with a bit of extra belly fat because your BMI didn’t trigger an alarm, it might be time to get yourself to the doctor for a check-up.
References
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Caleyachetty R, Barber TM, Mohammed NI, et al. Ethnicity-specific BMI cutoffs for obesity based on type 2 diabetes risk in England: a population-based cohort study. Lancet Diabetes Endocrinol. 2021;9(7):419-426. doi:10.1016/S2213-8587(21)00088-7
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