By Mac E. Lancaster, BS, and Ran D. Anbar, MD
The measurement we primarily use to determine if someone is obese, overweight, or metabolically healthy is called the body mass index (BMI). It can be calculated based on weight and height (Centers for Disease Control and Prevention).
Doctors and schools use the BMI to assess if someone is healthy because it’s quick, easy to measure, and easy to categorize. The BMI ranges from underweight (< 18.5) to healthy weight (18.5-24.9), overweight (25.0-29.9), and obese (more than 30.0). Because there is an abundance of BMI data available, this measure is frequently used in articles about metabolic health.
It is critical to note that obesity, especially involving visceral fat in the abdominal area, increases the risk of cancer, cardiovascular disease, and metabolic disorders like insulin resistance and type 2 diabetes (T2D) (Engin, 2017). The problem is that BMI fails to address adequately certain types of metabolic situations including,
Although BMI can help doctors make quick assessments, it is a flawed system of measurement because it overgeneralizes and is misleading for atypical cases or specific aspects of obesity that are related to metabolic disorders. (Denis & Hamilton, 2013).
For example, an individual with a BMI of 35 would be diagnosed on paper as obese. However, if you add that they are a bodybuilder with thick bones and only 8% body fat, it becomes apparent that this person is metabolically healthy.
Thus, BMI does not account for visceral fat and genetics, which are key factors contributing to metabolic disorders. Let's examine these two markers of health further to empower you with a better understanding of metabolism.
There are two types of body fat: visceral and subcutaneous. They have different locations, purposes, and health implications. BMI does not distinguish these types of fat.
While neither type of fat is healthy in excess, visceral fat poses a significantly greater health risk (Xu et al., 2022).
Visceral fat grows when cells enlarge or become hypertrophic (Horwitz & Birk, 2023). Hypertrophic fat cells are less responsive to insulin, have a reduced capacity to store fat, and are associated with releasing inflammatory molecules (Horwitz & Birk, 2023). As a result, excess visceral fat is stored in areas not normally involved in fat storage, including skeletal muscles, the liver, and the pancreas (Horwitz & Birk, 2023).
Visceral fat is linked to a higher risk of heart disease, high blood pressure, T2D, and several types of cancer, independent of BMI (Małodobra-Mazur et al., 2020; Horwitz & Birk, 2023).
Conversely, subcutaneous fat cells are more likely to stay small and grow by multiplying via hyperplasia, making them less metabolically damaging than visceral fat (Małodobra-Mazur et al., 2020). Subcutaneous fat is necessary for padding muscles and bones, controlling consistent body temperatures, and storing excess energy as a usable source during hunger, fasting, or diseases.
As an endocrine organ, subcutaneous fat secretes hormones that reduce your risk of cancer, stabilize your weight and blood pressure, bolster your immune system, increase your insulin sensitivity, and reduce inflammation (Coelho et al., 2013). Appetite regulation is also heavily influenced by some hormones produced by subcutaneous fat (Miller, 2017).
Sumo wrestlers represent edge cases with extremely high BMIs and healthy metabolisms, with normal blood glucose and relatively low visceral fat levels (Denis & Hamilton, 2013). They can prolong a healthy metabolic state because most of their fat is subcutaneous, and they maintain intense physical training (Denis & Hamilton, 2013). However, when these athletes stop their regular exercise regimen, they are at immediate risk of developing metabolic disorders.
Another factor BMI cannot account for is genetic variation. Compared to Westerners of the same BMI, South and East Asian people (Mittal, 2019) have a smaller body but are characterized as thin on the outside, fat on the inside (TOFI), with a higher average waist/hip circumference ratio (Leow, 2017).
Chinese people are more prone to visceral fat production and developing T2D (Wu et al., 2020). A recent study found that Asian people of Indian subcontinent descent living in Europe are two to four times more likely to develop T2D and are 40% more at risk of heart disease (Roberts et al., 2023). Another study found that even South Asian newborns had higher blood glucose levels and insulin, pointing toward TOFI characteristics (Karamali et al., 2015).
Another genetic factor is linked to differences in fat composition in males versus females. During and post-puberty, total body fat increases for females and males (Mittal, 2019). Females tend to accumulate more subcutaneous fat around the hips and thighs, while males store more visceral fat in the abdominal region (Mittal, 2019). Notably, pre-menopausal females show higher subcutaneous fat levels, while post-menopausal females show a higher ratio of visceral fat (Mittal, 2019).
