After discussing the social media story on antibiotics earlier, I now want to move on to another component that is often thought to be present in milk at harmful levels: hormones.
First of all, milk does contain hormones. All animals (and humans, if you want to see that as a separate category) make hormones and they are present in all our tissues and secretions. So all our animal-derived food (and also breast milk) contains hormones. For milk, the interest is in two groups of hormones: growth hormones and sex hormones.
When it comes to growth hormones, it depends on where you live. In Europe there is a general ban on growth hormones (BST, IGF-1) in animal husbandry. Still, low levels due to natural presence of these hormones occurs in milk. In other parts of the world, they can be used in animal husbandry, leading to these hormones being present at higher levels in the milk as well.
When it comes to sex hormones, these occur in milk, because they are naturally secreted. In dairy animals, there levels fluctuate over lactation, especially due to most cows being pregnant during lactation. In late lactation, sex hormone levels are therefore somewhat higher.
What happens to these hormones during processing? Research shows that the hormones stay largely intact, except for BST that is broken down during regular dairy processing. There is no difference in hormone levels between regular and organic milk (assuming both did not inject hormones in their dairy animals). For comparison, breast milk contains 10-20 times higher levels of sex hormones than bovine milk.
After processing, we consume the milk. So does this cause any harm? If we look at growth hormones, BST is naturally broken down in our digestive tract. IGF-1 is not broken down, but levels in milk are lower than normally occurring levels of IGF-1 in our body. Sex hormones are also not broken down during digestion. But if we look at sex hormone intake through milk (and animal products in general), they are lower than our daily maximum intake, even at high consumption levels (liters per day). So, in conclusion, although milk (and animal products in general) contribute to dietary hormone intake, it is at such low levels that health effects are not to be expected.
Kasper Hettinga, December 2016