The Dairy Dilemma: Part 4: Animal Protein

Protein from your diet is not all the same. Humans obtain protein through eating both animal-sourced foods and plant-sourced foods. The proteins from these differing sources vary in the amino acids that are their building blocks. Animal proteins, being very similar to our own proteins, are readily and rapidly available for use by human bodies. Plant proteins, with an amino acid content more dissimilar to human proteins, are absorbed more slowly and, in the past, been considered inferior and “incomplete”. We now know that this is an inaccurate point of view.

Specific plant proteins may lack one or more of the amino acids that human beings require but this is not a disadvantage for many reasons. Firstly, consuming a variety of plant proteins provides all the amino acids needed by human beings but with the benefit of slower assimilation into our bodies. Secondly, easy to absorb animal-sourced protein can result in the metabolism of high amounts of protein, putting stress on digestive organs such as the liver (1). Thirdly, higher protein intake does not lead to better health. In fact, animal-sourced protein can be quite destructive to human beings. Studies show that replacing animal protein in the diet with plant protein is linked to a 34% decreased risk of early death from all causes (2) and a recent review found that cardiovascular disease risk can be reduced by eating more plant sources of protein (3).

Two factors present in animal-sourced proteins that are responsible for some of its harmful effects in humans are IGF-1 and the activation of mTOR.

Ingesting Dairy Protein Can Increase IGF-1 Level

What is IGF-1?
IGF-1 (Insulin-like Growth Factor-1) is a natural human growth hormone that is important for normal growth during childhood but can promote abnormal growth such as cancers in adulthood. IGF-1 is produced primarily by the liver. Its production is initiated by growth hormone (GH) which is produced by the anterior pituitary gland. IGF-1 stimulates body growth both by producing new cells and by elevating growth in almost every cell of the body, but especially cells in skeletal muscle, cartilage, bone, liver, kidney, nerves, skin, hematopoietic cells (cells that produce components of the blood) and lung cells. The action of IGF-1 is independent of that of growth hormone although the two hormones can work synergistically as well (4). Much of the IGF-1 in the bloodstream is bound to IGFBP (Insulin-like Growth Factor Binding Protein) which reduces its activity. IGF-1 is produced throughout life but reaches its highest levels during the growth spurt of puberty (5).

Dairy Protein and IGF-1
Intake of protein increases IGF-1 levels in humans independent of calorie consumption (6). But further investigation reveals that it is the source of the protein that is the primary factor influencing levels of IGF-1 in the blood. Research has shown that IGF-1 levels are increased by consuming animal protein but not by eating plant protein (7).

Ingestion of milk and dairy products is linked to increased IGF-1 levels;
One study found that drinking three 8-ounce glasses of milk for 12 weeks resulted in a 10% increase in IGF-1 levels (8).
A 2014 study reported that an increase in daily milk intake by three servings (about 30 gm of protein) was associated with an average 18.6% increase in the blood level of free IGF-1 in women. Free IGF-1 is the most bioactive state of IGF-1 (9).
Another study found that high intake of skimmed milk (but not meat) increased both IGF-1 and IGFBP in eight-year-old boys. The boys were asked to consume either 1.5 liters of skimmed milk or 250 gm of low-fat meat daily for 7 days. The amount of protein contained in these two foods was similar. Protein intake increased by 61% in the milk-drinking group and by 54% in the meat-eating group. The higher milk intake elevated IGF-1 by 19% and IGFBP-3 by 13%. There were no increases in the meat group (10).

Consequences of high IGF-1 level
Early studies fed casein to rats primed with a dose of aflatoxin, a substance known to produce pre-cancerous lesion. Casein is the primary protein found in dairy. When casein levels were above 20% of energy from calories, the pre-cancerous lesions proliferated. Casein levels below 20% were associated with no lesion growth. The mechanism for this was considered to be the change in IGF-1 levels produced by the ingestion of casein (11).
Other animal studies show that blocking IGF-1 receptors prevents mammary cancers (12).

