Understanding body composition becomes increasingly vital as we age, with the risk of metabolic disorders and obesity-related complications rising over time.

As we delve into this topic, we’ll explore the relevance and utility of Body Mass Index (BMI), waist circumference, and waist-to-hip ratio — three key metrics that can offer insights into one’s health from the standpoint of longevity. Each of these indicators not only provides valuable information about weight and fat distribution but also hints at the risk factors for chronic diseases that may impact one’s lifespan and quality of life.

Body Mass Index (BMI)

How BMI is Measured.
BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters. The formula is straightforward and quick, making it a convenient measure for everyday use.

BMI Ranges.

• Underweight: Less than 18.5
• Normal weight: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese: 30 and above

Associations with Health.
BMI has been correlated with overall health risks. A higher BMI is associated with an increased risk for heart disease, diabetes, and certain cancers, while a BMI in the normal range is linked to a lower risk for these conditions.¹ Zhao, Yang et al. “Association of BMI with cardiovascular disease incidence and mortality in patients with type 2 diabetes mellitus: A systematic review and dose-response meta-analysis of cohort studies.” Nutrition, metabolism, and cardiovascular diseases : NMCD vol. 31,7 (2021): 1976-1984. doi:10.1016/j.numecd.2021.03.003 PubMed Source

Advantages.
BMI is easy to measure and is a good general indicator of disease risk for populations.

Disadvantages.
It doesn’t distinguish between muscle and fat mass and can misclassify those with significant muscle mass as overweight or obese. It also doesn’t take into account fat distribution.² Rothman, K J. “BMI-related errors in the measurement of obesity.” International journal of obesity (2005) vol. 32 Suppl 3 (2008): S56-9. doi:10.1038/ijo.2008.87 PubMed Source

In the next sections, we’ll compare BMI to other measures like waist circumference and waist-to-hip ratio to help you gain a comprehensive view of your Body Composition and its implications for your longevity journey.

How Waist Circumference is Measured.
Waist circumference is measured at the midpoint between the lower margin of the last palpable rib and the top of the iliac crest, using a stretch-resistant tape that provides constant tension.

Waist Circumference Ranges.
The risk of cardiovascular diseases and diabetes increases with a waist circumference greater than 94 cm (37 inches) for men and 80cm (31.5 inches) for women.³ Darsini, Darsini et al. “Health risks associated with high waist circumference: A systematic review.” Journal of public health research vol. 9,2 1811. 2 Jul. 2020, doi:10.4081/jphr.2020.1811 PubMed Source

Associations with Health.
A larger waist circumference can indicate a higher risk of heart disease, type 2 diabetes, and overall mortality. It reflects the amount of abdominal fat, which is a significant predictor of health risk.⁴ Siren, Reijo et al. “Waist circumference a good indicator of future risk for type 2 diabetes and cardiovascular disease.” BMC public health vol. 12 631. 9 Aug. 2012, doi:10.1186/1471-2458-12-631 PubMed Source

Advantages.
Waist circumference is increasingly recognized as a key indicator of health because it provides a direct estimate of abdominal fat, which includes both subcutaneous fat (found just under the skin) and visceral fat (found around internal organs). The latter is particularly associated with health risks. Visceral fat is metabolically active, meaning it can produce hormones and other substances that influence bodily functions.

High levels of visceral fat are linked to insulin resistance, diabetes, lipid disorders, and increased risk for heart disease. Unlike BMI, waist circumference can give a clearer picture of fat distribution and potential health risks, especially for individuals who are categorized as having a normal BMI but may have excess abdominal fat—a condition often referred to as “normal weight obesity” or “skinny fat”.

Disadvantages.
However, waist circumference measurements can be less consistent, as they may vary depending on the time of day, the tightness of the tape measure, or the technique of the person doing the measuring. Even minor variations in how the tape measure is applied can lead to different readings, which could potentially misclassify someone’s risk category.

Furthermore, while waist circumference is a good indicator of abdominal fat, it does not take into account a person’s overall body size or variations in height. Therefore, two individuals with the same waist circumference but different heights may have different health risks. This is why waist circumference is often used in conjunction with other measurements, like BMI, rather than as a standalone indicator.

How Waist-to-Hip Ratio is Measured.
The waist-to-hip ratio is measured by dividing the waist circumference by the hip circumference. The waist is measured at the narrowest point, while the hips are measured at the widest. When taking the measurements for the waist-to-hip ratio, you can use either centimeters or inches, as long as you use the same unit of measurement for both waist and hip measurements. The key is consistency in units.

Waist-to-Hip Ratio Ranges.

• Low risk: less than 0.90 for men, and less than 0.85 for women
• Moderate risk: 0.90 to 0.99 for men, and 0.85 to 0.95 for women
• High risk: 1.0 or higher for men, and 0.95 or higher for women

Associations with Health.
According to a recent study, waist-to-hip ratio may be a better tool for determining body composition and weight management than BMI.⁵ Khan, Irfan et al. “Surrogate Adiposity Markers and Mortality.” JAMA network open vol. 6,9 e2334836. 5 Sep. 2023, doi:10.1001/jamanetworkopen.2023.34836 PubMed Source

A higher waist-to-hip ratio suggests a higher concentration of visceral fat, which is associated with an increased risk of metabolic syndrome, heart disease, and type 2 diabetes.⁶ Czernichow, S et al. “Body mass index, waist circumference and waist-hip ratio: which is the better discriminator of cardiovascular disease mortality risk?: evidence from an individual-participant meta-analysis of 82 864 participants from nine cohort studies.” Obesity reviews : an official journal of the International Association for the Study of Obesity vol. 12,9 (2011): 680-7. doi:10.1111/j.1467-789X.2011.00879.x PubMed Source

Advantages.
The waist-to-hip ratio is a valuable measure because it considers the distribution of body fat, which is crucial in evaluating health risks. This ratio helps to differentiate between individuals with a similar BMI or waist circumference but different fat distributions.

For instance, individuals with a higher concentration of fat around the abdomen (‘apple-shaped’ bodies) are at a greater risk of metabolic syndromes, such as cardiovascular disease and type 2 diabetes, compared to those who carry more fat around the hips and thighs (‘pear-shaped’ bodies). This is because abdominal fat, particularly visceral fat, is more metabolically active, meaning it can more significantly affect insulin resistance and cholesterol levels.

Disadvantages.
However, there are several drawbacks to relying on the waist-to-hip ratio. Measuring the waist and hips accurately can be challenging and may result in inconsistencies. The waist must be measured at its narrowest point, and the hips at its widest, which can be difficult to determine and may vary between measurements or measurements. This variability can affect the reliability of the ratio and, consequently, the assessment of health risk.

Furthermore, the waist-to-hip ratio might not be as sensitive to changes over time as other measures. For instance, if both the waist and hip circumferences increase or decrease by the same proportion, the ratio will remain the same, potentially masking changes in body composition, such as an increase in both muscle and fat. It is also important to note that the waist-to-hip ratio does not account for changes in muscle mass, which can be a limitation in assessing overall health and fitness.

Conclusion

Ultimately, the “best” measure depends on the context and the individual. For a comprehensive assessment of health risks related to body composition, it’s advantageous to consider all three measures in conjunction with one another, rather than relying on a single metric. This multifaceted approach can provide a more complete picture of an individual’s health and help guide lifestyle modifications geared towards improving longevity. Regular monitoring can also track changes over time, offering a dynamic view of one’s health trajectory.