Many Canadians enjoy ingesting dairy products but, with the profusion of scientific investigations looking into possible risks of consuming dairy, this is a good time to examine the evidence available regarding dairy intake and our health.
One of the biggest potential dangers of ingesting dairy products is the hormones that they contain. This is a hazard even if your milk is organic because the hormones in milk are not added hormones, but ones that are naturally produced by cows. In particular, estrogen and progesterone levels in milk are of concern. Female mammals including cows produce estrogen cyclically, preparing their reproductive organs to nourish a new life should a successful mating occur. When a pregnancy is established, estrogen levels increase dramatically to support the development of the baby. One study found that estrone, a specific estrogen, increased from 30 pg/ml (picograms per milliliter) in non-pregnant cows to 151 pg/ml in pregnant cows at 40 to 60 days of gestation up to a maximum level of 1000 pg/ml in cows nearing the end of their pregnancy (1). A wild bovine animal (of which cows are a member) will not become pregnant while nursing a calf. However, today’s modern dairy cows not only become pregnant while lactating but have been bred to continue to produce milk throughout their pregnancies. Their excellent nutrition, consisting of scientifically-formulated feed mixes that encompass all the nutrients cows require to nourish both themselves and their developing calf, allows them to accomplish these two physiologically demanding tasks at the same time. Unfortunately, these increased hormone blood levels are mirrored in the milk the cow is producing and can affect the health of anyone who ingests it (2). Milk and other dairy products are the most significant source of exposure to female hormones for humans (3).
The high levels of estrogen in milk do indeed increase the hormone levels of those who drink it. The Melbourne Collaborative Cohort Study of women from Australia, New Zealand, the United Kingdom, Italy, and Greece found that dairy intake is significantly related to higher blood levels of estradiol in those ingesting dairy (4). A small study looked at estrogens in the diet of men, women and children. This study included 7 men, 6 prepubertal children and 5 women who drank one 600 ml glass of whole milk (containing at least 3.5% fat) derived from dairy cows. Urine and blood samples were taken for several hours after the drink. In collected urine samples, both estrogens and progesterones were significantly increased in all subjects including the men. The men also showed decreased blood levels of luteinizing hormone*, follicle-stimulating hormone** and testosterone (5).
* Luteinizing hormone is secreted by the anterior pituitary gland and stimulates ovulation in females and the synthesis of androgens (male hormones such as testosterone) in males.
**Follicle stimulating hormone is secreted by the anterior pituitary gland and promotes the formation of ova (eggs) in the female and sperm in the male.
Dairy Intake and the Onset of the Reproductive Cycle
Children’s bodies are sensitive to exposure to hormones whether they arise from the diet or the environment. Hormones in milk, such as estrogen or progesterone, have been linked to early puberty and lower age of first menstruation (8,9). It is worrisome that childhood exposure to the excess of estrogens in the “normal” amount of milk that they drink may be the cause of early sexual maturation (5).
Studies have determined that the number of girls reaching puberty at the early age of 7 or 8 has markedly increased between 1997 and 2010. Results show that 10.4% of girls who were seven-years-old in 2010 had reached puberty as indicated by breast development compared to only 5% of seven-year-olds in 1997. In 2010, 18% of eight-year-old girls had reached the same stage of puberty compared to 10.5% of eight-year-olds in 1997 (6). Why is this happening? It is hypothesized that increased consumption of animal products, higher fat intake, greater consumption of calories in general, increased body weight and decreased activity may all be contributing to this situation (7).
In very young children, higher intake of animal protein has been associated with earlier onset of menstruation while higher intake of vegetable protein is associated with a later start (10).
Early sexual maturation in girls has been correlated with higher BMI (Body Mass Index) and higher amounts of fatty body tissue. Since estrogen concentrates in fat, this earlier onset of puberty is thought to be partially due to the release of estrogen from fatty tissues into the bloodstream. Intake of commercial milk is one of the largest sources of exposure to estrogen and consequently may be a major cause of sexual maturation in children (7).
Endocrine-disrupting chemicals are chemicals that interfere with the production and function of estrogen. They are another concern connected with early puberty in girls and delayed puberty in boys (11). The structural similarity of endocrine-disruptors to estrogen allow them to bind with estrogen receptors and activate them to produce estrogenic effects. Endocrine-disrupting chemicals include BPA in plastics, dioxins, PCBs and DDT among others (12). They accumulate in fatty tissues of mammals such as their meat and the milk that they produce (13). Chemicals such as dioxins and PCBs persist in the environment for long periods of time and end up in the foods eaten by food animals, especially in areas contaminated by environmental pollution. Environmental food contamination is more likely to be a problem in milk than meat because dairy cows live longer than meat-producing animals and thus have more time to bio-accumulate such hazardous chemicals (11).
