Skimmed milk and blood pressure

“Drinking a glass of skimmed milk a day can cut blood pressure by up to a third,” reported The Daily Telegraph . The newspaper said that a study in the Netherlands has found that middle-aged people who consumed more “healthy dairy products, such as skimmed milk and low-fat yogurts” were less likely to develop high blood pressure later on.

This study investigated whether a diet low in saturated fat has a direct effect on blood pressure. It found that a higher consumption of dairy, and specifically low-fat dairy, lowered a person’s chances of having high blood pressure two years later. However, this association was not present at a six-year follow-up and there are other limitations with the study. This research does not prove that drinking skimmed milk lowers blood pressure or leads to a healthy heart. However, there is a large body of evidence that shows lower levels of saturated fat in the diet are better for health, and this study supports this idea.

Where did the story come from?

The research was carried out by Marielle F Engberink and colleagues from the Wageningen University and Research Centre and the Erasmus Medical Centre in the Netherlands. The study was published in the peer-reviewed medical journal American Journal of Clinical Nutrition.

What kind of scientific study was this?

This was a cohort study, which aimed to examine whether dairy products are associated with incidence of high blood pressure (hypertension) in older Dutch men and women.

The study assessed members of the Rotterdam Study, which is a population-based study looking at the incidence and progression of chronic diseases and their risk factors in people aged 55 or over. Participants in this age group were recruited between 1990 and 1993 from a suburb of Rotterdam. Anyone who fit the criteria was eligible to take part and 7,983 people (78% of those asked) agreed to participate. These people were interviewed and 89% of them were physically examined. The participants completed a checklist about what food and drink they had consumed in the previous year, their general food habits and their use of supplements. They were then interviewed by a trained dietitian, who used a 170-item semi-quantitative food-frequency questionnaire. The researchers say this was comparable to a two-week food diary. Dietary data was converted to daily total energy and nutrient intake using a standardised method. The participants were reassessed between 1993 and 1995 (79% response) and 1997 and 1999 (76% response).

The researchers calculated total dairy intake by adding up the intake of individual dairy items (excluding butter and ice cream) and then defining five categories of dairy foods: milk and milk products, cheese, low-fat dairy, high-fat dairy, and fermented dairy. For each of these five types of dairy, participants were grouped into four categories of intake from the lowest (about one serving a day or 164g) to the highest (about 4.5 servings a day or 691g).

Blood pressure was assessed at the beginning of the study and during follow-up assessments. Hypertension was defined as a systolic blood pressure of 140mmHg or above or a diastolic blood pressure of 90mmHg or above, or the use of blood pressure medication. Information on other health risk factors was collected at the assessments, including medical history, medications, smoking, alcohol, educational level, height and weight. The researchers specifically asked about history of heart attack or stroke, diabetes and blood cholesterol level. When analysing their results, the researchers adjusted their analysis for (took into account) other measured risk factors.

What were the results of the study?

The current research assessed 2,245 participants of the Rotterdam Study, who completed the food-frequency questionnaire, did not have hypertension at the beginning of the study and had blood pressure reassessed at follow-up.

A larger dairy intake was found to be associated with several other dietary factors, for example a lower consumption of meat, bread and coffee. A smaller dairy intake was more often seen in men, smokers, alcohol drinkers and those with higher total energy and saturated fat intake.

During the two-year follow-up, there were 664 new cases of hypertension. Risk of hypertension was found to decrease with an increasing intake of dairy. This was after the researchers took into account age, sex, BMI, educational level, smoking, total energy intake, alcohol consumption and several dietary factors (fruit, vegetable, meat, bread, coffee and tea consumption).

Low-fat dairy consumption had an inverse association with the risk of hypertension and the more that was consumed, the greater the decrease in risk of hypertension. Those consuming the highest quantity of low-fat dairy were calculated to have a 31% decreased risk compared to the lowest intake (the risk reduction figure quoted by the newspapers).

There were no significant associations between risk of hypertension and high-fat products or specific types of dairy intake, such as cheese or fermented dairy.

When the participants were reassessed at six years, 984 people had hypertension. There were then no significant associations observed between hypertension and total intake of dairy, intake of low-fat dairy or any other dairy products.

What interpretations did the researchers draw from these results?

The researchers concluded that intake of low-fat dairy products may contribute to the prevention of hypertension at an older age.

What does the NHS Knowledge Service make of this study?

Although an increased intake of low-fat dairy at the beginning of the study was found to lower a person’s chances of having high blood pressure two years later, this finding was not repeated at the six-year follow-up. This weakens the strength of the observations and conclusions that can be made.

Other features of the study’s design may limit its accuracy:

• The method of assessing food intake, frequency and quantity is likely to include some inaccuracy. The participants were required to estimate their usual intake of food for the past year, which is unlikely to remain constant and reflect lifetime patterns. As the researchers say, the food frequency questionnaire was not validated for assessing the intake of dairy products and different types of dairy (in other words, it is not an accepted method for assessment). Additionally, as the dairy categories were not mutually exclusive, there is possibly considerable overlap, misclassification and inaccuracy when separately grouping people into quantitative intakes of total dairy, low-fat, high-fat, cheese products, fermented dairy and milk and milk products.
• Although the researchers took into account many possible risk factors for hypertension, they did not take into account other medical conditions the participants may have had or their levels of physical activity.
• The study only represents about a quarter of the entire Rotterdam Study and different results may have been observed if a greater proportion had been assessed. 

The study does not prove that drinking skimmed milk lowers blood pressure or leads to a healthy heart. Milk contains other things than fat, including calcium and magnesium, and it could be these that contribute to the effect seen. However, there is a large body of evidence that shows lower levels of saturated fat in the diet are better for health, and this study supports this idea.