The Dark Side of Milk

  • 5 Jul 2009
  • Reading time 5 mins
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Should milk be a staple food? Does it increase cancer risk? Here I interview top scientist Professor Jeff Holly.

Q: What is IGF?

PJH: IGF stands for ‘Insulin-like Growth Factor’. As with the hormone insulin, it is a small protein. It is one of the most highly concentrated of the protein hormones in blood. Blood contains around a thousand times higher amounts of IGF than most other protein hormones and it has profound effects on stimulating growth and metabolism. It is present in all of us and is clearly a key hormone, influencing many aspects of our health. IGFs, for example, mediate the effects of growth hormone on childhood growth. There are two types, IGF-1 and IGF-II, but these are always present with one of six other proteins, IGFBP-1 to -6, and they activate a number of tissue receptors including IGF-1R, IGF-IIR and the insulin receptor. Unravelling their complex roles is one of the frontiers of endocrinology.

Q: How does drinking milk influence our IGF levels?

PJH: Milk contains lots of hormones and growth factors including IGF, although when we drink it most of these are digested. Only a very small amount, protected in the gut by the milk protein casein, makes it into our bloodstream. This isn’t, however, the only way in which milk creates an increase in blood levels of IGF. The cocktail of hormones, small proteins and amino acids in milk stimulates our own production of IGF, the net result being that IGF-1 levels in the blood increase as a result of drinking milk. To put this in context, if you were aged 20 to 30, you might normally have something like 170 ng/mL of IGF-1 in the blood. If your diet is high in dairy products you might have a level of 200 to 210 ng/mL. If you don’t drink milk your level might be 130-140ng/mL. IGF-1 levels are highest in teenage years, at the height of teenage sexual development, then decline rapidly in adulthood. In this graph, for example, you see the IGF-1 levels in men, aged 50-70, according to milk consumption. Those who have a pint or more of milk per day, compared to those with less than half a pint a day, have increased their IGF by about 25 ng/ml. This is a highly significant increase.

Q: Does this increase in IGF from milk drinking increase cancer risk?

PJH: It’s important to realise that IGF-1 is essential. It’s one of the most important hormone signallers in the body, present in relatively large amounts in the bloodstream compared to other hormones. If you have too little, which might happen on a diet completely devoid of all animal fat and protein, there is evidence of increasing risk of cardiovascular disease, type 2 diabetes, osteoporosis and cognitive decline. But if you have too much your risk for breast, prostate and colo-rectal cancer definitely goes up. Our research, and that of researchers at Harvard and Montreal, has shown that those in the top quarter for blood IGF-I levels have approximately a three to fourfold increase in risk of breast, prostate or colo-rectal cancer. This level of increased risk is in the same order as the risk of having cardiovascular disease from a high level of cholesterol. In the case of breast cancer the link with IGF-1 is strongest with pre-menopausal breast cancer than post-menopausal breast cancer. What we don’t yet know is to what extent the risk is set in puberty, or throughout life. Both may be important, which would imply that too much milk during puberty and adulthood may be a bad idea. The link between milk consumption and cancer risk is clearly shown from population studies. This graph, for example, shows the correlation between a country’s average consumption of milk and the risk of prostatic cancer. Similar associations exist for breast cancer.

We also know that there is a direct relationship between increasing IGF-1 levels and increasing levels of PSA, which is an indicator of prostate size in elderly men. Both of these cancers are on the increase. Genes, we know, account for as little as 2-5% of the risk, therefore the rest is down to environmental factors. The direct link between IGF-1 and breast and prostate cancer risk, certainly points to diet, specifically dairy consumption, and, to a lesser extent, high animal protein consumption, which also has as effect on circulating IGF-1, as a likely suspect. We are now seeing associations between rapid growth rates and tallness in girls, associated with diets high in animal protein, and with increased breast cancer risk.

Q: How does increased levels of IGF-1 lead to increased cancer risk?

PJH: Cancer is initiated by damage to DNA, for example from smoking or other sources of oxidants or carcinogens. Such DNA damage is occurring all the time and can usually be repaired. If it is not repaired, as the cell starts to misbehave, it is programmed to commit suicide. This is called apoptosis. In most cancers something goes wrong with either the repair process or the cell suicide process. IGF-1, in excess, strongly inhibits apoptosis in both prostate and breast cancer cells, so the cancer cells inappropriately survive and keep growing. In the case of these cancers it is likely that this suppression of normal apoptosis of cancer cells is a much more significant risk factor than our general exposure to carcinogens.

Q: What’s your advice on drinking milk?

PJH: Milk is designed to fill the gap between birth and a baby developing a mature digestive process, able to derive maximum benefit from nutrients in food. It is full of complex hormones not found in other food. It does not make sense to keep drinking it into adolescence or adulthood. No ......

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