Many individuals will be familiar with the almost-constant recommendations that come at us with regard to what we should be eating to reduce our risk of heart disease. A central theme in this advice, though not everyone would agree with it, is that the diet should be low in fat and high in carbohydrate. Sometimes, we have further refinement of this message where we are encouraged to eat less ‘saturated’ fat and more ‘polyunsaturated’ fat. Some food manufacturers, for example those who produce margarine and sell it as a ‘healthy’ alternative to butter, have been particularly keen to get it into our heads that a diet rich in polyunsaturated fat and low in saturated fat is the way to keep heart disease at bay. Many doctors and dieticians would agree. But how true is this assertion?
Well, the scientific veracity of these and other nutritional notions was recently reviewed in a study published in the Archives on Internal Medicine [1]. A group of researchers from McMaster University in Hamilton, Ontario, Canada, undertook a ‘systematic’ review of the evidence linking a wide variety of nutritional factors and heart disease. The authors of this review looked at two main types of evidence:
1. Cohort studies: this is a form of ‘epidemiological’ study in which the relationship between specific factors (in this case dietary factors) and a medical condition or conditions (in this case, heart disease) are assessed over a period of time.
2. Randomised controlled trials: where individuals are subjected to some sort of dietary change (e.g. lowered fat intake) and the result is compared with individuals who do not make this change.
Randomised controlled trials are good for sorting out cause-and-effect. For example, if eating a low fat diet was found to lead to a reduced risk of heart disease, that would strongly suggest that fat actually causes heart disease. On the other hand, cohort studies can’t generally be used to draw such conclusions. All they are really good for is to assess associations between things (and associations don’t prove causality).
However, certain criteria can be applied in judging cohort studies that may help to lend support to the idea that a risk factor that is associated with a disease is also a true cause of it. A specific set of criteria commonly used here is known as the ‘Bradford Hill’ guidelines (after the man who developed it). In this recent study, the Bradford Hill guidelines were used to judge each of a wide range of nutritional factors that might have some influence of heart disease risk.
Four primary criteria were applied. If the evidence fulfilled all four criteria, then the authors adjudged there to be ‘strong’ evidence of a cause-and-effect link between the specific dietary factor and heart disease. If 3 criteria were met, the evidence was regarded as ‘moderate’. Scores of 2 or less constituted, according to the authors, ‘weak; evidence of a causal link.
Here is a summary of this work:
Factors associated with a statistically significant reduced risk of heart disease included:
Higher alcohol consumption
Higher dietary beta-carotene consumption
Higher fibre consumption
Higher fish consumption
Higher marine-derived omega-3 fat consumption
Higher dietary folate consumption
Higher total folate consumption (from diet and supplements combined)
Higher fruit consumption
Higher vegetable consumption
Higher nut consumption
Higher monounsaturated fat consumption
Higher dietary vitamin C consumption
Higher total vitamin C consumption
Higher dietary vitamin E consumption
Higher total vitamin E consumption
Higher wholegrain consumption
Increased adherence to ‘Mediterranean’ dietary pattern
Please note the absence of ‘polyunsaturated’ fats from this list. In other words, the authors found no evidence that an increased consumption of these is associated with relative protection from heart disease.
Some dietary factors were associated with an increased risk of heart disease. These were:
A higher intake of trans-fatty acids
A higher intake of foods of high glycaemic index or high glycaemic load
Please note the absence of saturated fat in this list. In other words, this review shows the totality of the evidence does not support a link between saturated fat and heart disease. By the way, higher intakes of meat and eggs were not associated with an increased risk of heart disease either. And neither was total fat consumption.
Of all of these factors, the ones that fulfilled 4 Bradford Hill criteria (i.e. those where there was most evidence of cause-and-effect) were:
Mediterranean diet
Vegetable consumption
Nut consumption
Trans-fatty acid consumption
Consumption of foods of high GI or GL
Of these factors, only two turned out to have supporting evidence in the form of randomised controlled trials. These were:
Adherence to a ‘Mediterranean’ type of diet
Increased intake of marine (fish and seafood derived) omega-3 fat
What about cutting back on saturated fat though, surely there must be some evidence for the benefits of this (seeing as it perhaps the most commonly trumpeted nutritional dictum of all)? Well, it turns out that there were no appropriate randomised controlled trials on this that the authors could identify. Apparently, according to the authors, all of the studies that have cut back on saturated fat have employed one or more nutritional strategies (e.g. increased intake of omega-3 fats) at the same time.
In other words, according to this review, there are no appropriately conducted randomised controlled trials to support the notion that cutting back on saturated fat is good for the prevention of heart disease. Add that finding to the one which shows no link between saturated fat and heart disease from cohort studies and where does that leave us? Well, the only logical conclusion to be drawn is that there really is no good evidence to support the widespread recommendation to reduce saturated fat intake for the sake of heart health.
I have written about this here, where I attempt to summarise the evidence which shows that eating less saturated fat not only doesn’t benefit the heart, but does not have broad benefits in terms of anything else either.
And remember, this study showed good evidence for a cause-and-effect link between high GI/GL carbs and an increased risk of heart disease. You can read more about how and why this can be here.
Taken as a whole, I think the findings of this systematic review can be summarised as follows: a low fat, high carb diet is bad for the heart. And, again, a close inspection of the science gives us no reason at all to cut back on saturated fat.
References:
1. Mente A, et al. A Systematic Review of the Evidence Supporting a Causal Link Between Dietary Factors and Coronary Heart Disease. Arch Intern Med. 2009;169(7):659-669.
UPDATE – 18th January 2010
For an update on the science regarding saturated fat and heart disease see: Two major studies conclude that saturated fat does NOT cause heart disease
