Researchers have identified a potential biological mechanism that could explain why oral contraceptives may be less effective at preventing pregnancy in obese women, as some epidemiological studies have indicated.

    Although conventional oral contraceptives appear to eventually reach the effective blood concentrations needed in the body to prevent conception in obese women, it appears to take twice as long, leaving a "window of opportunity" every month where the contraceptive may not be at a high enough level to prevent a pregnancy.

    The findings are of particular importance, researchers noted in their study, because about 30 percent of all adults in the U.S. are obese and the birth control pill is one of the most popular forms of contraception in the nation.

    "We don't have enough data yet to recommend that physicians change their clinical practice for use of oral contraceptives with patients who are very overweight," said Ganesh Cherala, an assistant professor in the College of Pharmacy at Oregon State University. "However, until more studies are done, women may wish to consult with their physicians about this issue and consider a backup method of contraception at some times of the month."

    The study was just published in the journal Contraception, by scientists from OSU, Oregon Health and Science University, University of Colorado at Denver, Oregon National Primate Research Center, and the University of Southern California. The research was supported by the National Institutes of Health.

    The underlying problem, Cherala said, is that oral contraceptives, like most drugs, are initially tested in "healthy" people, which rarely includes people who are more than 130 percent of their ideal body weight.

    "When we first test drugs for safety and efficacy, we generally do not include people with a high body mass index," Cherala said. "But body weight and amounts of fat can seriously change the pharmacokinetics, or way drugs act and are processed in the body. There's a growing awareness that we need to more carefully consider obesity and other factors that affect drug absorption, distribution, metabolism and other factors."

    Conventional oral contraceptives, Cherala said, are thought to be relatively "lipophilic," or tend to concentrate in fat tissue. However, the researchers in this study said they were somewhat surprised to find that the affinity of these drugs for fat tissue was not significantly different between obese and normal body weight subjects.

    Rather, the researchers found that contraceptive drug levels in both obese women and those of normal weight eventually were about the same, but it took longer to achieve that level in very overweight women.

    The study showed it took an average of about five days for the drugs to achieve their maximum concentration in women of normal weight, an average of 10 days for obese women, and even longer than that for some individuals. One woman in the study took more than 20 days to reach a "steady state" drug concentration. Women of normal weight who follow their oral contraceptive directions should have appropriate protection against pregnancy. But the delay in reaching a steady state drug concentration raises questions about how well oral contraceptives may work for obese women.

    Increasing the drug dosage might help address this issue, Cherala said, but also adds other health concerns.

    In fact, the researchers noted in their report that many clinicians actually prescribe lower-dose oral contraceptives to obese patients in an effort to decrease their risk of venous thrombosis. These are blood clots in the legs or elsewhere that can increase the risk of stroke and heart attacks.

    The study was done with 20 women of ages 18 to 35, all of them healthy and seeking contraception, 10 of whom were of normal weight and 10 with a "body mass index" of more than 30 – a common measure of obesity.

    According to Dr. Alison Edelman, lead author of the study and assistant director of the Family Planning Fellowship at Oregon Health and Science University, the participants in this study were purposely selected for obesity in order to explore this issue. But several demographic studies have shown that even women just considered "overweight," with a body mass index of 25-30, may also be at increased risk of contraceptive failure.

    "Although our research has found this interaction between obese women and oral contraceptives, we don't have enough information yet to recommend changes in clinical practice, other than choosing a contraceptive option that works better for both normal weight and obese women, like an intrauterine device," Edelman said.

    For future work, she said, studies of contraception would be more useful if they included participants that reflect the general population, including women with different body mass indexes. The biological underpinnings of how oral contraceptives work, their effects on the hypothalamic-pituitary-ovarian axis, has only been studied in women of normal weight, the researchers noted in their study.

    At present, Cherala said, there is no readily available test that would tell a woman how long it would take for her to reach an effective concentration level of a particular contraceptive, and this does vary with the individual. However, scientists are continuing research on that issue, and they may ultimately develop tests or methods that would improve drug efficacy for women who wish to use oral contraceptives.

