Saturated fat and cardiovascular disease. The dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids (macronutrients). USDA 2005 report.
A closer look at the report.
In 2005, the USDA published it's report "dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein,
and amino acids". Chapter 8 deals with dietary fats (1).
On page 422, the USDA summarizes the evidence linking saturated fat intake to CHD:
| There is a positive linear trend between total saturated fatty acid intake and total and low density lipoprotein (LDL) cholesterol concentration and increased risk of coronary heart disease (CHD). A UL is not set for saturated fatty acids because any incremental increase in saturated fatty acid intake increases CHD risk. |
The USDA mentions the fact that research has shown that saturated fat increases both 'bad' LDL-cholesterol, and 'good' HDL-cholesterol levels (pages 481, and 483 of the report, respectively):
| Several hundred studies have been conducted to assess the effect of saturated fatty acids on serum cholesterol concentration. In general, the higher
the intake of saturated fatty acids, the higher the serum total and low density lipoprotein (LDL) cholesterol concentrations. Although all fats will increase serum high density lipoprotein (HDL) cholesterol concentration relative to carbohydrate, the increase attributable to saturated fats is greater than that observed for monounsaturated and polyunsaturated fatty acids. Serum HDL cholesterol concentration increases by 0.011 to 0.013 mmol/L for each 1 percent increase in saturated fat. |
The USDA mentions the fact that high LDL-cholesterol levels increase risk of CHD (page 483 report):
| Similar to that observed for saturated fatty acid intake and LDL cholesterol concentration, there is a positive linear relationship between serum total and LDL cholesterol concentrations and risk of coronary heart disease (CHD) or mortality from CHD. |
But the association between high HDL-cholesterol levels and CHD is not discussed.
The USDA states that most epidemiological studies show that saturated fat intake increases risk of CHD (page 483 report):
| A number of epidemiological studies have reported an association between saturated fatty acid intake and risk of CHD. The majority of these studies have reported a positive relationship between saturated fatty acid intake and risk of CHD and CHD mortality (Goldbourt et al., 1993; Hu et al., 1997, 1999a, 1999c; Keys et al., 1980; McGee et al., 1984). Ascherio and coworkers (1996) concluded that the association between saturated fatty acid intake and risk of CHD was not strong; however, saturated fat and the predicted effects on blood cholesterol concentrations did affect risk. No association between saturated fatty acid intake and coronary deaths was observed in the Zutphen Study or the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study (Kromhout and de Lezenne Coulander, 1984; Pietinen et al., 1997). |
Associations from these epidemiological studies as the USDA claims that were found, are compared to data from the original articles referred to, in the table below.
- The USDA claims increased risks were found in 7 articles, but this data clearly shows, increased risks were only found in 2 out of these 7 articles. Apparantly, P-values of ≥ 0.32 define an "association" according to the USDA. While a P-value of 0.09 (Kromhout D. 1984) is defined as "no association".
- The USDA failed to identify a large amount of other articles showing no association between saturated fat consumption and CHD (2)
- Noteworthy is the interpretation of data from one study. Ascherio et al (1996) found no association with CHD risk, but from the predicted effect based on blood cholesterol concentrations, it was concluded that an association was found anyway.
| Author | Cohort name | Relative Risk (RR) according to USDA | Relative Risk (RR) according to the original article |
|---|---|---|---|
| Hu FB (1999) | The Nurses' Health Study | A positive relationship with risk of CHD and CHD mortality. | RR = 1.00 (0.82-1.21; P = 0.60) for 4:0-10:0 saturated fatty acids, and RR = 1.04 (0.72-1.48; P = 0.47) for 12:0-18:0 saturated fatty acids. |
| Hu FB (1999) | The Nurses' Health Study | A positive relationship with risk of CHD and CHD mortality. | RR = 1.34 (0.82-2.21; P = 0.32). |
| Hu FB (1997) | The Nurses' Health Study | A positive relationship with risk of CHD and CHD mortality. | RR = 1.07 (0.77-1.48; P = 0.37). |
| Ascherio A (1996) | The Health Professionals Follow Up Study | Ascherio and coworkers (1996) concluded that the association between saturated fatty acid intake and risk of CHD was not strong; however, saturated fat and the predicted effects on blood cholesterol concentrations did affect risk. | RR = 0.96 (0.73-1.27; P = 0.69). |
| Goldbourt U (1993) | The Israeli Ischemic Heart Disease Study | A positive relationship with risk of CHD and CHD mortality. | CHD mortality rate per 10,000 = 49 for the highest- vs 61 for the lowest quintile of consumption. In multivariate analysis, the RR approached 1 (data not tabulated). |
| McGee DL (1984) | The Honolulu Heart Program | A positive relationship with risk of CHD and CHD mortality. | Saturated fat as a high % of calories significantly increased MI or CHD death (P = < 0.01). But no association was found of high vs low consumption (No P-value available). |
| Keys A (1980) | The Seven Countries Study | A positive relationship with risk of CHD and CHD mortality. | The average population intake of saturated fat was strongly related to 10- and 25-year population CHD mortality rates. |
| Pietinen P (1997) | The ATBC Study | No association with coronary deaths. | RR = 0.93 (0.60-1.44; P = 0.91). |
| Kromhout D (1984) | The Zutphen Study | No association with coronary deaths. | A nonsignificant protective effect (P = 0.09). |
Summary. The USDA 2005 report:
- States that saturated fat intake increases serum levels of both 'bad' LDL-cholesterol and 'good' HDL-cholesterol. It also states that LDL-cholesterol increases risk of coronary heart disease, but does not discuss the role of HDL-cholesterol in this process.
- States that the majority of epidemiological studies have reported a positive relationship between saturated fat intake and CHD. Increased risks were supposedly found in 7 studies. A closer look at these studies shows increased risks were only found in 2 out of these 7 studies.
- Failed to identify a large amount of epidemiological studies.
Conclusion: The USDA describes the effect of high saturated fat consumption on both LDL-, and HDL-cholesterol. But the predicted effect on CHD is
described only for LDL-cholesterol. The predicted effect of HDL-cholesterol is not taken into account when the association between saturated fat and CHD
is judged, and the decision for this choice is not motivated. Furthermore, the USDA statement that most epidemiological studies found an increased risk of CHD from
high saturated fat consumption, is based on incorrect citing of true results from the original articles. Also, the epidemiological data was not systematically reviewed.
Implication: If the effect of saturatef fat intake on serum HDL-cholesterol would be taken into account, and if the articles referred to were cited
correctly, no/inconclusive evidece would remain for a possible association between saturated fat and cardiovascular disease.
References:
1) Institute of Medicine. Chapter 8. Dietary fats: Total fat and fatty acids. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients) (2005).
Available at: Link. Accessed on Februari 14, 2011.
2) Consumption of dietary saturated fat and cardiovascular disease. Canceranddiet.nl.
Available at: Link. Accessed on Februari 14, 2011.