More recently, Chowdhury and colleagues published a separate meta-analysis in the Annals of Internal Medicine, and reached similar conclusions to that of Siri-Tarino and colleagues regarding the association between saturated fat and coronary heart disease.7 Unfortunately, this meta-analysis also failed to sufficiently address a number of important limitations that it shares with the meta-analysis by Siri-Tarino and colleagues. Furthermore, in this meta-analysis, although positively, but not significantly associated in the random-effects model, both dietary and total circulating concentrations of saturated fat were associated with a small, but statistically significant increased risk of coronary heart disease in the fixed effects model (RR=1.04 [95% CI, 1.01, 1.07] and RR=1.13 [95% CI, 1.03-1.25], respectively). These significant findings were however ignored in the conclusions of this study. Nevertheless, the media and proponents of popular Low-Carb and Paleo diets have repeatedly cited these meta-analyses as evidence to support a diet rich in saturated fat.
Saturated Fat and Coronary Heart Disease Mortality
|FIGURE 2. Risk ratios and 95% CIs for fully adjusted random-effects models examining associations between saturated fat intake in relation to coronary heart disease mortality. SAT, saturated fat intake.|
As coronary heart disease is the leading cause of death in the world, naturally these findings should be a cause for concern.28 Nevertheless, both the Siri-Tarino and Chowdhury meta-analyses are widely cited by proponents of Low-Carb and Paleo diets as providing compelling evidence in favor of a diet rich in saturated fat. It is important to note, however, that in the studies included in this meta-analysis, the difference for high vs low intake of saturated fat only ranged between about 5% and 10% of energy. This suggests that individuals following popular variants of these diets which often emphasize far higher intakes of saturated fat than recommended levels may be at a much greater risk of death.
It is important to note that the influence that saturated fat has on the risk of disease is not primarily determined by intake per se, but by which foods saturated fat is substituted for. As the intake of dietary fiber was universally low among subjects in these studies, this suggests that subjects consuming diets lower in saturated fat were substituting saturated fat primarily with lean animal foods and heavily processed foods.29 As dietary fiber was associated with a decreased risk of death from coronary heart disease in a number of these studies, this suggests that compared to fiber-rich foods, foods rich in saturated fat may be associated with an even stronger risk of coronary heart disease death.29
Although in this meta-analysis, the Israeli Ischemic Heart Disease Study appeared the least favorable of the hypothesis that saturated fat increases the risk of death from coronary heart disease, it should be noted that not only were the estimates controlled for serum cholesterol, in this study, saturated fat as a percentage of fat was actually associated with a statistically significant increased risk of death from coronary heart disease. In addition, subjects who were classified as being most adherent to religious Orthodoxy, which is typically accompanied by fasting periods in which the consumption of meat and other foods rich in saturated animal fat are prohibited, experienced a significantly lower death rate of coronary heart disease.11 This observation is supported by several other studies which found that Orthodox fasting is associated with improved cardiovascular risk factors, including blood lipids.30
The findings from this meta-analysis are also in agreement with numerous longitudinal ecological studies. For example, intake of saturated fat explained about 88% of the variance in death from coronary heart disease between the 16 cohorts in the 25-year follow-up of the Seven Countries Study.32 Similar estimates were also found for foods rich in saturated fat, including butter, meat, and animal foods combined.33 Similarly, in 1989, Epstein examined the changes in death from coronary heart disease in 27 countries during the previous 10 to 25 years, and noted that:
In almost all of the countries with major falls or rises in CHD mortality, there are, respectively, corresponding decreases or increases in animal fat consumption…
Epstein also noted that a number of other risk factors, such as smoking could not explain these findings, as the prevalence of smoking among women either remained largely unchanged or increased in most nations during this period, yet similar declines in death were often observed in both men and women.34 Epstein’s findings are further supported by a number of studies that have incorporated the IMPACT CHD mortality model, which has been shown to adequately explain which risk factors and treatments that have contributed most significantly to the changes of rates of coronary heart disease mortality throughout most parts of the world.35
