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Background
Epigenetic adaptations around conception can help organisms adjust to their future environment. Pre-conception cold exposure is thought to increase active brown fat mass, and as brown fat metabolizes stored energy to generate heat, this helps adjust to life in cold environments. We examine the implications of this process for human metabolic health.

Methods
We use data on 437,504 individuals born between 1934 and 1971 from the UK Biobank, and match these to historical temperature data. To isolate causal impacts of temperature, we utilize day-specific temperature deviations during the calculated pre-conception period relative to the long-term mean temperature for the same location and day of year. This approach leverages a quasi-random variation in temperature. Associations between pre-conception temperature exposure and adult metabolic health were estimated using regression models adjusted for relevant covariates.

Results
Individuals conceived when temperatures were lower than usual have lower body mass indices, smaller waist circumferences, and lower levels of triglycerides and total cholesterol in adulthood, while the effect on glycated hemoglobin appeared to be less strong.

Conclusions
Our study indicates that pre-conception environmental conditions can influence human metabolic health, potentially through epigenetic mechanisms linked to brown fat activity. These findings have implications for potential health effects of climate change, and, more strongly, of improved indoor insulation.