Milk is made, not simply filtered
Breast milk is not blood that has been lightly altered. Cells in the breast take building blocks from the bloodstream and assemble them into milk: they make milk sugar and proteins, and package fats for delivery. The body must continually send those ingredients to the breast as milk is removed and production continues.
That manufacturing work uses energy. So does moving nutrients through the body and maintaining the tissues involved in lactation. The cost accumulates across repeated feedings; it is not one burst of effort at the moment a baby latches.
A daily demand that varies
How much energy milk production requires depends partly on how much milk the body makes. Production generally responds to milk removal: frequent, effective feeding or pumping signals the breasts to keep producing, while less removal tends to reduce the signal over time. Prolactin supports milk production, and oxytocin helps move milk out of the breast. The hormones coordinate the process, but the energy is spent building the milk itself.
The body can meet that demand from food, stored energy, or a combination of both. That is why lactation may affect hunger and why its energy cost is often compared with a substantial bout of exercise, even though no muscles are visibly working. The comparison describes a hidden, repeated metabolic workload—not a workout that can be measured by how tired someone feels. The body is doing ongoing production work between feeds as well as during them.
