Cancer risk is strongly affected by diet.

Many studies have examined the relationship between dairy consumption and cancer.

Some studies indicate that dairy may protect against cancer, while others suggest that dairy may increase cancer risk.

The most commonly consumed dairy products include milk, cheese, yogurt, cream and butter.

This article reviews the evidence linking dairy products with cancer, looking at both sides of the argument.

How Do These Studies Work?

Before we continue, it is important to understand the limitations of the studies examining the link between diet and disease.

Most of them are so-called observational studies. These types of studies use statistics to estimate the relationship between dietary intake and the risk of getting a disease.

Observational studies can not prove that a food caused a disease, only that those who consumed the food were more or less likely to get the disease.

There are many limitations to these studies and their assumptions have occasionally been proven false in controlled trials, which are higher quality studies.

Yet, despite their weaknesses, well-designed observational studies are an integral part of nutrition science. They provide important clues, especially when coupled with plausible biological explanations.

Bottom Line:

Virtually all human studies on the connection between milk and cancer are observational in nature. They can not prove that dairy products cause a disease, only that consuming dairy is associated with it.

Colorectal Cancer

Colorectal cancer is cancer of the colon or rectum, the lowest parts of the digestive tract.

It is one of the most common types of cancer in the world (1).

Although the evidence is mixed, most studies indicate that eating dairy products may reduce the risk of colorectal cancer (2, 3, 4, 5).

Some components of milk may possibly protect against colorectal cancer, including:

  • Calcium (6, 7, 8).
  • Vitamin D (9).
  • Lactic acid bacteria, found in fermented dairy products like yogurt (10).
Bottom Line:

Most studies suggest that consuming dairy products is linked to reduced risk of colorectal cancer.

Prostate Cancer

The prostate gland is located just below the bladder in men. Its main function is to produce prostate fluid, which is a part of semen.

In Europe and North America, prostate cancer is the most common type of cancer in men.

Most large studies indicate that high dairy consumption may increase the risk of prostate cancer (11, 12, 13).

One Icelandic study indicates that high milk consumption during early life may increase the risk of advanced prostate cancer later in life (14).

Milk is a complex fluid containing a huge variety of bioactive compounds. Some of them may protect against cancer, while others may have adverse effects.

These include:

  • Calcium: One study has linked calcium from milk and supplements with an increased risk of prostate cancer (15), while some studies strongly suggest it has no effects (16, 17).
  • Insulin-like growth factor 1 (IGF-1): IGF-1 has been linked with an increased risk of prostate cancer (18, 19, 20). However, this may be a consequence of cancer rather than a cause (17, 21).
  • Estrogen hormones: Some researchers are concerned that the reproductive hormones in milk from pregnant cows may stimulate prostate cancer growth (22, 23).
Bottom Line:

The majority of studies suggest that high dairy consumption may increase the risk of prostate cancer. This may be due to several bioactive compounds found in milk.

Stomach Cancer

Stomach cancer, also known as gastric cancer, is the fourth most common cancer in the world (24).

Many major studies have found no clear association between dairy intake and stomach cancer (25, 26, 27).

Possible protective milk components may include conjugated linoleic acid (CLA) and certain probiotic bacteria in fermented milk products (28, 29).

On the other hand, insulin-like growth factor 1 (IGF-1) may promote stomach cancer (30).

In many cases, what cows feed on often affects the nutritional quality and health properties of their milk.

For example, milk from pasture-raised cows that feed on bracken ferns contains ptaquiloside, a toxic plant compound that may increase the risk of stomach cancer (31, 32).

Bottom Line:

In general, there is no clear evidence linking the consumption of dairy products with stomach cancer.

Breast Cancer

Breast cancer is the most common form of cancer in women (33).

Overall, the evidence indicates that dairy products have no effects on breast cancer (34, 35, 36).

In fact, some studies indicate that dairy products, excluding milk, may have protective effects (37).

Bottom Line:

There is no consistent evidence about dairy products affecting breast cancer. Some types of dairy may have protective effects.

How Much Milk Can You Safely Drink?

Since dairy may actually raise the risk of prostate cancer, men should avoid consuming excessive amounts.

The current dietary guidelines for dairy recommend 2–3 servings or cups per day (38).

The purpose of these recommendations is to ensure adequate intake of minerals, such as calcium and potassium. They do not account for a possible cancer risk (39, 40).

So far, official recommendations have not put a maximum limit on dairy consumption. There is simply not enough information for evidence-based recommendations.

However, it may be a good idea to limit your intake to no more than two servings of dairy products per day, or an equivalent of two glasses of milk.

Bottom Line:

Avoid excessive consumption of dairy products. Men should limit their intake to two servings of dairy products per day, or about two glasses of milk.

Take Home Message

Studies indicate that high dairy consumption increases the risk of prostate cancer.

Yet, at the same time, dairy products may reduce the risk of colorectal cancer.

For other types of cancer, the results are more inconsistent but generally indicate no adverse effects.

Keep in mind that most of the available evidence is based on observational studies, which provide suggestive evidence but not definite proof.

However, it is better to be safe than sorry. Consume dairy in moderation and base your diet on a variety of fresh, whole foods.