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Stroke

Synopsis

Although the mechanisms for how milk products may reduce the incidence of stroke remain to be fully explained, there is good evidence demonstrating a role for milk products in several conditions known to be risk factors for heart disease and stroke, including obesity, hypertension, metabolic syndrome and type 2 diabetes. Components of milk products that are implicated in reducing the risk of stroke include:

• Calcium,
• Magnesium,
• Potassium.

Contrary to the current hypothesis, emerging evidence also suggests that total dietary fat and saturated fat intake may not be detrimental to health and could even be protective with respect to stroke prevention.

The Evidence

A prospective cohort study published in 2009 followed 41,526 Japanese men and women from 1990 to 2003. The subjects were aged from 40 to 59 years and had no history of cardiovascular disease or cancer. The study found that both total and dairy calcium intakes are associated with a significantly lower risk of stroke incidence.¹

• Total calcium intake was associated with a significant 30% reduced risk of total stroke and intraparenchymal hemorrhage and a 28% reduced risk for ischemic stroke after adjustment for confounders;
• Dairy calcium was associated with a significant 31% reduced risk for total stroke and ischemic stroke;
• Non-dairy calcium was not significantly associated with risk for any end point.

From 1988 to 1990, a sub-cohort of the Japan Collaborative Cohort Study consisting of 21,068 men and 32,319 women with no previous history of stroke, coronary heart disease or cancer answered questionnaires pertaining to their milk, cheese and yogurt consumption.² According to the results:

• Individuals in the highest quintile of milk product consumption smoked less, drank less, had higher intakes of total energy and potassium and were less likely to have a history of hypertension than those in the lowest quintile;
• Greater dairy calcium intake was also associated with significantly lower mortality from stroke and cardiovascular disease, after adjustment for confounding factors;
• Total calcium had less of an impact than dairy calcium. Although total calcium did tend to be inversely associated with mortality from total and ischemic strokes in men and women, the only association to reach statistical significance was that of total stroke in women.

Relative risk in the highest vs. lowest quintiles of dairy calcium intake

Mortality from:	Men	Women
Total stroke	0.53 (p < 0.01)	0.57 (p < 0.04)
Hemorrhagic stroke	0.46 (ns)	0.51 (p < 0.04)
Ischemic stroke	0.53 (p < 0.01)	0.50 (ns)
Cardiovascular disease	0.73 (ns)	0.77 (p < 0.01)
ns = non-significant

In Japan, stroke remains the most prevalent cardiovascular disease. It has been observed that calcium intakes in this country are low compared to Western countries. According to national nutrition surveys, the mean dairy calcium intake by Japanese was 500 mg for men and 528 mg for women in 2004, whereas U.S. intakes were 966 mg and 765 mg respectively according to NHANES 1999 to 2000.² Studies conducted in the U.S. show similar findings to those conducted in Japan.

Data from the landmark prospective cohort Nurses’ Health Study, consisting of 85,764 women aged 34 to 59 years, demonstrated that intakes of calcium, potassium and magnesium were each inversely associated with age- and smoking-adjusted relative risks of ischemic stroke.³

• In a multivariate analysis, women in the highest quintile of calcium intake had an adjusted relative risk of 0.69, or a 31% reduced risk of ischemic stroke, p_trend = 0.03, compared to those in the lowest quintile;
• Further simultaneous adjustment for calcium and potassium intake suggested an independent association for calcium intake;
• The inverse association for calcium was stronger for dairy than for non-dairy calcium intake;
• Intakes of > 600 mg of calcium did not appear to reduce the risk further;
• Intake of calcium, potassium and magnesium were not related to risk of other stroke subtypes.

Similar results were found in the Honolulu Heart Program, a prospective cohort study of 3,150 older men of Japanese ancestry from 55 to 68 years residing on the Hawaiian island of Oahu and who were followed for 22 years.⁴ The results showed that:

• Men who did not consume any milk had double the incidence of stroke compared to men who consumed 16 oz./day or more (7.9 vs. 3.7 per 100 respectively, p < 0.05);
• Rates of thromboembolic stroke decreased significantly with increasing intake of dietary calcium from dairy sources (p < 0.01);
• Rates of thromboembolic stroke decreased significantly with increasing intake of dietary calcium from all sources and with increasing milk consumption (p < 0.05);
• Calcium intake from non-dairy sources was not related to stroke.

Potential Mechanisms

Although the mechanisms responsible for an inverse association between milk product intake and stroke are inconclusive, there nevertheless is an association between milk product consumption and the prevention of hypertension, a key risk factor for stroke.

For example, reductions in blood pressure of the magnitude achieved by the DASH diet—i.e., a diet rich in milk products, fruits and vegetables—could translate into a 15% reduction in ischemic heart disease and a 27% reduction in stroke.⁵

Milk products may also play an important role in healthy weight and the prevention of metabolic syndrome and type 2 diabetes which are also key risk factors for stroke.

Aside from calcium, other components of milk products that may be implicated include magnesium and the type of fat. In the Atherosclerosis Risk in Communities Study, dietary magnesium intake was inversely associated with ischemic stroke.⁶ Emerging evidence suggests that total fat and saturated fat intakes may not be detrimental to health and could even be protective. Data from the Framingham Heart Study of 832 men aged 45 to 65 years indicated that intakes of total fat, saturated fat and monounsaturated fat were significantly associated with a reduced risk of ischemic stroke.⁷ However, no significant associations were found between total fat and fat subtype consumption and the risk of stroke in the Health Professionals Follow-Up Study of more than 43,000 U.S. male health professional followed for 14 years.⁸

Conclusions

There is good evidence to date to indicate that adequate or higher milk product intake may significantly reduce the risk factors associated with heart disease and stroke and the incidence of stroke itself.

Randomized controlled trials and mechanistic studies are needed to provide more conclusive answers.

As emerging evidence indicates a potential role for the type of fat, more studies are needed on the role of specific milk products, including those that are higher and lower in fat content.

Keywords: stroke, cardiovascular disease

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