CLA may protect against prostate cancer by cancelling out the effects of the potentially carcinogenic growth factors found in milk, such as insulin-like growth factor-1 (IGF-1). This occurs naturally-and in identical forms-in both cows and humans. Because cows are milked during and after pregnancy-when growth factors are at their highest-scientists are concerned that consuming milk and dairy could raise IGF-1 levels in humans-perhaps by crossing the gut wall-and trigger an abnormal response leading to, for example, certain cancers.

Indeed, elevated IGF-1 levels have recently been linked to an increased risk of gastrointestinal and breast cancers (Int J Health Serv, 1996; 26: 173-85; Lancet, 1998; 351: 1393-6), lung cancer, childhood cancers, melanoma, and cancers of the pancreas and prostate (Ann NY Acad Sci, 1995; 766: 402-8; J Natl Cancer Inst, 2000; 92: 1910-7).

The association with the prostate appears to be particularly strong. In one study, men with the highest levels of IGF-1 had more than four times the risk of prostate cancer compared with those who had the lowest levels (Science, 1998; 279: 563-6).

Whether the IGF-1 in milk is the real culprit is not yet known. What is clear is that milk stripped of its natural fat is more likely to promote cancer, and the more of its natural fat-and CLA content-that milk retains, the more anticancer benefits it will have.

Infertility
But it's not only men who are at risk from the hazards of low-fat milk, and the problem isn't just cancer. Harvard scientists recently confirmed a link between low-fat dairy in the diet and an increased risk of infertility due to lack of egg release-also known as 'anovulatory infertility'.

This study monitored 18,555 American women aged 24 to 42, without a history of infertility, who were trying to become pregnant or had become pregnant between 1991 and 1999. It showed that women who ate two or more servings of low-fat dairy foods a day, such as skimmed milk or yoghurt, increased their risk of anovulatory infertility by more than 85 per cent compared with women who ate less than one serving of low-fat dairy a week.

Of the low-fat dairy foods, women who consumed one or more servings per week of skimmed or low-fat milk had a significantly higher risk of anovulatory infertility compared with those having less than one serving per week.

In contrast, adding a daily serving of whole milk reduced the risk of infertility by more than 50 per cent. Other high-fat dairy products, such as ice cream, were also associated with a lower risk.

Previous research has suggested that lactose, the sugar found in milk, might be involved in anovulatory infertility, but the present study found no such connection. Instead, the researchers believe that the presence of a fat-soluble substance, which improves ovarian function, might explain the lower risk of infertility from high-fat dairy foods. As with the prostate-cancer studies, there appears to be a substance vital for healthy ovaries that requires the presence of fat for it to be properly absorbed (Hum Reprod, 2007 Feb 28; Epub ahead of print).

This may also explain why studies that have looked at dairy intake and rates of ovarian cancer have found that only low-fat milk and skimmed milk, but not whole milk, were associated with an increased cancer risk. In the Brigham and Women's Hospital Nurses' Health Study, based on more than 80,000 women, those who consumed one or more servings of skimmed or low-fat milk daily had a 32-per-cent higher risk of any type of ovarian cancer-and a 69-per-cent higher risk of serous ovarian cancer, the most widespread form-compared with women who had three or fewer servings a month. Whole milk, on the other hand, had no such effect (Int J Cancer, 2004; 110: 271-7).