Why the Pill is Less Effective
Unfortunately, the exact reason as to why overweight and/or obese women are at greater risk of oral contraceptive failure is not completely known. However, several proposed theories point to biological factors that may account for increased risk:
• Hormone Levels: Modern-day birth control pills contain relatively low hormone levels (as compared to those first introduced decades ago). Oral contraceptive manufacturers have decreased hormone levels in an attempt to minimize unwanted side effects, such as risk of blood clots, weight gain, and headaches. In order to be effective, the hormones in the pill need to circulate through a woman’s bloodstream. If a woman has a larger body mass, it could be more difficult for adequate circulation to happen, especially given lower level of hormones found in most of today’s pills.
• Metabolism: Typically, heavier women have a higher metabolism, so the pill hormones may be metabolized faster. So, the more a woman weighs, the higher her basal metabolic rate; this can shorten the duration of the pill’s effectiveness.
• Liver Enzymes: overweight women may experience an up-regulation of liver metabolism; these higher levels of enzymes may help break down the hormones in oral contraceptives. Heavier women tend to have a greater circulating blood volume and body mass. Given that there is more tissue through which blood must circulate and the likelihood that the enzymes will break down the hormones faster, the levels of circulating hormones may be decreased.
• Hormone Storage: the hormones, estrogen and progestin, found in birth control pills are stored in body fat. Thus, the more fat cells a woman has, the greater her chances of the pill hormones becoming trapped in the fat instead of flowing through her bloodstream.
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