According to some studies, milk may increase the risk of diabetes. However, the reliability of these studies is questionable given the authors’ commercial interests.
In part I, “Milk and its influence on muscle growth and weight loss,” I discussed the general properties and benefits of milk regarding muscle growth and weight loss. Additionally, I wrote about certain groups of people who cannot tolerate milk and the physiological reasons behind it. In this second part, I will delve into the potential health hazards of milk in the long term. You will hear terms like “possible” and “alleged” more often in this piece because opinions vary widely and studies show diverse results.
New Zealand Study: “More Milk Consumption, More Cases of Type I Diabetes”
Most alleged health hazards caused by milk are related to its effect on insulin. Therefore, it might be useful to first read the piece I wrote some time ago about carbohydrates and the function of insulin.
The concerns about the dangers of most types of milk mainly come from Australia and New Zealand. An initial impetus was a report published in 1992 after a symposium in Paris a year earlier [1]. This report described the discovery that children in Polynesian islands like Western Samoa had much lower rates of type I diabetes than Polynesian children in Auckland (New Zealand’s largest city). In children with type I diabetes, they found more milk antibodies, and the differences in the number of diabetes cases between the islands and Auckland correlated with the difference in milk consumption.
However, that’s not the whole story. As mentioned, opinions differ regarding the influence of milk on diabetes.
Boston 2002 Study: “Dairy Intake Reduces Risk of Insulin Resistance”
A large-scale American study conducted at Harvard University on the influence of milk on insulin resistance (often a precursor to diabetes and a cause of obesity) concluded that dairy intake actually reduces the risk of insulin resistance and all its symptoms [2]. They concluded, among other things, that daily dairy consumption reduces the risk of IRS (Insulin Resistance Syndrome) by 21%.
Increased dairy intake, regular or reduced fat, was significantly associated with a lower incidence of IRS and each of its components (obesity, hypertension, abnormal glucose and dyslipidemia) among individuals who were overweight (BMI ³25 kg/m2) at baseline. Ten-year incidence of IRS was lower by more than two-thirds among overweight participants in the highest category of dairy intake compared to the lowest category. The associations were dose-dependent. Each daily occasion of dairy consumption was associated with a 21% lower odds of IRS (odds ratio = 0.79).
-M.A. Pereira, Children’s Hospital Boston, Harvard
This was about dairy in a broad sense, whereas in New Zealand, it was specifically about milk. Additionally, the Americans looked at insulin resistance, not cases where diabetes (type I) was already present. However, it’s striking that the conclusion here is the opposite: the more dairy, the lower the risk of insulin resistance. However, this was only true for overweight individuals. They found no correlation in slim people.
Below you can see the risk of insulin resistance compared to the number of times milk is consumed weekly or other dairy products are eaten/drank. With larger quantities, you indeed see the risk decreasing. However, the graph also shows that consuming dairy 10 to 16 times a week (let’s say 1.5 to 2 times a day) offers a 15% higher risk of IRS than consuming dairy 0 to 10 times a week (0 to 1.5 times a day). However, this difference does not seem statistically significant. Otherwise, one would think you should either not consume dairy at all or consume it at least twice a day (but not in between).
An important point is that they looked at dairy in a broad sense and not just milk. Even more important is that the amount per serving was not taken into account. In my opinion, this makes the data less relevant. The number of times dairy is eaten does not necessarily reflect the total amount of dairy consumed.
Boston 2010 Study: “Low-Fat Dairy Reduces Risk of Type 2 Diabetes in Women. No Effect of Milk”
In 2010, researchers from UCLA examined the possible relationship between dairy and type 2 diabetes [3]. Out of over 37,000 participants who did not have diabetes at the start of the Women’s Health Study, their dairy intake was recorded. They were followed for 10 years. During this period, 1603 women developed type 2 diabetes. Even after considering other risk factors such as smoking and weight, there was a negative association between dairy intake and the presence of type 2 diabetes. Each daily dairy intake reduced the risk of type 2 diabetes by 4%.
the relative risk for type 2 diabetes among women in the highest quintile of dairy intake was 0.79 (95% CI 0.67-0.94; P for trend = 0.007) compared with those in the lowest quintile. Each serving-per-day increase in dairy intake was associated with a 4% lower risk (0.96 [0.93-1.01]). The inverse association with type 2 diabetes appeared to be mainly attributed to low-fat dairy intake; the multivariate relative risks comparing the highest to the lowest quintiles was 0.79 (0.67-0.93; P for trend = 0.002) for low-fat dairy.
