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Type 2 Diabetes

Synopsis

While the exact mechanisms of this relationship remain to be elucidated, it also appears that adequate or higher milk-product intakes are associated with a significant and substantial reduction in the risk of developing metabolic syndrome, an important risk factor for both type 2 diabetes and cardiovascular disease. Several components or aspects of milk products may be responsible for this beneficial role, such as their low glycemic index and their content in the following nutrients:

• Calcium,
• Magnesium,
• Vitamin D.

The Evidence

A systematic review with meta-analysis, published in 2008, which examined the evidence from 4 prospective cohort studies in relation to milk product intake and the incidence of type 2 diabetes indicated that the relative risk for type 2 diabetes is almost 10% lower in people who had a high milk intake.¹

In another systematic review with meta-analysis, published in 2007, calcium, vitamin D and milk products were implicated with a reduced risk for development of type 2 diabetes and metabolic syndrome, an important risk factor in the development of type 2 diabetes.²

• Data from prospective cohort studies indicate that an adequate or higher intake of milk products significantly reduces the risk of developing type 2 diabetes, summary odd ratio (OR) = 0.86 (95% CI, 0.79-0.93) for the highest versus lowest milk product intake (i.e. 3 to 5 servings/day versus < 1.5 servings/day);
• The prevalence of metabolic syndrome was 0.71 (95% CI, 0.57-0.89) for the highest milk product intake (3 to 4 servings/day) versus the lowest (0.9-1.7 servings per day);
• Low calcium intake is consistently found to be inversely associated with incident type 2 diabetes or metabolic syndrome;
• The summary OR was 0.82 (95% CI, 0.72-0.93) for the highest versus lowest calcium intakes (661-1200 mg/day versus 219-600mg/day);
• The evidence from observational studies suggests an association between low vitamin D status and calcium intakes (including low milk products intake) and risk of type 2 diabetes or metabolic syndrome;
• After multivariate analysis, it was observed in the Nurses Health Study that women with the highest calcium (>1200mg/day) and vitamin D (> 800 IU.day) intakes had a 33% lower risk of type 2 diabetes compared with women with the lowest calcium (< 600 mg/day) and vitamin D (< 400 IU/day) intakes;
• The lower risk seen with the combined intake of calcium and vitamin D was more than that seen with the highest intake of each nutrient separately, which highlights the importance of both nutrients as potential modifiers of type 2 diabetes risk.

Data from the Health Professionals Follow-up Study, an ongoing longitudinal study of male health professionals (mean age of 53 years at baseline) initiated in 1986, indicated that, after adjustments of important confounders, a higher total intake of milk products reduces the risk of developing type 2 diabetes in men.³ The following data were obtained after 12 years of follow-up:

• In a cohort of 41,254 participants with no history of diabetes, cardiovascular disease or cancer, there were 1,243 incident cases of type 2 diabetes;
• Men in the highest quintile of total milk product intake (≥ 2.9 servings low and high fat, excluding butter) had a 23% lower risk of developing type 2 diabetes compared with those in the lowest quintile (< 0.9 servings), after adjustment for potential confounders such as body mass index (BMI), physical activity and known dietary risk factors. This reduction represents a relative risk (RR) of 0.77 (p_trend = 0.003);
• Each serving-per-day increase in total milk product intake was associated with a significant 9% reduced risk of developing type 2 diabetes (RR = 0.91, p_trend < 0.001);
• The corresponding RR was 0.88 for intake of low-fat milk products, while high-fat milk products conferred neither risk nor benefit with respect to the risk of developing type 2 diabetes (RR = 0.99).

