Can the hormones in milk affect breast cancer?

We think you are asking about a hormone given to cattle to increase milk output. This hormone is called 'bovine somatotropin' or BST. It is banned in the UK and Europe but has been licensed in the US since 1995. BST is given to cows to make them produce more milk. The European Union ban was made permanent on January 1st 2000. BST was banned on animal welfare grounds and not because there is any proven effect on human health. Some environmental campaigners and health groups are concerned about this hormone. Among other things, there are suggestions that it could increase breast cancer risk. There is no evidence at the moment that this is the case. A European Union Scientific Committee report has stated that there is no scientific evidence that this hormone was a health risk.

BST is actually a natural hormone made by all cows. Like some human hormones, we can now make BST articifically. It is the laboratory made BST that is given to cows in the USA and many other countries. There is a human version of this hormone, called human somatotropin or HST. BST is not the same shape as HST and cannot fit into hormone receptors in the human body in the same way as HST. So there is unlikely to be a direct health risk that way.

Milk from cattle that have been given somatotropin contains higher levels of a growth factor called IGF-1 (insulin growth factor 1). This growth factor has recently been the subject of a research study into breast cancer risk. Women with high levels of IGF-1 in their blood may have a higher risk of breast cancer. Some scientists think that high IGF-1 levels may also encourage an existing breast cancer to spread, but this has not been proved. There are also reports of increased risk of colon cancer with high IGF-1 levels. But we don't know if the high IGF-1 levels come from milk and dairy products in the diet, or from some other cause.

IGF-1 is also a naturally occuring hormone that we (and cows) all make. The IGF-1 levels in milk vary greatly depending on many things - mostly how long the cow has been producing milk. Levels in cows that have recently calved are much, much higher. The normal IGF-1 level in humans is 100 times higher than the level in the average pint of milk. It may be that IGF-1 is important in the development of cancer. But it may be that our natural levels and how they vary between people that is the issue. So, we don't know yet whether drinking milk with high IGF-1 levels can increases IGF-1 levels in humans. As UK and European cattle don't get this hormone, our milk does not contain high IGF-1 levels. If you are worried, you may want to avoid foods imported from the US that contain milk or dairy products, as many US cattle are given this drug.