Dairy Intake and the End of the Reproductive Cycle
Ingesting dairy products can delay the onset of menopause in women. Although some may consider this a benefit, later menopause is linked to increased risk for cancers such as breast and endometrial cancer. Studies have looked at the association between consumption of dairy foods and age at natural menopause. A prospective analysis with 20 years of follow-up in over 46,000 participants of the Nurses’ Health Study showed that women consuming low-fat dairy products nearly every day reached menopause significantly later than those ingesting milk less often. For every one-year increase in age before a woman reaches menopause her risk for breast cancer increases by about 2.5% (14).
Changes in the reproductive cycle have consequences for both males and females. Early puberty is linked to greater risk for breast cancer, teen pregnancy, heart disease and diabetes. There can also be psychological risks such as depression (15,16). Late menopause is associated with increased risk of some cancers. Awareness of the potential of milk products to alter hormonal levels should constitute a warning to us to reduce our intake of dairy products or avoid them altogether.
1 Heap, R.B., Hamon, M. Oestrone sulfate in milk as an indicator of a viable conceptus in cows. Br. Vet. J. 1979; 135: 355–363.
2 Malekinejad, H., Rezabakhsh, A. Hormones in Dairy Foods and Their Impact on Public Health – A Narrative Review Article. Iran J Public Health. 2015 Jun; 44(6): 742–758.
3 Ganmaa, D., Sato, A. The possible role of female sex hormones in milk from pregnant cows in the development of breast, ovarian and corpus uteri cancers. Med Hypotheses. 2005;65(6):1028-1037.
4 Brinkman, M.T., Baglietto, L., Krishnan, K., English, D.R., Severi, G., Morris, H.A., Hopper, J.L., Giles G.G. Consumption of Animal Products, their Nutrient Components and Postmenopausal Circulating Steroid Hormone Concentrations. Eur J Clin Nutr; 2010 Feb; 64(2):176-183.
5 Maruyama, K., Oshima, T., Ohyama, K. Exposure to exogenous estrogen through intake of commercial milk produced from pregnant cows. Pediatr Int. 2010 Feb; 52(1): 33-38.
6 Biro, F.M., Galvez, M.P., Greenspan, L.C., Succop, P.A., Vangeepuram, N., Pinney, S.M., Teitelbaum, S., Windham, G.C., Kushi, L.H., Wolff, M.S. Pubertal assessment method and baseline characteristics in a mixed longitudinal study of girls. Pediatrics. 2010 Sep;126(3): e583-590.
7 Kaplowitz, P.B. Link between body fat and the timing of puberty. Pediatrics 2008;121 Suppl 3:S208-217.
8 Aksglaede, L., Juul, A., Leffers, H., et al. The sensitivity of the child to sex steroids: possible impact of exogenous estrogens. Hum Reprod Update 2006;12:341-349.
9 Wiley, A.S. Milk intake and total dairy consumption: associations with early menarche in NHANES 1999-2004. PloS one 2011;6:e14685.
10 Gunther, A.L., Karaolis-Danckert, N., Kroke, A., et al. Dietary protein intake throughout childhood is associated with the timing of puberty. J Nutr 2010;140:565-571.
11 Roy, J.R., Chakraborty, S., Chakraborty, T.R. Estrogen-like endocrine disrupting chemicals affecting puberty in humans—a review. Med Sci Monit 2009;15:RA137-145.
14 Carwile, J.L., Willett, W.C., Michels, K.B. Consumption of Low-Fat Dairy Products May Delay Natural Menopause . The Journal of Nutrition. October 2013; 143(10):1642–1650. https://doi.org/10.3945/jn.113.179739
15 Biro FM1, Greenspan LC, Galvez MP, Pinney SM, Teitelbaum S, Windham GC, Deardorff J, Herrick RL, Succop PA, Hiatt RA, Kushi LH, Wolff MS. Onset of breast development in a longitudinal cohort. Pediatrics. 2013 Dec;132(6):1019-1027.
16 Greenspan, L., Deardorff, J. The New Puberty: How to Navigate Early Development in Today’s Girls. Published by Rodale Books; October 20, 2015.