Similar findings were demonstrated in women according to data from the Women’s Health Study. In a prospective cohort study of 37,183 women (mean age of 55 years at baseline) with no heart disease, stroke or cancer, 1,603 incident cases of type 2 diabetes were documented during 10 years of follow-up. A higher total intake of milk products was associated with a significantly lower risk of developing type 2 diabetes after adjusting for important confounders.³

• Women in the highest quintile of total milk product intake (> 2.9 servings low and high fat, excluding butter) had a 21% lower risk of developing type 2 diabetes compared with those in the lowest quintile (< 0.85 servings), after adjustment for potential confounders including total energy, total fat, calcium, vitamin D, magnesium and glycemic load. This reduction represents an RR of 0.79 (p_trend = 0.007);
• Each serving-per-day increase in total milk product intake was associated with a 4% lower risk for type 2 diabetes;
• This inverse association appears to be mainly due to lower fat milk products. Women in the highest quintile of lower fat milk product intake had a 36% lower risk of developing type 2 diabetes compared to those in the lowest quintile (RR = 0.64, p_trend = 0.0001);
• Intake of yogurt was inversely associated with the risk of type 2 diabetes. Women who consumed 2 or more servings/week had an 18% lower risk of developing type 2 diabetes compared with women who consumed < 1 serving/month (RR = 0.82, p_trend = 0.03).

Similarly, a prospective cohort study involving 41,186 U.S. Black women (the Black Women’s Health Study), higher milk product intake, was associated with a significant reduction in the risk of developing type 2 diabetes.⁵

• Women who consumed one serving per day or more of lower fat milk products had a 13% lower risk of developing type 2 diabetes compared to women who consumed less than one serving of lower fat milk products per week after adjusting for important confounders;
• A higher magnesium intake was significantly associated with a lower risk of type 2 diabetes;
• A higher calcium intake was not independently associated with risk of type 2 diabetes;
• Of the food sources, consumption of whole grains and lower fat milk products were associated with a lower risk of type 2 diabetes.

The aforementioned studies have been conducted in the context of a western lifestyle. A study published in 2009 assessed the intakes of calcium and magnesium as well as milk consumption in a prospective cohort of middle-aged Chinese women.⁶ According to the Shanghai Women’s Health Study, a prospective study of 64,191 middle-aged (40 to 70 years) women who were free from type 2 diabetes or other chronic diseases at baseline, an inverse association between the intake of calcium (all sources) and magnesium (milk and animal sources only) and the risk of type 2 diabetes was observed.⁶ Milk intake was also associated with a reduced risk of developing type 2 diabetes and, in the case of fresh as opposed to powdered milk, a dose-response was observed.⁶

• Women in the highest quintile of calcium intake (649.9 mg/day from all sources) had a significantly lower risk (RR = 0.73, p_trend < 0.0001) for developing type 2 diabetes compared to women in the lowest quintile (277.5 mg/day);
• Women with the highest intake of calcium from milk (208 mg/day) compared to no calcium from milk had a significant 36% reduced risk of developing type 2 diabetes;
• Women with the highest intake of magnesium from milk (22 mg/day) compared to no magnesium from milk had a significant 34% reduced risk of developing type 2 diabetes;
• Women who consumed milk had a lower risk of developing type 2 diabetes than those who consumed no milk. There was a 36% to 40% reduction in risk between those who consumed milk compared to those who did not consume any milk, with a greater reduction in those who consumed the most milk (> 200 g/day);
• Although the fat content of milk was not assessed, most participants in this population have access only to whole milk.

Potential Mechanisms

The mechanisms behind the inverse association of milk product consumption and type 2 diabetes remain to be elucidated.

Calcium and Vitamin D may reduce the risk of type 2 diabetes via their role in modulating pancreatic beta cell function, insulin resistance and inflammation, some of the key mechanisms believed to be involved in the etiology of type 2 diabetes.²

Milk products as a whole have also been associated with metabolic syndrome, a very important risk factor for the development of type 2 diabetes.

Conclusions

There is very good, consistent evidence to date to suggest that adequate or higher intakes of milk products may significantly reduce the risk of developing type 2 diabetes.

Randomized controlled trials, as well as mechanistic studies, are needed to provide more definitive answers.

Keywords: type 2 diabetes, metabolic syndrome

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