-S. Liu, Brigham and Women’s Hospital, Boston
Here too, we are talking about dairy and not specifically about milk. However, the data to make this distinction is available. The researchers found that low-fat dairy such as yogurt and skimmed milk was primarily responsible for this effect. Looking at milk specifically, they found a positive correlation, but it was too small to be statistically significant (could have been due to chance).
Cardiff Study: “Milk has no effect on the risk of diabetes”
Researchers from Cardiff University examined men aged 45-59 years (from the “Caerphilly cohort study”) for metabolic syndrome, a condition affecting energy storage and consumption [4]. This includes insulin resistance and thus increases the risk of diabetes [5]. Like in the above study, they compared cases of metabolic syndrome with the diets as recorded in surveys. Their research showed that milk intake actually reduced the risk of metabolic syndrome. When specifically looking at diabetes, they found no association.
Milk intake showed no significant trend with incident diabetes…
…The consumption of milk and dairy products is associated with a markedly reduced prevalence of the metabolic syndrome, and these items therefore fit well into a healthy eating pattern.
-P.C. Elwood, Cardiff University
Similar conclusions were reached by researchers from Iran [6]. They suspected this was related to the calcium in dairy.
Dairy consumption is inversely associated with the risk of having metabolic syndrome. It seems that this relation is somewhat attributed to calcium.
-L. Azadbakht, Shaheed Beheshti University of Medical Sciences
A1 Beta-casein: The “Diabetes Protein”?
Various studies have looked into a possible specific culprit in milk (and the risk of) diabetes. These point to the influence of a type of protein found in milk. As mentioned in part I, 80% of the protein in milk is casein. Casein can be further divided into different types, including Beta-casein. Originally, there was one type of Beta-casein called A2, but thousands of years ago, a natural mutation caused some cows to produce milk with a different variant of Beta-casein, called A1 Beta-casein.
Several studies have “shown” that A1 Beta-casein can actually increase the risk of diabetes and heart diseases. I came across the book “Devil in the Milk,” in which Professor Woodford from Lincoln University in Christchurch, New Zealand, cited over a hundred studies to demonstrate that the A1 variant is harmful to health [7]. According to Woodford, removing A1 beta-casein from milk is feasible. He points to A2 Corp., a company from New Zealand that can test cows for this A1 mutation. A2 Corp. can help dairy manufacturers produce milk with the harmless A2 variant. This takes about two generations or 10 years.
According to Woodford, type 1 diabetes and cardiovascular diseases are more common in countries with the highest concentrations of A1 in milk. He studied this among rodents. He gave rats A1 beta-casein or A2. In the group that received A2, none of the rats developed diabetes, whereas 50% of the rats in the A1 group developed diabetes.
Commercial Interests in the A1 & A2 Milk Debate
However, one might question how independent Woodford is. A2 Corp has once sued a major dairy manufacturer because they believed that the (supposed) dangers of their A1 milk should be stated on the packaging. Clearly, this is a marketing ploy to sell more A2 milk. Actions like this immediately make me skeptical about firm statements on A1 milk, especially when they come from the Australia and New Zealand region.
This is especially true for research funded by A2 Corp itself, such as McLachlan’s [8]. He compared the consumption of A1 milk in countries like Ireland, France, and Germany with type I diabetes.
…beta-casein A1 consumption also correlates strongly with type 1 diabetes incidence in 0-14-year-olds, suggesting that IHD and diabetes may share at least one causative risk factor.
