Durk Pearson & Sandy Shaw’s®
Life Extension NewsTM
Volume 15 No. 6 • October 2012


Measuring Adiposity: Body Mass Index, Percent Body Fat, and Leptin

A new paper1 provides very useful information on the best lab test for adiposity (fatness). The researchers explain that BMI (body mass index = weight in pounds/(height in inches)2 x 703), the most commonly used measure for fatness, underestimates body fat, especially in women with high levels of the hormone leptin (>30 ng/ml). However, BMI is popular because of its convenience, safety, and minimal cost, and thus is widely used, despite not being able to distinguish lean body mass from fat mass.1 The BMI, developed nearly 200 years ago, is not actually a measure of adiposity, but an imprecise mathematical estimate.1

The authors explain: “BMI ignores several important factors affecting adiposity. Greater loss of muscle mass leading to sarcopenic obesity in women occurs increasingly with age. BMI does not acknowledge this factor, exacerbating misclassifications. Furthermore, men’s BMI also does not consider the inverse relationship between muscular strength and mortality. It fails to take into account that men lose less muscle with age than women.”1

The researchers explain further that DXA (duel-energy x-ray absorptiometry) is a direct measure of body fat, but is not a disease correlate. Thus, scientists have investigated various factors that might explain disparities between BMI and direct fat measurements, factors that include leptin, insulin, ghrelin, and adiponectin.1 “Leptin, a 16kDa peptide secreted primarily by adipocytes [fat cells], regulates the body’s energy balance by acting as a negative feedback adiposity signal, decreasing food intake and increasing energy expenditure.”1 Unfortunately, obese individuals have high leptin levels but are leptin insensitive with the result that the normal leptin signaling is blunted. Leptin insensitivity is associated with a state of “obesity with normal weight” (people with normal weight but with metabolic defects usually associated with obesity that includes chronic inflammation, type II diabetes, hypertension, and myocardial injury).1

A Strong Relationship Between Increased Leptin and Increased Body Fat

The bottom line is that a better measure of body fatness (adiposity) is needed to predict the risk of obesity-associated medical conditions and to help guide the result of therapies used to reduce obesity-associated disease risks.

“Since a recent study showed that the significant lowering of leptin impacts long term weight control, the idea of utilizing leptin as a component in the national* attack on obesity might be considered.” “… lowering elevated leptin has been associated with improved obesity and clinical outcomes.”1


* We are not fans of a “national” attack on obesity (if that means government programs) but are interested here in the use of leptin measurements as part of a physician/patient attack on obesity. Indeed, there is NO power authorized in the Constitution for a “national” attack on obesity (take a look at your copy) so that any attempt to carry out programs in pursuit of this goal would be unconstitutional.


The authors studied the relationship between BMI, percent body fat (as measured by DXA), and leptin in a population of 1,393 adult patients (63% of which were women, 37% were men). They found that “while there was agreement [between obesity based on BMI versus percent body fat] for 60% of the sample, 39% were misclassified as non-obese based on BMI, while meeting obesity criteria based on percent body fat … A total of 48% of women were misclassified as non-obese by BMI, but were found to be obese by percent body fat. In sharp contrast, 25% of men were misclassified as obese by BMI, but were in fact non-obese by percent body fat (i.e. the muscular body morphology).”1 They also found that 91% of their patients with high leptin levels were women.

Where You Can Get Your Leptin Level Measured

Before we wrote this article, we investigated where it would be possible to get commercial lab tests for leptin, as it would clearly be helpful in evaluating one’s adiposity. Here’s what we found out. Leptin tests:

http://www.econolabs.com/product_p/lc-146712.htm

http://www.labsmd.com/view_all_blood_tests.php

http://www.walkinlab.com/leptin_serum_test.html?category_id=1&search_string=leptin&search_category_id=1

Note: These tests require a venous blood sample.

Reference

  1. Shah and Braverman. Measuring adiposity in patients: the utility of body mass index (BMI), percent body fat, and leptin,” PLoS ONE 7(4):e33308 (Apr. 2012); www.plosone.org

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