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August 27, 2011
For decades now, physicians and dieticians have warned against the perils of consuming foods high in saturated fat. These recommendations are based on years of epidemiological evidence that saturated fat intake is associated with an increased risk of numerous diseases–particularly cardiovascular disease. The majority of the general public has taken this advice as gospel, demonizing saturated fat as the nutritional enemy.
Recently, however, some researchers have begun to challenge the accepted dogma that saturated fat intake actually increases cardiovascular risk. A recent systematic review by Siri-Tarino and colleagues in the American Journal of Clinical Nutrition sought to investigate the subject. The review used a technique called meta-analysis to investigate the association of saturated fat and cardiovascular disease. In case you don’t know, meta-analysis involves pooling the results from multiple research studies so as to improve statistical power and arrive at a more definitive interpretation of data. The review identified 16 studies that evaluated the association between saturated fat consumption and heart disease and 8 studies that evaluated the association of saturated fat and stroke. After performing statistical analyses, the researchers concluded that “there is insufficient evidence from prospective epidemiologic studies to conclude that dietary saturated fat is associated with an increased risk of coronary heart disease, stroke, or cardiovascular disease.” A pretty definitive statement, wouldn’t you say?
Based on the results of the study, it would seem that previous recommendations have been off-base and that we should feel free to slap the butter on thick and have that extra serving of bacon for breakfast. After all, a systematic review is considered the gold-standard in evidence-based practice. What more info do we need? Well, not so fast…
As is often the case, things aren’t necessarily as simple as they may seem. In the same journal issue, Dr. Jeremiah Stamler provided a detailed critical assessment of the Siri-Tarino et al. study that exposes issues with some of the methodologies of the review. While I don’t necessarily agree with all of Stamler’s points, he nevertheless does make some interesting observations.
First and foremost, there are potential biases in the method of dietary analysis in the studies evaluated by Siri-Tarino et al. Namely, 4 of the studies relied on a single 24-hour dietary recall to determine nutrient intake. This method of data collection is highly subject to recall bias and research shows it “provides a very inadequate estimate of usual intake of individuals”. Interestingly, only 1 of these studies showed an increased relative risk for saturated fat intake with respect to heart disease. On the other hand, of the 5 studies that used dietary history or multiday food records–a more accurate method of determining food intake–all 5 showed an increased relative risk.
Stamler also notes that results of the 11 studies that investigated “hard” fatal coronary heart disease (i.e. deaths from heart attack) showed a significant increase in relative risk from saturated fat intake as opposed to the 5 studies looking at “soft” data (those who experience increased total cardiovascular risk). This suggests that saturated fat intake may have a greater effect on causing cardiovascular fatalities irrespective of increasingly markers of cardiovascular disease. In other words, it is possible that a high saturated fat intake might result in a greater incidence of death from heart disease than lower intakes even if overall risk for disease is not significantly increased (i.e. progression of the disease is worse).
So what do we make of this data? Do we believe the results of the systematic review–a gold standard in evidence-based practice–that saturated fat intake is benign? Or do we dismiss it because of its inherent limitations? My view on the subject is somewhere in the middle. People are often all-too-quick to pounce on new research as the be-all, end-all on a particular topic. This is invariably a mistake and it certainly would be here. Research will always have inherent limitations and biases; this is the nature of the beast. Still and all, we must be open to new information and not rigidly adhere to previously held beliefs. As such, we must sift through the evidence and then use logical reasoning to try to make sense of what we know.
There is very good evidence that saturated fat intake increases cholesterol production, including increased LDL levels (the so-called “bad” cholesterol). However, recent research indicates that LDL levels may not be as important as once thought in promoting atherosclerosis. Rather, it is suggested that the size of the LDL particles are more indicative of cardiovascular risk, with the smaller, denser particles being atherogenic and the “large, fluffy” particles being benign. While this hypothesis is interesting, it remains largely untested in controlled research. We simply don’t know as much as we previously thought on the topic and more research is needed to draw firm conclusions.
There is emerging evidence that chronic inflammation is involved in the development of cardiovascular disease. It is therefore possible that the combination of high inflammatory markers and a high saturated fat intake may exacerbate plaque build up in the arteries and ultimately lead to an increased risk of a cardiovascular event. Again, while this hypothesis is interesting, it requires further research.
Considering the best available evidence, my advice is that you are probably better off limiting saturated fat consumption. If nothing else, saturated fat intake does little if anything for biological function (it does have a positive effect on HDL and testosterone, but so do the more biologically active monounsaturated fats). Moreover, emerging research suggests that replacing saturated fat with unsaturated fat can reduce cardiovascular risk. Given that omega-6 fats are pro-inflammatory, you’re best bet is to focus on monounsaturated fats (like those found in avocado, olive oil, almonds, etc) and omega-3s (like those found in cold water fish such as salmon, sardines, etc). This is considered to be one of the key components of the cardioprotective effects of the Mediterranean Diet. The omega-3 fats, in particular, have been shown to be heart-healthy (as well as potentially conferring many other health benefits for virtually every organ system in the body).
On the other hand, you don’t need to be anal about counting every gram of saturated fat consumed. For the majority of people, having a modest amount of saturated fat in your diet is not likely to have a significant effect on cardiovascular risk. Nutrition is highly individualized and it may well be that some people are more susceptible to negative effects from saturated fat intake than others. Hopefully the day is approaching when we will be able to customize nutritional regimens based on an assay of our individual genetic code. Until then, a balanced approach is warranted.
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