What about effects of IGF-1 in humans?
IGF-1 has been implicated in many forms of cancer (13,14).
People with low levels of IGF-1 show greatly reduced risk of cancer development (15).
A 2004 symposium examined the accumulating evidence that risk of human cancers of the colon, pancreas, endometrium, breast and prostate are related to blood levels of insulin, IGF-1 or both. Results showed that both insulin and IGF-1 stimulate growth and can promote tumour development by increasing proliferation of tumour cells and inhibiting cancer cell death (16).
A meta-analysis from 2009 illustrated that raising levels of IGF-1 circulating in the bloodstream is associated with increased prostate cancer risk (17).
A 2010 study showed that increased IGF-1 blood levels are associated with higher risk of breast-cancer (18).

Reducing IGF-1
Development of drug therapies aimed at reducing the effect of IGF-1 on cancer have been disappointing, prompting researchers to call for further research in this area (19). However, there are other steps that can be taken to reduce IGF-1 levels. Investigations have discovered that IGF-1 activity can be lessened by reducing animal protein in the diet, increasing plant protein in the diet and increasing physical activity (16,20,7).
Completely plant-based diets show decreased levels of circulating IGF-1 with associated lower risks for common “Western” cancers such as breast cancer, endometrial cancer, colon cancer, prostate cancer, pancreatic cancer, ovarian cancer and lung cancer. Conversely, high intake of animal products is linked to higher IGF-1 levels and higher risk of these cancers (7). One study showed that vegan diets are associated with 13% lower levels of IGF-1 and 20 to 40% higher levels of IGFBP-1 and IGFBP-2 (IGFBP is Insulin-like Growth Factor Binding Protein that ties up IGF-1, reducing its effects) (21).

Ingesting dairy Protein can activate mTOR

What is mTOR?
mTOR stands for “mammalian target of rapamycin” and it is an enzyme that appears to play a major role in how long we live. (Rapamycin is a drug that can inhibit mTOR. Unfortunately, it has only modest benefits for human disease and many adverse effects that can be severe (22).) mTOR is a promotor of normal growth in childhood but in adulthood it fuels aging. mTOR evolved in human beings long before we were living into our 70s and 80s. Its goal was to keep us alive by encouraging rapid growth, preventing early death by predation and allowing us to live to the age of reproduction. And so, when food was plentiful, mTOR encouraged cell division and growth. On the other hand, when food was lacking, mTOR slowed growth and promoted “autophagy” (directly translated as “self eating”). Autophagy shifts the energy source from food to damaged and unneeded substances in our bodies such as dead or malfunctioning cells and proteins. In autophagy these “garbage” substances can be used as fuel or for the production of new healthy cells, proteins and other molecules necessary for life (23). These mTOR activities may have helped our distant relatives survive but today, in an age of over-consumption of food, mTOR is a hazard.

Dairy protein and mTOR
Milk from a mammal such as a cow is much more than simply one out of many beverage choices. Cow’s milk acts as an endocrine-signalling system that activates mTOR and promotes cell growth and proliferation. Though this is beneficial in infancy and childhood when we need our cells to proliferate so that we grow, sustained signals to cells to divide and grow in later life may be a factor in the development and growth of cancer (24).

Certain amino acids promote greater mTOR activity. The amino acid that appears to have the most effect on mTOR is leucine. Leucine is abundant in animal-sourced foods including meat, chicken, fish, eggs and dairy products. Plants foods generally have far lower leucine content (25).

Consequences of high mTOR activity
mTOR activity has been implicated in several disease processes including cancer; type-2 diabetes; diseases of the eye such as macular degeneration and diabetic retinopathy; neuro-degenerative disorders; and obesity (26,27).

Evidence from epidemiological studies reveals that high consumption of dairy protein is a major dietary promoter for the risk and development of prostate cancer. The increase in mTOR activity caused by consuming milk from cows combined with constant exposure to the increased estrogen levels in today’s commercial cow’s milk may explain the association of ingesting cow’s milk and the increased risk of prostate cancer observed in Western societies (24).

Higher expression of mTOR has been observed in breast cancer as well and is associated with more aggressive disease and worse overall survival (28).

Reducing the activity of mTOR
Inhibition of the mTOR signalling pathway appears to play a pivotal role in increasing lifespan in mammals including humans (26). Controlling mTOR might also improve the diseases that it promotes and ongoing research is looking for such therapies that might work in human beings. However, because mTOR activity is central to growing and dividing cells, strong inhibition of mTOR might very well be problematic for human beings (27).