– C.N. McLachlan, A2 Corporation Ltd
Maybe I’m too skeptical, but as soon as I read that the researcher works for a company that has a commercial advantage in the study’s conclusion, I usually don’t even bother to read further. I’ve only mentioned this here for completeness.
I also raised questions about the following research, which also comes from New Zealand [9]
They raise the possibility that intensive dairy cattle breeding may have emphasized a genetic variant in milk with adverse effects in humans. Further animal research and clinical trials would be needed to compare disease risks of A1-free versus ‘ordinary’ milk.
-M. Laugesen, Health New Zealand
Health New Zealand? The above researcher does not work for a university but for Health New Zealand. It turns out that this is an organization committed to raising political awareness of health-related issues. They have been particularly active as an anti-lobby for the tobacco industry. Currently, they are asking for sponsors for various studies on their website. I can imagine that A2 Corp would be eager to finance some studies on A1 milk. So, I searched for the full text and eventually found it. I immediately scrolled down to where any conflicts of interest are usually disclosed, and… yes:
Conflicts of interest: The IHD section of this paper was funded by a grant from A2 Corporation, Auckland, to the first author, who is a minor shareholder in A2
Corporation. Both authors are directors of the NZ Milk Institute Ltd, which owns a patent related to A1-free milk.
It’s even worse than I thought. Both researchers involved in the study are directors of the NZ Milk Institute, which holds a patent for producing A1-free milk. Moreover, the lead researcher is a shareholder in A2 Corp! I’m not 100% sure, but I believe the first study I showed in this article is from the same Elliott who is a director of the NZ Milk Institute [1].
Conclusion
It’s hard to draw a conclusive stance on the potential impact of milk on the risk of diabetes (and its precursors) when the debate is tainted by commercial interests. If you have to conduct a study just to check the researcher’s interests, things don’t get any clearer.
Many of the (independent) studies end with the sentence that more research is needed to provide clarification. While this might seem like a standard line in many studies, here it’s more than justified.
I can’t rule out that milk has an impact on the risk of diabetes and whether this impact is positive or negative. The studies point in all directions. Personally, I found the studies showing a negative association (and especially the researchers conducting them) not convincing enough to outweigh the benefits of milk that have been much better established (see part I).
References
- Elliott RB. (1992) Epidemiology and Etiology of Insulin-Dependent Diabetes in the Young InEpidemiology and Etiology of Insulin-Dependent Diabetes in the Young eds Levy-Marchal C & Czernichow P, Vol 21, pp 66−71 Basel: Karger.
- Pereira MA, Jacobs DR, Van Horn L et al. Dairy Consumption, Obesity, and the Insulin Resistance Syndrome in Young Adults. The CARDIA Study. JAMA 2002;287:2081-2089.
- Liu S, Choi HK, Ford E, Song Y, Klevak A, Buring JE, Manson JE. A prospective study of dairy intake and the risk of type 2 diabetes in women. Diabetes Care.
2006 Jul;29(7):1579-84. - Elwood PC, Pickering JE, Fehily AM. Milk and dairy consumption, diabetes and the metabolic syndrome: the Caerphilly prospective study. J Epidemiol Community Health. 2007 Aug;61(8):695-8.
- Wannamethee SG, Shaper AG, Lennon L, Morris RW. Metabolic syndrome vs Framingham Risk Score for prediction of coronary heart disease, stroke, and type 2 diabetes mellitus. Arch Intern Med. 2005 Dec 12-26;165(22):2644-50.
- Azadbakht L, Mirmiran P, Esmaillzadeh A, Azizi F. Dairy consumption is inversely associated with the prevalence of the metabolic syndrome in Tehranian adults. Am J Clin Nutr. 2005 Sep;82(3):523-30.
- Keith Woodford. Devil in the Milk: Illness, Health and the Politics of A1 and A2 Milk Paperback – March 9, 2009
- Laugesen M, Elliott R. Ischaemic heart disease, Type 1 diabetes, and cow milk A1 beta-casein. N Z Med J. 2003 Jan 24;116(1168):U295.
