Over recent decades, chronic disease has trended upwards, associated with the mismatch between our modern nutritional environment and our paleolithic genome. The beverage industry has exploded in sync with chronic conditions, including bladder symptomatology and obesity(1,2).  At the same time, societal messaging to avoid dehydration and hydrate beyond thirst, contrary to current medical recommendation to drink to thirst, has influenced fluid intake and non-pathological changes to bladder state. As a population subset, postpartum women are especially motivated to maintain best health in the interest of their newborn. Lactating women are exposed to relatively heightened societal messaging and beliefs (logical though unsubstantiated) that there is sizable maternal fluid lost in breastfeeding. Evolutionarily, women only need to drink to thirst to maintain adequate hydration for breastfeeding. The equilibrium in breastfeeding fluid dynamics predicts no difference in voiding frequency in relation to fluid intake when comparing breastfeeding and non breastfeeding women.
For proof of concept, we test in a natural experiment design study that conditions of higher ( breastfeeding women) vs baseline (non-breastfeeding women) social messaging about fluid intake (with baseline messaging still elevated but deemed nearer to evolutionarily evolved fluid needs of the human body) will result in meaningful differences in outcomes of 1) total beverage intake, and 2) number of bladder voids per day.

A natural experiment exemplar proof of concept uses the logic and paradigm of evolutionary medicine to consider the modern beverage culture as a cause of higher frequency void, often associated with overactive bladder prevalence (17-31%, age dependent). We used an existing dataset from the Evaluating Maternal Recovery from Labor and Delivery (EMRLD) study. This longitudinal, cohort study recruited postpartum participants from January 2004 to April 2012  and all participants gave informed consent to be in the parent EMRLD study. This study is a secondary analysis with main outcome measure as void frequency. In this natural experiment, the preventative treatment group is considered to be 39 women who at 8 months postpartum are not breastfeeding and hence not receiving same degree of societal messaging to hyper-hydrate, as compared to 52 breastfeeding women.

The preventative treatment group (non-breastfeeding women) show significantly lower daily beverage intake (62.6 oz.) than the breastfeeding group (77.6 oz.), p = 0.02. Results also show a significantly lower average number of daily voids in the preventative treatment group who have baseline societal messaging compared to the control group with heightened societal messaging (6.3 voids/day versus 7.2 voids/day respectively, p = 0.04). (This secondary analysis as proof of concept did not have a power calculation).

In accordance with evolutionary theory the expectation for small increase in fluid intake in lactating women would be sufficient to maintain the same hydration level, and consequently the same number of daily voids, as non-breastfeeding women. However, in this study, situated within the milieu of the beverage-driven society, the condition of heightened-messaging-condition (breastfeeding women) resulted in significantly higher total daily beverage volume intake than baseline-messaging condition (non-breastfeeding women). We can speculate that the women who were breastfeeding were responding to societal messaging to hyper-hydrate for the purpose of breastfeeding. The compensatory production of breast milk did not account for this fluid change, and women instead experienced increased urinary frequency.

This natural experiment provides first evidence-based proof of concept for the argument that in terms of evolutionary perspective, the new (half century) cultural milieu of the beverage focused society may be driving contemporaneous increase in voiding frequency, and that prevalence and progression of these chronic conditions may lessen without conscious individual effort if restraint is imposed on societal and marketing messaging to drink regardless of thirst.

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