Dietary Patterns and Coronary Heart Disease Mortality
|FIGURE 3. Risk ratios and 95% CIs for fully adjusted random-effects models examining associations between low-carbohydrate diets in relation to cardiovascular disease mortality. ¹Studies that included adjustments for saturated fat intake.|
As coronary heart disease is the number one cause of cardiovascular death in the nations where these studies were carried out, this provides indirect evidence that diets rich in saturated fat, at least in the context of a carbohydrate restricted diet, increases the risk of coronary heart disease. Furthermore, the difference in intake of saturated fat between the low and high low-carbohydrate scores was generally smaller than the difference of intake between popular low-carbohydrate diets and recommended levels, suggesting that individuals who follow more extreme variants of these diets may be at an even greater risk of death. As reviewed previously, these findings may be explained, in part, by a number of adverse effects that carbohydrate restricted diets have been shown to exert on cardiovascular risk factors. For example, recent meta-analyses of randomized controlled trials have found that compared to diets rich in nutrient poor, low-fiber carbohydrates, carbohydrate restricted diets raise LDL cholesterol and impair flow-mediated dilatation.42
|FIGURE 5. Risk ratios and 95% CIs for fully adjusted random-effects models examining associations between omnivorous diets in relation to coronary heart disease mortality.|
It is important to note that the omnivores in these studies had a relatively low intake of meat, suggesting that individuals following popular meat based diets may be at a greater risk of death. This suggestion is supported by a recent meta-analysis of prospective cohort studies which found that an increment of 1 mg/day of heme iron, found only in animal tissue is associated with a 27% increased risk of coronary heart disease.48 As reviewed previously, in these studies, the degree of reduction in risk of death from coronary heart disease observed in vegetarians in these studies was generally in proportion to the expected reduced risk based on the differences in levels of total and non-HDL cholesterol. This suggests that these results may, in part, be explained by differences in intake of saturated fat.
Saturated Fat is a Major Problem
The findings reviewed here support the hypothesis that saturated fat increases the risk of coronary heart disease mortality. Furthermore, as reviewed previously, evidence also suggests that the hazardous effects of diets rich in saturated fat are also applicable to diets rich in organic, grass-fed animal foods. However, saturated fat is only one of a number of problems as far as chronic diseases are concerned. The effect that a particular food has on the risk of coronary heart disease cannot be fully explained by saturated fat content alone, but rather by multiple nutrients that likely operate together in a complex manner to modify the risk of disease. Therefore, it may be more appropriate to focus attention on recommending healthy dietary patterns that are naturally low in saturated fat, while rich in dietary fiber and other beneficial nutrients; primarily, minimally processed, plant-based diets. Such a focus may be more effective to help lower the intake of saturated fat, while simultaneously improving overall dietary quality compared to the more contemporary reductionist approach of focusing on modifying single nutrients.
In forthcoming parts of this review, I will examine both the effects of dietary and total circulating concentrations of saturated fat on the risk of total incidence of coronary heart disease. In addition, I will examine a number of other important limitations of the studies included in these meta-analysis that may have bias these findings towards null.2 3 4 5 6
Study acronyms: ATBC, Alpha-Tocopherol Beta Carotene Study; BLSA, Baltimore Longitudinal Study of Aging; EPIC-Greece, European Prospective Investigation into Cancer Greece; EUROASPIRE, European Action on Secondary and Primary Prevention through intervention to reduce events; FHS, Framingham Heart Study; HLS, Health and Lifestyle Survey; HPFS, Health Professionals’ Follow-Up Study; IBDH, Ireland-Boston Diet Heart Study; IIHD, Israeli Ischemic Heart Disease Study; JACC, Japan Collaborative Cohort Study; LRC, Lipid Research Clinics; MALMO, Malmo Diet and Cancer Study; NHS, Nurses’ Health Study; SHS, Strong Heart Study; SWLHC, Swedish Women’s Lifestyle and Health Cohort; ULSAM; Uppsala Longitudinal Study of Adult Men; VIP, Västerbotten Intervention Program; WES, Western Electric Study.
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