Dairy and Cancer

Full-Fat Dairy and Cancer

Today I’d like to discuss the possible effects of full-fat dairy consumption on different types of cancer. While my previous posts had positive evidence for full-fat dairy on inflammation, type 2 diabetes, CVD, and weight loss, there is mixed data for cancer. Today I will look at the evidence for three different types of cancer: breast cancer, prostate cancer, and colorectal cancer.

First some fast facts on cancer in the United States. After heart disease, cancer is the second leading cause of death and accounts for 1 in 4 deaths each year! About 1.5 million people will develop cancer each year, costing the US $265 billion in medical costs and lost productivity. Prostate cancer and colorectal cancer are the second and third leading causes of death from cancer for males (if you’re curious, lung cancer is number 1). For females, breast cancer and colorectal are the second and third leading causes, respectively, of death from cancer (lung cancer is also number 1 for females). Altogether, these three types of cancer accounted for more than 100,000 deaths in 2015.

Cancer is a pressing issue that will likely affect someone close to use. Therefore, it is important to consider factors that may play a role in its development and ways to reduce its occurrence. I will be assessing the role of diet, specifically high-fat dairy intake on cancer incidence today.

Let’s start with breast cancer first.

Why might high-fat dairy have a negative influence on breast cancer development? Estrogen is a hormone that is created and stored in fat cells. It is important for regulation of the menstrual cycle, breastfeeding, bone density, and raising HDL cholesterol. However, too much estrogen stimulates breast cell growth, specifically the growth of hormone-receptor-positive breast cancer cells.

About 60-80% of estrogens consumed come from dairy products. Dairy products in the US are believed to be very high in estrogen because the milk is produced by pregnant cows. The farther along a cow is in her pregnancy, the more hormones appear in her milk. Milk from a cow late in pregnancy has more than 33 times as much estrogen than milk from a non-pregnant cow. Further, it is estimated that these natural estrogens from cows are more than 100,000 times stronger than environmental counterparts, like estrogen compounds found in pesticides1.

Low-fat dairy products may be a better alternative because most of the fat has been removed and they therefore have lower levels of estrogen.

In one study, researchers examined the association between high- and low-fat dairy intake on the recurrence and mortality after being diagnosed with breast cancer. 1,893 women were included from a cohort who had been diagnosed with early-stage breast cancer and followed up 5-6 years later for outcomes.

The researchers found that overall dairy intake was unrelated to recurrence of breast cancer, but was positively related to overall mortality. In other words, the more dairy products one consumes dairy, the higher the risk for death from breast cancer. Low-fat dairy was unrelated to recurrence or survival, but high-fat dairy intake was positively associated.

Compared to those who consumed 0 to <0.5 servings of high-fat dairy/day, individuals who consumed more than 1 serving a day were 50% more likely to die from breast cancer. Although there was also an increased risk of recurrence with high-fat dairy consumption, the results were not statistically significant. This present interesting data in terms of exploring future avenues to see if different types of diary play a different role in the risk of recurrence. For example, researchers should examine differences between the type of high-fat dairy consumed, pudding and ice cream versus greek yogurt, and the source (grass-fed vs. regular).

However, there are some limitations to the study. The women filled out a food frequency questionnaire (FFQ) that asked them to report an entire year of dairy consumption, which is quite hard to remember. Also, the researchers didn't specify if they looked at what else the women were eating or whether they controlled for variables such as weight and exercise. The final point the researchers didn't note is how many women had hormone-receptor-positive breast cancer. These women would have been prescribed hormonal therapy to reduce risk of recurrence. However, if some women stopped treatment early, this could have influenced the results of the study.

Another study conducted a prospective analysis between dietary fat intake and breast cancer risk among 100,000 women aged 26-46. After 8 years of follow-up, the researchers found that women with the highest fat intake had a 25% increased risk of breast cancer. This increase was associated with animal fat intake (red meat and high-fat dairy foods), but not vegetable fat (olive oil). Further, women who consumed more than 2 servings of high-fat dairy/day had a 36% increased risk of developing breast cancer compared to those who didn't consume any high-fat dairy. Therefore, this study provides evidence that high intake of animal fat, mostly from high-fat dairy foods and red meat, is associated with increased risk.

Like the previous study the dietary assessment was conducted with a FFQ, which could have under-reporting of certain dietary intake. However, the FFQ was completed twice, once in 1991 and 1995 so this helped reduce reporting error.

Overall, however, I think that both these studies provide evidence that consumption of high-fat dairy foods increases risk for breast cancer development, recurrence, and mortality, although more research should be conducted exploring the different variables I discussed above.

The results for prostate cancer were not as conclusive. In one study, a cohort of 4000 men diagnosed with prostate cancer were followed and dairy intake was assessed from repeated FFQs. Men who consumed higher amounts of low-fat dairy products had 40% decreased risk of lethal prostate cancer compared to those who didn’t consume any low-fat dairy.

On the other hand, men who consumed high amounts of full-fat dairy had 30% increased risk! Further, men with the highest intake of whole milk (>4 servings/week) were more than twice as likely to have lethal progression of prostate cancer compared to those who consumed 0-3 serving of whole milk/month.

Another study on prostate cancer did not find similar results. After following 12,000 men for 11 years and investigating intake of fat, meat, and dairy foods, the researchers found that fat was not associated with development or risk of lethal prostate cancer. Similar to the previous study, the researchers found that dairy foods protected against aggressive cancer, with a 60% decreased risk in current smokers! The researchers concluded that it was not dietary fat that increased risk for prostate cancer, but lifestyle factors such as family history and smoking. The researchers concluded that although some of these dairy products consumed may be high in fat, it also contains vitamin D and conjugated linoleic acid (CLA) which may have protective effects against prostate cancer2,3,4.

What is CLA? It is a fatty acid found in meat and dairy products that is derived from ruminants, and intake may help reduce body fat and have other important implications for disease. This brings me to my final study, commenting on colorectal cancer! This study was another prospective cohort that evaluated ~60,000 women aged 40-76, with high-fat dairy food consumption assessed at baseline and at follow-up. After adjustment for age and other confounders, the researchers found that women who consumed at least 4 servings/day of high-fat dairy foods such as whole milk, full-fat cultured milk, cheese, cream, and butter had a 40% decreased risk of developing colorectal cancer compared to women who consumed less than 1 serving a day!

When the researchers looked at CLA only (the fatty acid found in most full-fat dairy foods), they found an inverse relationship between intake and development of colorectal cancer: those who consumed more CLA, from full-fat dairy foods, were 30% less likely to develop colorectal cancer. This data provides more evidence that full-fat dairy may have a protective effect on colorectal cancer development. My one limitation with this study, however, is that it was conducted in Sweden, where there is a lack of CAFOs. Cows are grass-fed and pasture-raised, increasing their CLA content and decreasing their omega-6 PUFA content. This study may have had more significant results because of this and similar results may not be seen on the average American in the US because most cows are grain-fed and unable to graze freely, negatively altering their fat content. This is something I will further discuss in my next post.

Overall, I would say the evidence on full-fat dairy consumption and cancer is mixed, with greater associations seen in breast cancer, mixed results in prostate cancer, and protective effects for colorectal cancer.

Although this was my last informative post looking at the cold, hard evidence, my next post will be a conclusion on my stance regarding full-fat dairy consumption and recommendations, based on my previous posts! Look out for my last post, you won’t want to miss it!