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October 2009

 Asthma risk and waist size in women

Does enlarged waist makes women more vulnerable to succumb to asthma? Some previous studies did find positive statistical link between overweight/obesity and asthma risk. A recent study indicates that even wider waist alone, without overweight factor, increases that risk (Obesity, waist size and prevalence of current asthma in the California Teachers Study cohort, Behren et al, 2009).

Is there plausible explanation for that?

It doesn't seem too hard to come by. The study analyzed data on as many as 88,304 female teachers and school administrators in California, 13% of which were classified as obese (30-40 BMI) and 1.5% extremely obese (BMI over 40). Incidence of adult-onset asthma was 40% higher for overweight woman (25-30 BMI) than for those below 25 BMI, more than doubled for obese, and more than tripled for extremely obese.

Among obese women, the risk was 50% higher for those with waist over 88cm. Even women with BMI below 25, but waist over 88cm had 37% higher asthma incidence. Also, asthma severity was generally higher among obese and overweight women.

The indication that larger waist alone is a risk factor can be fairly reliably seen as a consequence of less than adequate accuracy of BMI (Body Mass Index, expressed as weight in kg per height in meters, as kg/m2) as a measure of overweight/obesity. A light-frame person can have significant fat accumulation around waist, and still pass as "normal", with BMI under 25.

This inadequacy of the criterion applied can distort statistical data, but doesn't work in alleviating abdominal-fat-related asthma risk. As the study author Julie Von Behren states, this fat - also called visceral, or intra-abdominal fat - is biologically "more active" than subcutaneous (beneath the skin) or intramuscular fat, hence more capable of affecting body's biochemistry. It is already positively linked to heart disease, diabetes, stroke and hypertension.

Just what does abdominal fat do biologically?

Turns out, adipose tissue in general - and abdominal fat in particular - is not merely a layer of fat put aside by the body as an energy store and insulator. Apparently, evolution voted for a different model, in which fat tissue is not just a passive presence, but

actively participates in body's metabolism.

Fat tissue consists of several type of cells, the predominant being adipocytes, constantly storing and releasing fat droplets. It contains many small blood vessels, which not only serve for the transit of lipids in and out of the tissue, but also carry adipose hormones - adiponectin (part of glucose regulation and fatty acid catabolism), angiotensin (blood vessel constrictor), plasminogen activator inhibitor 1 (inhibiting breakdown of blood clots), tumor necrosis factor (TNF, pro-inflammatory/immune cells regulating cytokine), interlukine-6 (another immune cell regulating cytokine), leptin (hormone assisting nervous system in regulating food consumption and energy expenditure) and aromatase used for production of estradiol (estrogenic steroid hormone) - out to the rest of the body.

In effect, adipose tissue in general - and visceral fat in particular - are a functioning

endocrine organ with significant role
in regulating body metabolism.

 Accumulation of body fat is likely to cause some form of disturbance in its function, that may result in, or contribute to various body dysfunctions, from insulin resistance to hypertension.

The role of visceral fat in body's inflammatory mechanism and immune system regulation hints at how it could be related to asthma symptoms.

As always, specific health effects of body fat accumulation vary with the complexities of individual biochemistries and exposures. What applies to everyone, however, is that body fat has an active, important role in regulating body's metabolic processes, which evidently become compromised as a result of excessive body fat accumulation.

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