Instead of looking for drugs to affect mTOR activity, perhaps we should turn our attention to less drastic measures. Simply restricting calories can cause mTOR activity to slow down (29,30). And so does the restriction of protein. In fact, the mTOR-slowing effects of restricting calories may be derived exclusively from the restriction of protein that results from cutting down on calories (31). Studies have illustrated this with the discovery that reducing protein, even without any change in calorie level, can suppress mTOR (32). There are a couple of other bonuses here as well. First of all, restricting protein is much easier to do than restricting calories. Secondly, restricting protein, especially animal protein, also suppresses IGF-1 (33).

Plants themselves contain natural inhibitors of mTOR, introducing another great reason to eat more of them. Some plant foods that are especially good at decreasing mTOR activity include cruciferous vegetables such as broccoli; onions; grapes; strawberries; mangoes; green tea; and turmeric (34).

In summary….

We once believed that protein derived from animals was superior to plant protein for human beings. Research though is telling us otherwise. The protein from cow’s milk increases both IGF-1, a substance linked to greater cancer risk, and mTOR activity, a pathway that accelerates aging through a variety of pathways. Decreasing our intake of dairy foods and other sources of animal protein along with increasing plant protein in our diet is not difficult and may provide us with benefits that include lower risks of chronic diseases and more chance of living a long, healthy life. The effects created by protein restriction may help to explain the unusually long and healthy lifespans of civilizations such as those in the Blue Zones located around the world.



2 Song, M., Fung, T.T., Hu, F.B., Willett, W.C., Longo, V.D., Chan, A.T.,Giovannucci, E.L. Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality.
JAMA Intern Med. 2016;176(10):1453-1463.

3 Richter, C.K., Skulas-Ray, A.C., Champagne, C.M., Kris-Etherton, P.M. Plant Protein and Animal Proteins: Do They Differentially Affect Cardiovascular Disease Risk?. Adv Nutr. 2015 Nov; 6(6): 712–728.

4 Laron, Z. Insulin-like growth factor 1 (IGF-1): a Growth Hormone. Mol Pathol. 2001 Oct; 54(5): 311–316.

5 Yakar, S., Rosen, C.J., Beamer, W.G., Ackert-Bicknell, C.L., Wu, Y. et al. Circulating levels of IGF-1 directly regulate bone growth and density. Journal of Clinical Investigation. 2002; 110 (6): 771–781.

6 Levine, M.E., Suarez, J.A., Brandhorst, S., Balasubramanian, P., Cheng, C.W., Madia, F. et al. Low protein intake is associated with a major reduction in IGF-1, cancer, and overall mortality in the 65 and younger but not older population. Cell Metabolism. 2002; 19 (3): 407–417.

7 McCarty, M.F. Vegan proteins may reduce risk of cancer, obesity, and cardiovascular disease by promoting increased glucagon activity. Med Hypotheses. 1999 Dec; 53(6):459-485.

8 Heaney, R.P., McCarron, D.A., Dawson-Hughes, B., Oparil, S., Bergam, S.L., Sterns, J.S., Barr, S.I., Rosen, C.J. Dietary Changes Favorably Affect Bone Remodeling in Older Adults. J Am Dietetic Ass. Oct 1999; 99(10): 1228-1233.

9 Beasley, J.M., Gunter, M.J., LaCroix, A.Z., Prentice, R.L., Neuhouser, M.L., Tinker, L.F., Vitolins,M.Z., Strickler, H.D. Associations of Serum Insulin-like Growth Factor (IGF-I) and IGFBP-3 Levels Biomarker-Calibrated Protein, Dairy, and Milk Intake in the Women’s Health Initiative.
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10 Hoppe, C., Mølgaard, C., Juul, A., Michaelsen, K.F. High intakes of skimmed milk, but not meat, increase serum IGF-I and IGFBP-3 in eight-year-old boys. European Journal of Clinical Nutrition 2004; 58: 1211-1216.