    研究人員發現了一種導致肥胖女性服用口服避孕藥失效的生物機制，以解釋流行病學研究發現的上述現象。

    雖然傳統的口服避孕藥可以幫助肥胖女性達到有效的血液濃度以防止懷孕，其所需的時間卻是一般人的兩倍。因此每個月就存在一定的漏洞時期，肥胖女性體內的口服避孕藥濃度不夠而導致懷孕。

    上述發現異常重要，研究者在其報告中表示，因為美國大約30%的成年人為肥胖癥患者，而口服避孕藥是最為普遍的避孕方法。

    "現階段我們還沒有足夠的數據支持來建議醫生改變對其肥胖病人使用口服避孕藥的臨床實踐，"Ganesh Cherala,俄勒岡州立大學藥學院的副教授說。"然而，在我們取得更多數據以前，那些需要進行避孕的肥胖女性，可以咨詢她們的醫生在每個月的一些時候采取一些備份的避孕方法。"

    這項研究結果刊登在避孕雜志中，由俄勒岡州立大學、美國俄勒岡保健科學大學、丹佛科羅拉多大學、俄勒岡國家靈長類動物研究中心和南加州大學的科學家共同發布，并由國家衛生院資助。

    問題的根本在于，Cherala說，如其他藥物一樣，對口服避孕藥的最初檢測對象是健康人群，基本沒有包括那些超重30%的人。

    "在首次進行藥物安全和有效性檢測時，我們一般部考慮那些身體質量指數高的人群，"Cherala說，"但是體重和脂肪的數量會嚴重影響藥代動力，也就是藥物在體內的作用效果以及起效過程。人們越來越多的意識到，其應該認真研究肥胖以及其他因素對藥物的吸收、分布、代謝和其他方面的影響。"

    Cherala說，傳統的口服避孕藥相對而言具有嗜油脂性，也就是說會在脂肪處聚集。然而，令本次研究人員感到意外的是，這些藥物的嗜油脂性在肥胖以及一般被試中沒有明顯區別。

    相反，研究人員發現，口服避孕藥的最終濃度在肥胖和正常體重的被試中是相同的，但是超重女性達到最終濃度的時間較長。

    研究發現，正常體重的女性，口服避孕藥發揮藥效需要大概5天，超重女性大概是10天，而一些特殊人群則需要更長的時間。一個被試女性用了超過20天才達到避孕所需的穩定濃度。正常體重的女性使用口服避孕藥就能夠達到避孕效果，但這些藥在肥胖女性體內達到穩定濃度所需的較長時間，讓人們不得不考慮它們的有效性。

    增加藥量金額以解決這個問題，Cherala說，然而這將會產生其他健康問題。

    事實上，根據研究報道，許多臨床醫生通常減少肥胖女性的口服避孕藥劑量以降低靜脈血栓（在大腿或其他部位的可以增加中風和心臟病發作風險的血塊）形成的風險。

    研究的被試由20名健康并需要希望采取避孕措施的女性組成，年齡為18~35歲。其中10人體重正常，另外10人的身體質量指數超過30（肥胖癥的一般標準）.

    Alison Edelman醫生，本研究的骨干，俄勒岡保健科學大學計劃生育協會的副主席表示，本次實驗的被試是特別選定的。然而一些人口研究的結果顯示，那些身體質量指數超過25,不到30的女性也存在不斷增加的避孕風險。

    "雖然，我們研究了肥胖女性和口服避孕藥之間存在的關系，我們還沒有足夠的數據支持來做出臨床醫學改革建議。我們現在只能建議那些對于正常和超重女性都較為有效的避孕方法，比如子宮環，"Edelman說。

    對于未來的研究工作，她說，最好是選取代表性好的樣本，能夠包括不同身體質量指數的女性。研究報告中顯示，對于口服避孕藥起作用的生物學基礎以及其對下丘腦-垂體-卵巢的影響，我們只是針對正常體重的女性進行了研究。

    目前為止，還沒有現成的實驗數據顯示口服避孕藥再女性體內起效的具體時間，而這個時間也因人而異。然而，科學家正在對此進行深入研究，希望有朝一日他們能發現改善口服避孕藥效果的實驗或方法，給那些希望使用者帶來福音。