In this preliminary prospective study, breast milk is sampled surrounding 4 religious fast days to determine the effect of a more than 24-hour fast on breast milk composition. The participants are 48 healthy women nursing healthy babies between 1 and 6 months of age. Samples are collected within 2 days before the fast (baseline), immediately after the fast, and 24 hours after fast completion. Samples are tested for sodium, calcium, phosphorus, triglycerides, total protein, and lactose. From baseline to immediately after fast, mean sodium, calcium, and protein levels increase (P = .013, P < .0001, and P < .0001, respectively) and mean phosphorus and lactose levels decrease (P < .0001 and P = .003, respectively). Mean triglycerides are unchanged. Twenty-four hours after fast, parameters are no longer significantly different from baseline except for elevated mean protein levels (P = .022) and lactose that is still reduced (P = .017). A fast of this nature is statistically associated with certain biochemical changes in breast milk.