BMI is an easy way to measure a patient’s health on the surface level. However, it excludes variables that make its reliability as a sole determining factor of health misleading. Because everyone's metabolism is different, learning about the factors and measurements associated with metabolism—and there are many more than the few mentioned above—is an important first step toward empowering you to take control of your health.
Mac Lancaster obtained his undergraduate degree in Cognitive and Behavioral Neuroscience at UC San Diego.
References
Centers for Disease Control and Prevention. (2022, September 2). Adult BMI Calculator. Centers for Disease Control and Prevention. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html
Centers for Disease Control and Prevention. (2022, June 3). Assessing Your Weight. Centers for Disease Control and Prevention. https://www.cdc.gov/healthyweight/assessing/index.html#:~:text=If%20your%20BMI%20is%20less,falls%20within%20the%20obese%20range.
Coelho, M., Oliveira, T., & Fernandes, R. (2013). Biochemistry of adipose tissue: an endocrine organ. Archives of Medical Science, 9(2), 191–200. https://doi.org/10.5114/aoms.2013.33181
Denis, G. V., & Hamilton, J. A. (2013). Healthy obese persons: how can they be identified and do metabolic profiles stratify risk?. Current Opinion in Endocrinology, Diabetes, and Obesity, 20(5), 369–376. https://doi.org/10.1097/01.med.0000433058.78485.b3
Engin A. (2017). The definition and prevalence of obesity and metabolic syndrome. Advances in Experimental Medicine and Biology, 960, 1–17. https://doi.org/10.1007/978-3-319-48382-5_1
Horwitz, A., & Birk, R. (2023). Adipose tissue hyperplasia and hypertrophy in common and syndromic obesity-The case of BBS obesity. Nutrients, 15(15), 3445. https://doi.org/10.3390/nu15153445
Karamali, N. S., Ariëns, G. A., Kanhai, H. H., de Groot, C. J., Tamsma, J. T., & Middelkoop, B. J. (2015). Thin-fat insulin-resistant phenotype also present in South Asian neonates born in the Netherlands. Journal of Developmental Origins of Health and Disease, 6(1), 47–52. https://doi.org/10.1017/S204017441400052X
Leow M. K. (2017). Characterization of the Asian phenotype - An emerging paradigm with clinicopathological and human research implications. International Journal of Medical Sciences, 14(7), 639–647. https://doi.org/10.7150/ijms.18880
Małodobra-Mazur, M., Cierzniak, A., Pawełka, D., Kaliszewski, K., Rudnicki, J., & Dobosz, T. (2020). Metabolic differences between subcutaneous and visceral adipocytes differentiated with an excess of saturated and monounsaturated fatty acids. Genes, 11(9), 1092. https://doi.org/10.3390/genes11091092
Miller G. D. (2017). Appetite regulation: Hormones, Peptides, and neurotransmitters and their role in obesity. American Journal of Lifestyle Medicine, 13(6), 586–601. https://doi.org/10.1177/1559827617716376
Mittal B. (2019). Subcutaneous adipose tissue & visceral adipose tissue. The Indian Journal of Medical Research, 149(5), 571–573. https://doi.org/10.4103/ijmr.IJMR_1910_18
Roberts, M. J., Hamrouni, M., Wadley, A. J., & Bishop, N. C. (2023). Adipose inflammation: South Asian ethnicity and central obesity are independently associated with higher immune cell recruitment to adipose-specific media: A pilot study in men. Physiological Reports, 11(22), e15883. https://doi.org/10.14814/phy2.15883
U.S. Department of Health and Human Services. (n.d.). Insulin Resistance & Prediabetes - Niddk. National Institute of Diabetes and Digestive and Kidney Diseases. https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/prediabetes-insulin-resistance
Wu, Z. E., Fraser, K., Kruger, M. C., Sequeira, I. R., Yip, W., Lu, L. W., Plank, L. D., Murphy, R., Cooper, G. J. S., Martin, J. C., & Poppitt, S. D. (2020). Metabolomic signatures for visceral adiposity and dysglycaemia in Asian Chinese and Caucasian European adults: the cross-sectional TOFI_Asia study. Nutrition & Metabolism, 17(1), 95. https://doi.org/10.1186/s12986-020-00518-z
Xu, F., Earp, J. E., Blissmer, B. J., Lofgren, I. E., Delmonico, M. J., & Greene, G. W. (2022). The demographic specific abdominal Fat composition and distribution trends in US adults from 2011 to 2018. International Journal of Environmental Research and Public Health, 19(19), 12103. https://doi.org/10.3390/ijerph191912103
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