The research, published in Physiological Reports, examined 49 healthy females aged 18 to 45 years, dividing them into three groups: those with natural menstrual cycles, those taking second-generation combined oral contraceptive pills, and those on third-generation formulations. The findings were striking. Fasting glucose concentrations were approximately 10 percent higher in second-generation pill users and roughly 12.5 percent higher in third-generation users compared to naturally cycling women. Even more dramatic, fasting triglycerides were elevated by 56 to 70 percent in pill users relative to their naturally cycling counterparts. These are not trivial differences for a population of young, healthy, normal-weight women.

What makes this study particularly valuable is the rigor with which the researchers controlled for variables that have plagued previous investigations. Earlier studies on oral contraceptives and metabolism have produced wildly inconsistent results—some reporting benefits, others harm, and still others no effect at all. The McMaster team identified several reasons for this confusion. Many prior studies failed to account for the phase of the pill cycle or menstrual cycle at which blood samples were drawn. Others compared short-term users to long-term users without accounting for the body’s adaptation over time. And critically, several lacked a proper control group of naturally cycling individuals, making it impossible to disentangle the effects of the pill from natural hormonal variation.

To address these methodological gaps, the researchers implemented what they describe as rigorous temporal controls. Participants attended two laboratory visits timed to coincide with their low-hormone and high-hormone phases. For naturally cycling women, this meant the early follicular phase (days 3 to 7) and the mid-luteal phase (approximately 7 days after ovulation, confirmed with at-home ovulation kits). For pill users, visits were scheduled during the placebo week and during the third week of active pill use—the highest hormone week for those on triphasic formulations. Blood samples were collected in a fasted state at consistent times of day, and all participants underwent dual-energy X-ray absorptiometry (DXA) scans to assess body composition with precision.

The results extended beyond glucose and triglycerides. The researchers also found that appendicular lean mass index—a measure of skeletal muscle mass in the arms and legs normalized to height—was approximately 10 percent lower in both pill-user groups compared to naturally cycling women. However, when lean mass was expressed as a percentage of total body mass rather than height, this difference disappeared, suggesting that the naturally cycling group may simply have had larger and leaner lower bodies rather than a fundamentally different muscle distribution. Leg lean mass also trended toward being lower in pill users, though this did not reach statistical significance.

One of the most intriguing findings emerged from an exploratory sub-analysis. When all participants were pooled into a single group, the researchers observed a moderate negative correlation between appendicular lean mass index and fasting glucose concentrations—but only during the high-hormone phase. This suggests that skeletal muscle mass may play a more active role in glucose regulation during certain hormonal contexts than previously appreciated. Skeletal muscle is responsible for as much as 75 percent of glucose uptake from the bloodstream following a carbohydrate-containing meal, making it a critical player in metabolic health. Previous research has shown that lower muscle mass predicts impaired fasting glucose in elderly women, and the current findings hint that this relationship may extend to younger populations under specific hormonal conditions.

The mechanisms behind these metabolic differences likely involve the synthetic estrogen ethinyl estradiol, which is contained in both second- and third-generation oral contraceptives. Endogenous 17β estradiol—the naturally produced form of estrogen—plays a beneficial role in energy metabolism. It facilitates insulin secretion, stimulates glucose uptake from the bloodstream, reduces hepatic glucose release, promotes the shift from lipid storage to lipid oxidation, and lowers circulating triglyceride concentrations. Crucially, chronic exposure to ethinyl estradiol dramatically suppresses the body’s own estrogen production. The researchers previously confirmed that circulating 17β estradiol concentrations were significantly lower in both pill-user groups compared to naturally cycling women during the high-hormone phase. It is therefore plausible that pill users simply had less natural estrogen available to perform these metabolic protective functions.

The divergence between pill generations adds another layer of complexity. Third-generation users had approximately 40 percent higher HDL cholesterol concentrations compared to second-generation users, and there was a trend toward a lower total cholesterol to HDL ratio in the third-generation group. This difference likely relates to the type of progestin used in each formulation. Second-generation pills contain androgenic progestins that may antagonize the lipid effects of ethinyl estradiol, while the less androgenic progestins in third-generation pills may instead enhance them. This finding underscores the importance of distinguishing not just between pill users and non-users, but between different generations and formulations of the pill itself.

Importantly, the researchers caution that the magnitude of these differences, while statistically significant, remains modest in absolute terms. Average fasting glucose in all three groups remained well below the clinical threshold for concern, and only one participant across the entire study met the criteria for metabolic syndrome. For young, healthy, normal-weight women, the pill is unlikely to meaningfully increase the risk of developing metabolic disease. However, the researchers note that these subtle shifts could become clinically relevant for individuals with preexisting risk factors. A woman whose fasting glucose sits at 5.1 millimoles per liter before starting the pill, for example, could cross the threshold into the elevated range with the kind of increase observed in this study.

As metabolic syndrome rates continue to rise worldwide—particularly among women aged 18 to 45, the demographic most likely to use oral contraceptives—these findings carry significant public health implications. They also suggest that researchers may need to reconsider how they design studies involving female participants. Rather than excluding pill users to control for hormonal variability, the authors propose that expanding eligibility criteria to include both naturally cycling women and pill users could actually increase female participation in physiological research while improving the relevance and generalizability of the results. Future work, they note, should also examine how newer long-acting hormonal contraceptives, such as hormonal intrauterine devices, influence metabolism and body composition, as their popularity has surged over the past decade.

Subject of Research: The effects of combined oral contraceptive pills on fasting glucose, lipid metabolism, and body composition in young healthy females

Article Title: Elevated fasting glucose and triglycerides in combined oral contraceptive users compared to naturally cycling females

Article References: Karaguesian, C., Williams, J. S., MacDonald, M. J., & Bell, K. E. (2026). Elevated fasting glucose and triglycerides in combined oral contraceptive users compared to naturally cycling females. Physiological Reports, 14(19), Article e71135. https://doi.org/10.14814/phy2.71135

Image Credits: AI Generated

DOI: 10.14814/phy2.71135

Keywords: oral contraceptives, fasting glucose, triglycerides, metabolic syndrome, estrogen, body composition, HDL cholesterol, insulin resistance, menstrual cycle, women's health, DXA, skeletal muscle