Five reasons milk isn’t healthy for adult cows

  1. Cows are not biologically designed to drink milk after weaning
    Like all mammals, calves produce the enzyme lactase to digest lactose — but after weaning, enzyme activity declines sharply. Adult cows, like adult humans without lactase persistence genes, can experience bloating, diarrhea, and gut dysbiosis from milk sugar.

    • Reference: Lactase activity decline in bovines and other mammals – Journal of Dairy Science, 1999 (doi.org/10.3168/jds.S0022-0302(99)75292-3)

  2. Excessive calcium and protein intake would cause metabolic imbalance
    Adult cows on balanced feed already get optimal calcium–phosphorus ratios. Milk adds excessive calcium and casein protein, which can interfere with phosphorus absorption, causing metabolic alkalosis and kidney stress.

    • Reference: Goff, J.P. “Mineral disorders of the transition cow.” Veterinary Clinics of North America: Food Animal Practice, 2004.

  3. Milk elevates insulin and IGF-1, disrupting energy metabolism
    Cow milk naturally contains growth-promoting hormones (IGF-1, insulin, estrogens). For adult cows, ingesting milk can cause metabolic confusion, promoting fat deposition and insulin resistance.

    • Reference: Collier et al., Journal of Dairy Science, 2017 (IGF-1 and bovine metabolism link)

  4. Fat and casein overload can harm rumen function
    The adult bovine digestive system is built for fiber fermentation, not liquid dairy fat and casein. Drinking milk bypasses normal cud processing, which can lead to ruminal acidosis and microbial imbalance.

    • Reference: Nagaraja & Titgemeyer, “Ruminal acidosis in dairy cattle,” Veterinary Clinics of North America: Food Animal Practice, 2007.

  5. No evolutionary or behavioral precedent
    Cows, in nature, do not seek out or drink milk after weaning. When offered milk, adults often refuse or develop gastrointestinal distress — much like lactose-intolerant humans. Evolutionarily, milk is a growth fluid for infants, not a maintenance food.

    • Reference: Haenlein, G.F.W. “Relationship of milk consumption to adult health,” Small Ruminant Research, 2004.

Then how could it be good for humans?

Humans are the only species that drinks milk after infancy — and from another species. While some populations evolved lactase persistence, that doesn’t necessarily mean it’s healthy; it just means they can digest it.

Studies that question milk’s benefit for humans:

  • Fraser, G. (2020). “Dairy milk and risk of breast cancer.” International Journal of Epidemiology, 49(5):1537–1549.

  • Willett, W.C. (2018). “Dietary fats and risk of chronic disease.” American Journal of Clinical Nutrition. (Notes saturated fat and IGF-1 issues).

  • Harvard School of Public Health: “Calcium and Milk: What’s Best for Your Bones?” — finds no clear evidence that milk protects adult bone health. (Harvard.edu link)

In short:
If adult cows can’t healthily drink their own milk — a fluid meant for infants and full of growth hormones, lactose, and high calcium — it’s worth asking:

“If milk isn’t healthy for the species that makes it, why assume it’s ideal for us?”