Dairy Products Shown To Protect Against Breast Cancer.
Dr. Leán O'Flaherty,
Senior Nutritionist, The National Dairy Council.
13th November 2000
Jane Plant, author of 'Your Life in your Hands', described how she overcame breast cancer at the recent AGP AGM. Her statement linking dairy products to breast cancer deserves to be challenged. There is no scientific basis for her argument and her statements are based on one case study that has not been peer reviewed in the scientific press. A study describing just a single case history, particularly when it is the author's own, is scientifically unacceptable on statistical grounds and on the grounds of objectivity.
It is impossible to link breast cancer risk with any one single dietary component, this would be far too simplistic. Risk of breast cancer is multi-factorial, involving family history, environmental and lifestyle factors. Dietary factors have also been proposed, however scientific research has been inconclusive.
Prospective cohort studies offer a valuable tool to test a hypothesis. The Nurses' Health Study is one of the most powerful prospective cohort studies and has provided important information on many disease types. Data from nearly 2,000 women with invasive breast cancer from the Nurses' Health Study found that individuals with higher intakes of dairy foods are more likely to survive than those with low intakes (Holmes et al 1999). The authors concluded that increasing intake of dairy products, fish, poultry, and vegetables may increase survival in women who have been diagnosed with breast cancer.
The reason for the observed protective effects may be that dairy foods are good sources of conjugated linoleic acid (CLA)- a potent cancer preventive agent. CLA was one of the topics discussed at 'Nutrition Week 2000', an international nutrition conference that took place in Dublin at the end of May. Dr. Clement Ip, from the Roswell Cancer Institute in New York, presented data from several published studies carried out in his laboratory on CLA enriched butter. Dr. Ip has consistently shown CLA to significantly reduce tumour growth in models of breast cancer, by as much as 50% (Ip et al, 1999). It is widely believed among the scientific community that CLA may explain many of the cancer protective effects that have been demonstrated for dairy products.
In twenty years of research, scientists have failed to find simple relationships between diet and cancer. Recently, a review published in Nutrition Research concluded that there is no significant association of dairy foods with cancer of the breast (Jian et al, 1998). This particular review examined data from 14 separate epidemiological studies, 8 of which showed that dairy products were negatively associated with breast cancer, 4 were positive, and 2 were unable to demonstrate any association. However, many of these studies were flawed in that they failed to adjust for confounding factors. Just two of the 14 studies carried out appropriate adjustments, and both of these found intake of dairy to be negatively associated with risk of breast cancer. For those with the highest intakes of dairy products, the relative risk of developing breast cancer was 0.59 (Toniolo et al, 1994) and 0.42 (Knekt et al, 1995).
Looking at the balance of evidence therefore, there is no scientific basis for linking dairy products with breast cancer. Indeed, there are several studies to indicate a protective role. In her presentation, Jane Plant failed to acknowledge any of these published studies. Neither did she appear to have any knowledge of the cancer protective agent found in milk, CLA. Rather, she gave results of an unpublished, single case history. The crux of her theory linking dairy products with breast cancer centres around two points; that dairy products are a source of IGF, which may cause breast cancer, and that Asian women have a lower incidence of breast cancer and have lower intakes of dairy products. No evidence was offered to support the IGF theory, indeed there is no published data to suggest that IGF is absorbed from milk and enters the blood stream. In fact, the majority of circulating IGF is produced endogenously and is not derived from food sources. It is too simplistic to equate the incidence of breast cancer in Asian women to a lack of dairy products in the diet because compared to women in Western countries, Asian women are less overweight, consume less alcohol, and have their babies earlier. All of these factors have been implicated in breast cancer risk.
Many women are concerned about breast cancer, and rightly so. But it is vitally important that any advice given to them is well founded and backed by scientific evidence. Dairy products have many positive health benefits to offer consumers. They are the main contributor to calcium intake in the Irish diet. Dietary calcium is critical for bone health and the prevention of osteoporosis, a condition that affects one in four Irish women by the age of 60. It is imperative that all women consume three servings from the milk group daily (equivalent to one pint of milk) to meet calcium requirements. To meet calcium requirements on a dairy free diet is extremely difficult. Soy products contain oxalate and phytate, which inhibit the absorption of calcium. It has recently been observed that calcium absorption from imitation soy milk is just 75% that of cow's milk (Heaney et al 2000).
The National Dairy Council therefore warns against following Jane Plant's advice. There is no evidence to promote the exclusion of specific foods to protect against breast cancer and it is important not to highlight one person's personal experience to the detriment of strong proven health benefits.
Dr. Leán O'Flaherty
Senior Nutritionist
National Dairy Council
Grattan House
Lower Mount Street
Dublin 2
Phone: 01 6619599
Fax: 01 6620379
E.mail: ndc@iol.ie
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