Want bacon and cheese on that burger?
For several decades, there has been a consensus in the medical field that eating foods high in saturated fat raises the risk of heart disease and other related health problems. In my view, the confidence level behind this statement is about half as strong as it was several years ago. In fact, those who would like to raise “reasonable doubt” about the link between saturated fat and heart disease may now do so without being dismissed as crazy. I do believe that saturated fat, starch and sugar are the main drivers of heart disease, but the saturated fat story is more confusing than ever.
To prove that eating saturated fat causes heart disease, we would need a clinical trial where people without heart disease were randomly assigned to be fed a diet high in saturated fat, or another diet that was otherwise identical, except free of (or significantly lower in) saturated fat, and followed for many years on their assigned diet to determine the number and rate of new heart disease cases in each group.
Unfortunately, this is practically impossible for many reasons, including the fact that it is not possible to reduce saturated fat without changing something else about the diet, such as the amount of unsaturated fat, calories, protein, carbohydrate, etc. When multiple things (rather than one thing) differ between the two diet groups, then it becomes unclear what aspect of the dietary change or dietary difference was the cause of any difference in heart disease rates.
The closest we have to such a clinical trial was the Finnish Mental Hospital Study involving over 10,000 men and women. In the 1960′s, in two mental hospitals in Finland, one group of residents had their whole milk replaced with skim milk plus soybean oil (homogenized in), and butter was replaced by margarine so that the total fat and calorie intake was the same, but the ratio of saturated fat to unsaturated fat was lower. Over a period of 6 years, the group with less saturated fat intake and higher unsaturated fat intake had about 45% fewer heart attacks and heart disease deaths (53% fewer in men, 34% fewer in women) than the usual diet group. There are two additional similar studies where saturated fat, largely from dairy products, was reduced and replaced with unsaturated fat, and doing so led to reduced heart attacks.
Together, these studies form the strongest evidence that saturated fat accelerates heart disease.
Other types of evidence that link saturated fat to heart disease are less convincing due to weaknesses in the ability to make inferences. For example, in certain laboratory animals, we can produce heart disease by feeding them saturated fat. However, this does not prove that saturated fat causes heart disease in humans. Likewise, studies that show that the rate of heart disease in a given country is correlated with the amount of saturated fat consumed in that country (so-called “geographic studies”) do not serve as proof because many aspects of diet (and lifestyle factors) correlate with inter-country differences in saturated fat intake. For example, countries with high saturated fat intake also tend to have higher sugar intake, lower fiber intake and less exercise, etc.
One would think that if high saturated fat intake accelerated heart disease, then individuals who eat lots of saturated fat for many years would go on to have higher heart attack rates than folks who eat less saturated fat. If, in fact this was true, then the case would be closed: “guilty as charged.”
And for years, health professionals have believed this to be the case. However, as it turns out, it is not obvious from observational studies that individuals who eat lots of saturated fat are at increased risk for heart disease! For example, for every large study that indicates some link between saturated fat and heart disease, there are two other studies that seem to find no relationship. In fact, when researchers line up about 2 dozen well-designed scientifically valid observational studies, and when they use statistical methods to try to account for differences in body weight, age, gender, etc. then there is exactly zero relationship between the amount of saturated fat intake and risk of heart disease.
Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease
Patty W Siri-Tarino, Qi Sun, Frank B Hu and Ronald M Krauss
Background: A reduction in dietary saturated fat has generally been thought to improve cardiovascular health.
Objective: The objective of this meta-analysis was to summarize the evidence related to the association of dietary saturated fat with risk of coronary heart disease (CHD), stroke, and cardiovascular disease (CVD; CHD inclusive of stroke) in prospective epidemiologic studies.
Design: Twenty-one studies identified by searching MEDLINE and EMBASE databases and secondary referencing qualified for inclusion in this study. A random-effects model was used to derive composite relative risk estimates for CHD, stroke, and CVD.
Results: During 5–23 y of follow-up of 347,747 subjects, 11,006 developed CHD or stroke. Intake of saturated fat was not associated with an increased risk of CHD, stroke, or CVD. The pooled relative risk estimates that compared extreme quantiles of saturated fat intake were 1.07 (95% CI: 0.96, 1.19; P = 0.22) for CHD, 0.81 (95% CI: 0.62, 1.05; P = 0.11) for stroke, and 1.00 (95% CI: 0.89, 1.11; P = 0.95) for CVD. Consideration of age, sex, and study quality did not change the results.
Conclusions: A meta-analysis of prospective epidemiologic studies showed that there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD. More data are needed to elucidate whether CVD risks are likely to be influenced by the specific nutrients used to replace saturated fat.
Holy cow! How can this be? These observational studies, when taken as a whole, seem to indicate that heart disease risk has nothing to do with saturated fat intake! Is there truly no relationship, or are these studies flawed?
Well, I contend that how you interpret the result has a lot to do with your preconceived notions about how you would like the truth to be. If one is invested, for whatever reason, in the notion that there is no link, then it is easy to trumpet these results and show little skepticism. If one is invested, for whatever reason, in the notion that there is a causal link, then it is easy to be skeptical and to show interest in discrediting these findings. Scientists pride themselves on being “objective” and “neutral” with no stake in the outcome of a study, but the truth is that the more one cares about whether there is a relationship or not, the less objective one can be.
An important sentence in the conclusion reads, “More data are needed to elucidate whether CVD risks are likely to be influenced by the specific nutrients used to replace saturated fat.” What does this mean? It means that if eating less saturated fat tends to also mean eating something else that affects heart/cardiovascular disease rates, then the study might actually fail to pick up a true association between saturated fat and heart/cardiovascular disease.
In an opinion piece, the authors indicate there is increasing evidence that increases in carbohydrate intake, especially refined or high glycemic carbohydrate intake, often used to replace the fat and saturated fat intake, may have entirely offset the health benefits of reduced saturated fat intake. They did not come right out and say it, but their study methods were unable to test that hypothesis, which they seem to believe is likely to be correct. They would expect to find, perhaps, that increased saturated fat intake is associated with increase risk, once the amount and type of carbohydrate is accounted for, which they were unable to do.
To complicate matters further, two of the authors receive significant financial support from the dairy industry. Cheese, cream, ice cream, and butter are major sources of saturated fat, so it seems like a good strategy for the dairy industry to try to support scientists whose research might help dispel the link between saturated fat and the most common cause of death in developed countries.
Is it plausible that this study never would have been done if not for dairy industry interests? Is it plausible that the authors would have spelled out more clearly the possibility that a true link between saturated fat and heart disease was masked due to refined carbohydrate intake? Would they have been more “skeptical” of their own findings if funding from dairy was not a factor? Two of the authors are not linked to dairy industry funding, and the study methods are fairly objective. Is that enough to eliminate any concerns? I don’t have the answers to these important questions.
I will conclude by saying the water is muddier than it used to be, and that I believe all scientific evidence to date is consistent with the notion that saturated fat, sugar and starch are all important and potentially synergistic accelerators of heart disease and related diseases, especially in people with obesity and insulin resistance.
- Michael Dansinger, MD
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