The Dairy Dilemma: Part 3: Cancer

In 2015 the results of an interesting cohort study from Sweden were published on the subject of dairy intake and cancer (1). Researchers looked at 22,788 lactose-intolerant participants and their immediate family members (who were not lactose-intolerant). Because individuals with lactose intolerance usually follow recommendations to avoid milk and other dairy products they can be considered subjects that consume only small amounts of dairy products or none at all. Study results showed that the lactose-intolerant participants were 45% less likely to develop lung cancer, 21% less likely to develop breast cancer and 39% less likely to develop ovarian cancer than their non-lactose-intolerant family members who had cancer rates similar to the Swedish population as a whole. These results strongly suggest a link between dairy intake and certain cancers.
The researchers suggested possible mechanisms for these results (1);
1 Dairy intake causes increased IGF-1 levels. Lower dairy consumption would result in decreased IGF-1 levels. (IGF-1 is Insulin-like Growth Factor that encourages tissue growth and cell division in adults and is known to play a role in cancer development.)
2 Dairy products are high in saturated fat and hormones, both of which are associated with increased incidence of cancer.
3 Dairy consumption is associated with increased calorie intake and higher risk of obesity, a known risk factor for breast and ovarian cancer.
4 Lactose-intolerant people might be substituting dairy milk with soy milk that offers a cancer- protective benefit.

There have been numerous studies looking at dairy consumption and cancer risk. Although many of them, such as the one described above, show strong associations between dairy intake and cancer development, others have conflicting results. Continuing research should clarify the effects of dairy intake on cancer development. However, we can look at the present state of scientific knowledge on this subject.

 

Prostate Cancer:

The strongest evidence linking intake of dairy products with cancer is seen in prostate cancer. Several meta-analyses over the last two decades have analyzed the data. A meta-analysis from 2004 including 11 studies found that consumption of milk was associated with an average 68% increased risk of prostate cancer (2). A 2007 meta-analysis again illustrated that the higher the intake of milk, the greater the risk of prostate cancer (3). A 2015 meta-analysis of 32 previous studies revealed links between total dairy product intake, total milk intake, low-fat milk intake and cheese intake with increased prostate cancer risk. Conversely, non-dairy calcium sources such as leafy greens were found to be protective, suggesting that the harmful factor responsible for the increased cancer risk was not calcium but another component of dairy such as animal protein (4).

A 2012 study from Iceland examined the milk consumption of men in their early, mid- and current life and followed them for over 24 years. Researchers found that adolescent males who drank milk daily were linked to a 320% increase in the risk of advanced prostate cancer in later life compared to men drinking milk less than daily. (No, that is not a typo! It is a 3.2-fold increase.) (42)

In 2013 the Physicians’ Health Study published the results from 28 years of following 21,660 participants. Those who consumed more than 2.5 servings of dairy products a day had significantly increased risk of prostate cancer compared to those consuming less than ½ a serving a day (5).

A 2015 study looked at ingesting dairy products after a prostate cancer diagnosis and found that men consuming more than three servings a day of total dairy products had a 76% higher risk of death from all causes and a 141% higher risk of death from prostate cancer (6).

A prospective study from 2018 found that men with previously diagnosed prostate cancer who consumed more than 4 servings of milk a week had a 73% increased risk of prostate cancer recurrence compared to men consuming zero to 3 servings a month. Men with a BMI of greater than 27 showed a 3-fold higher risk of prostate cancer recurrence associated with 4 servings of milk a week compared with zero to 3 servings per month. Low fat milk and other dairy foods were not associated with prostate cancer recurrence (7).

 

Breast Cancer

Researchers state that there are a variety of mechanisms through which dairy consumption might increase breast cancer risk. These include estrogenic hormones, fat, insulin-like growth factor (IGF-1), calcium, vitamin D, conjugated linoleic acid and chemical contaminants (8,9,10,11,12). Because of the strong influence of estrogens on growth of breast tissue, the estrogens contained in milk and dairy products may play the major role in breast cancer development. The main sources of estrogens from the human diet are milk and dairy products (13).

A study from 2017 found an association between higher intake of milk and cheese with increased risk of breast cancer (14).

Metabolism of estrogen produces substances called estrogen metabolites and these have been associated with increased breast cancer. A 2009 analysis found considerable amounts of estrogen metabolites in a variety of commercial milks (whole, 2%, skim and buttermilk). Concentrations were lowest in skim milk. Soy milk contained no estrogen metabolites. (15)

Because estrogens reside primarily in fat, the relationship between risk of breast cancer and high-fat milk has been extensively studied. A 2003 study found that high-fat dairy intake of all types is associated with increased risk of breast cancer (16). In 2013 another study showed that those consuming larger amounts of high-fat dairy had 20% higher mortality from breast cancer and greater mortality from all causes. This study also found a link between consumption of high-fat dairy and breast cancer recurrence although it was not statistically significant (17).

