The Food Magazine - Click to return to the home page

Milk – with extra oestrogen?

28th October 2005

Two-thirds of our milk supply comes from pregnant cows, but we don’t know what that means for our health. Tim Lobstein continues investigating the impact of modern farming on the quality of the food we buy.

The UK population guzzles milk in nearly the same quantities as our cars guzzle petrol. On average, every household buys about five litres of milk every week. And that is just the domestic scene: more milk is drunk at school, at the fast food restaurant, in canteens and offices, while travelling and even in the hospitality zone of the Food Commission’s local bank.

And if you add in the cheese, yogurt, crème fraiche and occasional Ben & Jerry’s ice cream, the total comes to the equivalent of 4.4 litres a week for every member of the population, about a pint a day. (Since you ask, the UK’s petrol consumption for cars is about 6 litres a week per person).

The figures also show that the UK imports the equivalent of 3 billion litres of milk each year, and exports 2.5 billion litres – quite crazy, but that’s another story.

There are many aspects of milk that have concerned consumers and nutritionists over the years, of which the most worrying for health is probably the fatty acid profile, especially the high levels of myristic acid – a saturated fatty acid which is particularly potent at stimulating a rise in LDL blood cholesterol, the form which is a major risk factor in cardiovascular disease.

While attempts have been made to lower the fat content of milk, and while most consumers have been making significant changes to their diets by replacing full-fat milk with reduced-fat versions, other concerns remain. Among these is the problem of hormones.

Nearly two decades ago, the government started allowing trials of bovine somatotropin (BST), a synthetic growth hormone produced using genetic engineering and designed to increase the efficiency with which cattle turned their feed into milk.

The hormone worked only too well, with udders full to bursting, putting immense stress on the cow with consequent increases in the incidence of mastitis (infection of the udder) and other ailments such as lameness. The treatment significantly raised the levels of a hormone, insulin-like growth factor-1 (IGF1), in the milk, to levels which contributed to the European Union’s decision to ban the use of BST for commercial milk production.

The point of this story is that it opened up a new area of concern for consumers, namely the hormone content of milk supplies. And yet, surprisingly, very little research has been published on the topic. And, even more to the point, there is even less information about how changes in modern farming practices may influence the quantities and types of hormones found in milk.

What we do know

The average lifespan of modern dairy cows is only about five years. In normal circumstances they can live to an age of 25-30 years, but physiological stress and a farmer’s sharp eye for a cow whose yield is declining, means that the animals are turned into meat and bone meal in just a few years – equivalent to a teenager in human terms.

GraphModern dairy farmers hope their cows will to produce over 6,000 litres of milk during their annual 10 month lactation, with some prize cows producing 12,000 litres, equivalent to more than 40 litres (70 pints) a day during peak production. The graph (opposite) shows the average for the UK, which has risen from 16 pints per day to 39 pints per day in just a few cattle-generations.

A dairy cow raised by industrial farming methods is expected to give birth to a calf at least every 12 months, to keep her producing milk. A pregnancy is nine months, so cows are simultaneously lactating and pregnant for a significant part of the cycle. After calving the cow is ‘open’ and available for a new conception, which will usually take place within three months (a cow’s menstrual cycle is 21 days).

About two months before she gives birth, the milking is stopped and she is ‘dried off’. Within days of giving birth the milking routine starts again. Within three months of giving birth, the cow is made pregnant again. As the diagram of the cow’s year shows (below), the cow is pregnant for about seven out of ten months lactation, from which we can deduce that at least two thirds of our milk is extracted from pregnant cows.


In order to maximise yields still further, a farmer may be tempted to shorten the open period and the drying off period, so that there are fewer months when the cow is not lactating. The effect of this would be to increase the proportion of time that the milk is being collected while the cow is pregnant.

Taking milk while a cow is pregnant, and especially during the last few weeks of her pregnancy, raises questions about hormones in milk.

During pregnancy, the cow’s ovaries secrete high levels of progesterone. Her placenta secretes high levels of oestrogen. These hormones, plus others, including corticosteroids, growth hormones and prolactin, target the mammary gland to stimulate lactation.

GraphThe graph on the right shows the hormone cycles in cattle during a menstrual cycle (around 21 days) followed by pregnancy and calving. The curves show blood levels of these hormones, but milk can be expected to follow the pattern. Indeed, one of the tests for whether a cow is pregnant is to examine the progesterone levels in milk. The answer is ‘yes’ if the progesterone exceeds 10 microgram per litre of milk1, which is typically higher than the blood levels.

New techniques are being explored to reduce the ‘dry’ period before she gives birth. There are moves towards using a one-month dry period,2 and researchers are now demonstrating that a zero dry period is possible.3 Yet these last few weeks of pregnancy are when circulating hormones can rise to their highest levels.

The quantities of hormones in our milk supply are not equivalent to pharmceutical amounts, and their absorption from the alimentary tract may be poor. But there is a surprising lack of evidence about the possible effects on our health of consuming cattle hormones in small quantities, every day for decades. All we know is that a string of research papers has highlighted various concerns that appear to be linked to milk and dairy food consumption (see box).

Infants will be especially vulnerable, partly because they drink a lot of milk, partly because they are still growing and may be more vulnerable to hormonal interference, and partly because, in the first few months especially, their gut walls are more permeable to larger molecules such as hormones.

Milk is a food that is rapidly expanding its market base as diets in the Eastern and Southern hemispheres become increasingly westernised. Its production has become intensified and its hormone content increased, yet we know next to nothing about its potential impact on health. And we haven’t even started to look at the other components of milk, such as its enzymes, antibodies or nucleotides.

How milk hormones affect health?

Milk may contain varying amounts of many hormonal substances, with an unknown effect on our own endocrine systems. When pregnant, a cow’s hormone levels may rise, but 70% of our milk comes from pregnant cows. This may rise to 80% or even 90% using new farming methods.

Yet the research evidence, which is very sparse, points to several possible influences on our normal endocrine functions and on childhood growth and development. These are summarised in the box, but it cannot be stressed too highly that the research base is extremely poor, and we should be seeking better evidence.


Milk may contain…
Which may be linked to…
Prostate cancer
Colon cancer
Cortisone and other adrenal steroids
Endometrial cancer, breast cancer
IGF-1 growth hormone
Cardiovascular disease
Thyroxine and triiodothyronine
Acne and pubertal development

Source: PubMed searches (


1) P Rioux, D Rajotte. Progesterone in milk: a simple experiment illustrating estrous cycle and enzyme immunoassay. Adv Physiol Educ 2004, 28:64-67.

2) KC Bachman. Milk production of dairy cows treated with oestrogen at the onset of a short dry period. J Dairy Sci 2002, 85:797-803.

3) Washington State University: Dry Period – Does the Cow Need One? See