No simple link between dairy products and heart disease
People who drank a lot of milk had a greater risk of heart attack and death than those who didn’t. But people who ate a lot of cheese had a lower risk, a new Norwegian study shows.
Research in recent years has pointed to fruit, vegetables and whole grains as being protective against heart disease. But when it comes to dairy products like milk, cheese and yogurt, the research is much less clear.
Many dairy products contain saturated fat, which has been linked to increases in blood cholesterol levels. High LDL cholesterol has been linked to an increased risk of cardiovascular disease.
Nevertheless, researchers have not found a clear connection between consumption of dairy products and heart disease.
Some studies have linked dairy products to an increased risk of disease, while others have suggested no connection or a lower risk. This was the background for a group of researchers who decided to undertake a new look at the question.
Relied on data from an old study
Vegard Lysne from Haukeland University Hospital and colleagues from Bergen, Oslo and Sweden have examined data from the West Norwegian B-vitamin Intervention Trial (WENBIT), which was conducted between 1999 and 2004.
This was a study of people with angina, which is pain caused by narrowing of the heart's coronary arteries. The participants were on average 62 years old, and most were men.
When the study started, the participants filled out a comprehensive questionnaire about their lifestyle and diet, including their consumption of dairy products. Lysne and his colleagues have now re-evaluated information from almost 2,000 of the participants.
The researchers compared the participants' diets with how their health actually fared in the years following the survey. They looked for possible connections between milk products and cases of heart attack, stroke, death as a result of heart disease, and death in general.
More disease with a lot of milk, less with a lot of cheese
The results show that there does not appear to be a simple link between dairy products and heart disease.
“We see that there are different connections between different dairy products and disease risk, even in different directions,” Lysne said to sciencenorway.no.
The data showed, for example, that the risk of heart attack was lowest for people who consumed moderate amounts of dairy products. But the risk was increased for those who consumed more milk or butter.
The results for cheese consumption, however, were the exact opposite: The more cheese the participants ate, the lower the risk of heart attack.
There was also a difference between dairy products when it came to the risk of stroke and death. When milk and dairy products were considered together, the risk increased in line with intake.
But there was much greater variation for butter and especially cheese. Here, the risk could be unchanged, or even lower with more consumption.
Agrees with previous results
“The main message is that there is a difference between dairy products. They should be studied individually and not just combined in one group,” Lysne said.
This is not entirely surprising. Several previous studies have shown the same trend, says clinical nutritionist Tine Sundfør, who was not involved in the study.
The results from previous observational studies have differed substantially, she wrote to sciencenorway.no.
But the conclusion of a summary of these studies from 2021 agrees with Lysne's results: The data indicated that cheese is linked to a lower risk of heart disease, while whole milk is linked to a higher risk.
So what might be the reason for this difference among milk products?
Here it is important to understand that this type of study cannot tell researchers what leads to what.
Can see connection, but not cause
“This is a cohort study, which means it’s an observational study of groups that are most often followed forward in time. In this type of study, you can study how the frequency of a disease, condition or risk factor changes over time, and you can study connections and possible causes,” Sundfør wrote.
That makes it possible to find connections, such as that people with angina who drank a lot of milk had a greater risk of heart attack and death, while those who ate a lot of cheese had a lower risk.
But it is impossible to say with certainty whether it was the milk or the cheese itself that affected people’s health, or whether the connection could be explained in other ways.
One possible explanation could be that there are other differences between the individual groups.
People who eat a lot of cheese, for example, often belong to social groups with higher income and social status. Previous research has shown that they often have better health than the average.
Another issue is what the participants ate instead of dairy products.
Those who ate fewer dairy products probably had to eat more of something else. But what? Could part of the effect on heart health be due to the foods that replaced the dairy products?
Works differently in the body
At the same time, it’s also possible that the milk products themselves influenced the risk of illness and death.
If so, why might different dairy products have different effects?
It does not appear that the difference is related to lean and fatty foods, as many have traditionally thought. Many types of cheese, for example, contain a lot of fat, but were linked to a lower risk of heart disease.
It’s possible that the structure and chemistry of the products means that the body processes the fats in the products differently, Lysne said.
“There have been some interesting studies comparing different dairy products with the same amount of fat and looking at the effect they have on the levels of cholesterol in the blood. The results have shown that the response is different,” he said.
Experiments — intervention studies — from 2011 and 2017, for example, have shown that cheese does not increase LDL cholesterol, but that butter does, Sundfør wrote.
There are several possible explanations for this.
Crushed fat globules
“It may be linked to whether the fat comes with a membrane,” Lysne said.
The fat in milk is contained in small globules with a thin skin — a membrane — of proteins and fats on the outside.
In milk that comes straight from a cow, these fat globules are intact. But when milk is homogenized, the fat globules burst. Homogenization is what keeps the fat from floating to the top of the carton, and what causes it to be distributed evenly in the milk.
Fat globules are also crushed in the process of churning butter. But they are more intact in cream and cheese. Some studies have shown that the fat in milk is absorbed more easily when the fat globules are crushed.
It’s possible to speculate whether this could be the cause of some of the health differences associated with milk and cheese, Lysne and his colleagues wrote in an article that has now been published in the European Journal of Preventive Cardiology.
Other substances and fermentation
Another possible reason could be that the composition of other substances is different in different dairy products.
“Fat does not work in isolation. It has been shown that substances such as calcium and protein can affect the body's response after a meal. Cheese has much more calcium and protein than other dairy products,” Lysne said.
A third factor might be fermentation with bacteria and fungi. Dairy products such as yoghurt, sour cream and many types of cheese are fermented. This changes both the structure and composition of substances in the food and the way it acts on our own intestinal flora.
Some research has suggested that fermented food has a different effect on the body than other food. But there’s a lot that isn’t known, Lysne said.
Currently, it’s not known whether any of these factors are the reason for the results from Lysne’s survey. But they offer interesting possibilities to explore in future studies.
Don't change dietary guidelines
So what do the results mean for us butter and cheese eaters now?
“The study was conducted on a group of elderly men with heart disease and therefore does not apply to the entire population. The data are also more than 20 years old. Today, our diets are generally different, and it’s not certain that these effects would be the same now,” Lysne said.
He believes the new study should not result in changes to the current Norwegian recommendations that people eat three portions of dairy products a day.
“But it remains an open question as to whether some products should be eaten rather than others. Our results indicate that this may be the case and that cheese in particular is exciting to research further,” he said.
In general, the study is in keeping with a number of other investigations that show that all dairy products and sources of saturated fat cannot be studied as a unit, Lysne said.
“These results are yet another indication that it matters where saturated fat comes from,” he said.
Translated by Nancy Bazilchuk
Reference:
Anthea Van Parys, Vegard Lysne et.al.: The association between dairy intake and risk of cardiovascular disease and mortality in patients with stable angina pectoris. European Journal of Preventive Cardiology, 2022. Summary.
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Read the Norwegian version of this article at forskning.no
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