The risk for breast cancer is known to be at least partially determined during childhood and adolescence (18). Susceptibility to risk factors such as hormones and dietary fat is higher at this early stage of life. Several studies suggest that factors involved in this increased breast cancer risk include more rapid growth in height during childhood, being thinner in childhood, being taller in adulthood and earlier onset of menstruation (19,20,21). High milk intake is associated with more rapid growth in children, increased rate of growth in height and greater total height in older girls as well as early onset of puberty and breast development (22), thereby introducing some of the factors that are associated with greater risk of breast cancer. An observational study from 2010 found that women who ate the highest amount of fat in adolescence had 35% increased risk of suffering from breast cancer later in life (23).

Benign breast disease is a condition that leads to elevated breast cancer risk. A study showed that increased risk of benign breast disease is linked to being thinner and taller during childhood and adolescence (24). Girls experiencing the most rapid increase in height also showed higher risk of developing benign breast disease (18).

In contrast to these studies, there are some whose results are conflicting, finding less risk of breast cancer associated with dairy intake or no association at all (25,26,27,28,29) and making it difficult to come to a firm conclusion about the relationship between dairy intake and breast cancer. More research is needed.

 

Ovarian cancer

In a 2004 investigation, women who consumed more than one serving of skim or low-fat milk per day were found to have a 32% higher risk of developing ovarian cancer and a 69% higher risk of that cancer being serious compared with women consuming less than 3 servings of milk monthly (30).

Galactose and lactose, two sugars found in dairy products, have been linked to ovarian cancer. It is thought that galactose may be toxic to ovarian germ cells (the cells that are destined to divide and differentiate into eggs). A case-control study from 2000 found that certain genetic types of galactose metabolism can influence the risk for ovarian cancer (31).

A 2004 study from Sweden involving over 61,000 women examined the link between intake of lactose from dairy products and the risk of ovarian cancer. The study found that women who consumed more than 4 daily servings of any dairy product doubled their risk of certain types of ovarian cancer compared to women who consumed less than 2 servings a day. Each 10 gm increase in lactose intake (approximately the amount in one glass of milk) was associated with a 20% greater risk of ovarian cancer. Milk was the dairy product with the strongest association with ovarian cancer (32).

A meta-analysis from 2005 looked at the relationship between dairy products and lactose consumption and the risk of ovarian cancer. Prospective cohort studies in this analysis consistently showed that for each 10 gm daily increase in lactose intake there was a corresponding 13% increase in the risk of ovarian cancer (33).

In spite of these associations, the role of dairy food intake in ovarian cancer risk remains inconclusive. A 2015 meta-analysis of 19 studies found a slightly increased risk of ovarian cancer with high intake of whole milk but the increase was not statistically significant. Researchers point out the need for larger studies to validate the association between dairy-derived food intake and ovarian cancer (34).

 

Endometrial and Uterine cancer

Here again, though some investigations have found clear associations between dairy intake and endometrial and uterine cancer, more study is needed to support a causal relationship.

Estrogen seems to be a strong promoter of uterine cancer. Higher consumption of animal-derived food, especially the dairy products of today with their elevated levels of estrogen and progesterone, has been related to the development of uterine cancer (13).

High intakes of milk and dairy products are associated with increased risk of endometrial cancer, especially among postmenopausal women not using hormone therapy. A prospective cohort study from 2012 with over 68,000 participants showed that increasing dairy intake was linked with higher rates of endometrial cancer (35).

 

Testicular cancer

Though studies such as the following are indicating a link between dairy intake and testicular cancer development, more study is necessary to strengthen this association.

In 2002 incidence and mortality of testicular cancer in 42 countries was correlated with dietary practices. The food item most closely associated with development of testicular cancer was cheese, followed closely by animal fats and milk (36).

A 2003 Canadian study found that men who regularly consumed generous portions of cheese were almost 90% more likely to develop testicular cancer than men who ate little or no cheese (37).

In 2006 another study found that regular intake of 20 servings of milk per month in adolescents was associated with a 37% rise in the risk of testicular cancer. Researchers hypothesized that the hormone galactose and the fat present in dairy products may be responsible for cancer promotion (38).

 

Soy protein does not promote cancer development

Isoflavones in soybeans act as phytoestrogens. They are similar in structure to the human female hormone, 17- ß-estradiol, allowing them to bind to estrogen receptors. However, they produce only very weak estrogenic activity and so act as estrogen blockers. Studies show that even high soy intake has no feminizing effects on men (39). Other studies have found that consuming soy foods is actually protective against breast cancer (40,41). A 2008 meta-analysis of 8 studies showed that those who ate the most soy foods had a 30% reduction in the risk of breast cancer compared to those who ate the least soy foods (43).

 

In Summary…

The constituents of cow’s milk present a conundrum when it comes to its ingestion. On the one hand we have been told throughout our lives that milk is “nature’s perfect food”. But scientific investigations are discovering potentially injurious effects of these foods on health. The nutrients that milk offers to us can be easily obtained from other readily available foods. It may be prudent to start turning to these other sources in order to reduce our exposure to the potential threats concealed in dairy foods.

 

 

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