Although lactation is accompanied by increased nutrient demands for milk synthesis, many species of bears, true seals, and baleen whales fast for much or all of lactation. Large body mass in these species confers the advantage of greater stores of fat and protein relative to rates of milk production. Given the constraints on substrate availability during fasting, the milks of fasting mammals are predicted to be low in carbohydrate, protein, and water and to be high in fat. The milks of bears, true seals, and baleen whales conform to this prediction. Mammals that lactate while fasting may lose up to 40% of initial BW. The production of milk entails the export of up to one-third of body fat and 15% of body protein in the dormant black bear and in several seal species, which greatly depletes maternal resources and may represent a physiological threshold, because higher protein and fat outputs have only been measured in species that start feeding. The low K:Na ratio of seal and whale milks and the low Ca:casein and inverse Ca:P ratios in seal milks are unusual and warrant further study.