11 Dunaif, G.E., Campbell, T.C. Relative contribution of dietary protein level and aflatoxin B1 dose in generation of presumptive preneoplastic foci in rat liver. J Natl Cancer Inst. 1987 Feb; 78(2):365-369.

12 Kleinberg, D.L., Wood, T.L., Furth, P.A., Lee, A.V. Growth hormone and insulin-like growth factor-I in the transition from normal mammary development to preneoplastic mammary lesions. Endocr Rev. 2009 Feb;30(1):51-74.

13 Arnaldez, F.I., Helman, L.J. Targeting the insulin growth factor receptor 1. Hematology/Oncology Clinics of North America. June 2012; 26 (3): 527–42, vii–viii.

14 Yang, Y., Yee, D. Targeting insulin and insulin-like growth factor signaling in breast cancer. Journal of Mammary Gland Biology and Neoplasia. Dec 2012; 17 (3–4): 251–261.

15 Gallagher, E.J., LeRoith, D. Is growth hormone resistance/IGF-1 reduction good for you?. Cell Metab. April 2011; 13 (4): 355–356.

16 Kaaks, R. Nutrition, insulin, IGF-1 metabolism and cancer risk: a summary of epidemiological evidence. International Agency for Research on Cancer, Lyon, France. Novartis Found Symp. 2004;262:247-60; discussion 260-68.

17 Rowlands,M.A., Gunnell, D., Harris, R., Vatten, L.J., Holly, J.M., Martin, R.M. Circulating insulin-like growth factor peptides and prostate cancer risk: a systematic review and meta-analysis. Int J Cancer. 2009 May 15;124(10):2416-2429.

18 Endogenous Hormones and Breast Cancer Collaborative Group; Key, T.J., Appleby, P.N., Reeves, G.K., Roddam, A.W. Insulin-like growth factor 1 (IGF1), IGF binding protein 3 (IGFBP3), and breast cancer risk: pooled individual data analysis of 17 prospective studies. Lancet Oncol. 2010 Jun;11(6):530-542

19 Girnita, L., Worrall, C., Takahashi, S., Seregard, S., Girnita, A. Something old, something new and something borrowed: emerging paradigm of insulin-like growth factor type 1 receptor (IGF-1R) signaling regulation. Cellular and Molecular Life Sciences. July 2014; 71 (13): 2403–2427.

20 Fontana, L., Klein, S., Holloszy, J.O. Long-term low-protein, low-calorie diet and endurance exercise modulate metabolic factors associated with cancer risk. The American Journal of Clinical Nutrition, Dec 2006; 84(6): 1456–1462.

21 Allen, N.E., Appleby, P.N., Davey, G.K., Kaaks, R., Rinaldi, S., Key, T.J. The associations of diet with serum insulin-like growth factor I and its main binding proteins in 292 women meat-eaters, vegetarians, and vegans. Cancer Epidemiol Biomarkers Prev. 2002 Nov; 11(11):1441-1448.


23 Blagosklonny, M.V. TOR-driven aging: Speeding car without brakes. Cell Cycle. 2009 8(24):4055 – 4059.

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28 Wazir, U., Newbold, R.F., Jiang, W.G., Sharma, A.K., Mokbel, K. Prognostic and therapeutic implications of mTORC1 and Rictor expression in human breast cancer. Oncol Rep. 2013 May;29(5):1969-1974.

29 Pallavi, R., Giorgio, M., Pelicci, PG. Insights into the beneficial effect of caloric/ dietary restriction for a healthy and prolonged life. Front Physiol. 2012 3:318.

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34 Jung, C.H., Kim, H., Ahn, J., Jeon, T.I., Lee, D.H., Ha, T.Y. Fisetin regulates obesity by targeting mTORC1 signaling. J. Nutr. Biochem. 2013 24(8):1547 – 1554.

Promoting a healthy adventurous lifestyle powered by plants and the strength of scientific evidence.

My name is Debra Harley (BScPhm) and I welcome you to my retirement project, this website. Over the course of a life many lessons are learned, altering deeply-rooted ideas and creating new passions.


  1. Carolyn on July 5, 2019 at 4:31 pm

    Nice summary

    • Deb on July 15, 2019 at 8:10 am

      Thanks so much Carolyn.

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