A current affairs program broadcast last week posed the question: ‘Is low fat milk unhealthy?’ And then followed up with ‘For more than two decades we’ve been encouraged to go low fat or even no fat when it comes to milk, but stunning new results may change all that.’ Really? What are these stunning new results?
Whenever new scientific findings are announced via current affairs programs on television my scepticism antennae start to twitch. It’s a safe assumption that a public relations firm has been engaged to push the story through to the general public. Let’s look at the science behind it.
The trigger for the program was a review published in the European Journal of Nutrition exploring possible links between the consumption of high fat dairy foods and risk for cardiovascular disease and obesity. This is one of a series of reviews of epidemiological evidence relating to dairy foods published over the last few years.
Dairy foods and coronary heart disease
On the important issue of whether dairy foods or dairy fat affect the risk for heart disease all the recent reviews say much the same thing – the epidemiological evidence is inconsistent. But how should we interpret this? Is any link between dairy fat and heart disease weak? Or is the available evidence of such poor quality that it is impossible to come up with any strong conclusion?
Gibson and colleagues from South Australia summed up the issue nicely in their excellent 2009 review: “… the studies available for examining the effect of dairy food consumption on [coronary heart disease] are too varied in design, quality and dietary assessment methodology to evaluate the nature of the relationship.” So it’s a matter of messy data providing inconclusive results.
These reviewers detailed the problems. Some of the available studies are small and have limited power. There are also different approaches to adjusting for confounders in different studies. Most of the studies commenced in the 1970s when full-fat dairy foods were the norm and dietary data were not updated as the swing to lower fat dairy products took hold. Very few studies allowed for comparisons between those subjects who ate low fat dairy products and those who ate high fat dairy products.
Gibson and colleagues identified just one cohort study by Hu and others in which there was both updated dietary data and a comparison between higher fat and lower fat dairy products. In this prospective cohort study of 80,000 women followed up for 14 years, the ratio of high fat to low fat dairy products was associated with increased risk for coronary heart disease. When types of dairy products were looked at separately, high fat dairy intake tended to be associated with higher risk and low fat dairy intake with lower risk.
What mechanisms are at play?
That’s the epidemiology. The next question is whether there is a plausible mechanism that would explain why high fat dairy foods are associated with increased risk for heart disease. The concern about dairy fat has always been its high content of saturated fatty acids which raise the level of LDL-cholesterol in the blood. There is strong evidence that increasing LDL-C increases the risk for heart disease.
Saturated fat has other undesirable effects on the cardiovascular system. A trial at CSIRO showed that saturated fat worsened the ability of blood vessels to expand and contract normally. This was confirmed in a trial at the Heart Research Institute in Sydney, which also found that saturated fat lowered the anti-inflammatory action of HDL. Saturated fat is just bad for arteries.
Recently, there has been considerable debate about saturated fat and heart disease but this has revolved around the ideal replacement for saturated fat in the diet. A meta-analysis of randomised controlled trials confirmed that replacing saturated fat with polyunsaturated fat lowers coronary heart disease risk, confirming findings from epidemiology. The latest Dietary Guidelines for Americans recommends that saturated fat should be replaced by unsaturated fats, not carbohydrate.
Low fat, low risk?
If fat were the only issue low fat dairy foods would be expected to have a neutral effect on the risk for heart disease. But low fat dairy foods aren’t neutral; they are protective. How can this be explained? The answer probably lies with a beneficial effect of low fat dairy foods on blood pressure.
Australian researchers recently published a meta-analysis of epidemiological studies into dairy foods and elevated blood pressure and found that low fat dairy foods were associated with lower risk. The mechanism is unclear but is thought be related to the calcium content of dairy foods. However, high fat dairy products were not associated with protection against elevated blood pressure, despite their calcium content. It appears that the adverse effect of saturated fat on blood vessel function offsets the beneficial effect of calcium. Saturated fat strikes again.
The recent review also found no evidence that the fat content of dairy foods affects the risk for obesity. This will come as no surprise to readers of this blog (see ‘Do calorie-rich foods make you fat?’). In short, the idea that the fat content or the energy density of the diet or individual foods increases the risk for obesity is an outdated hypothesis. It’s just not true.
The television program also suggested that eating low fat dairy foods lowered intake of fat-soluble vitamins, especially A and D, which is fair enough. However, the latest advice for heart health is not to eat a low fat diet but to replace saturated fats with unsaturated fats. One key recommendation is to replace butter with soft table margarines, which are enriched with vitamins A and D, as well as being high in unsaturated fats.
So is low fat milk unhealthy?
Rather than there being any ‘stunning new results’, there is simply no evidence to suggest that low fat milk is unhealthy. On the contrary, choosing low fat dairy products in preference to high fat dairy products is likely to have beneficial effects on blood lipids and blood pressure, lowering risk for coronary heart disease. I guess that’s why low fat dairy products are recommended in dietary guidelines all over the world.