The 2015 Scientific Report from the Dietary Guidelines Advisory Committee (DGAC) was released in February. The DGAC convenes every five years to review U.S. dietary guidelines. The committee submits a list of recommendations to the Secretary of the Department of Agriculture and the Secretary of the Department of Health and Human Services.
The 2015 report has a significant finding on fat consumption listed in the Executive Summary:
“The DGAC also found that two nutrients—sodium and saturated fat—are overconsumed by the U.S.population relative to the Tolerable Upper Intake Level set by the IOM or other maximal standard and that the overconsumption poses health risks.”
To understand the impact of the DGAC finding, it’s helpful to first get an update on cholesterol.
Much of the confusion about fat, cholesterol and cardiovascular disease comes from lumping two different substances, cholesterol and lipoproteins, under the one term ‘cholesterol’.
Cholesterol is an essential building block for a wide range of substances in the body, from cell walls to Vitamin D.
Lipoproteins are transport vehicles for carrying triglycerides and cholesterol and can be further divided into High Density Lipoprotein (HDL) and Low Density Lipoprotein (LDL). LDL acts as delivery vehicle for cholesterol. HDL picks up excess cholesterol and transports it back to the liver.
For decades there was a hypothesis that cardiovascular disease was caused by saturated fat and cholesterol accumulating along the walls of arteries. Since LDL delivers cholesterol it was labeled “bad” and HDL was labeled “good” because it collects cholesterol.
The understanding of cardiovascular disease has improved considerably along with the ability to identify the different types and concentrations of lipoproteins in circulation.
LDL can further be divided into two types, large buoyant particles and small dense particles. Blood samples can be measured to see if there is a larger percentage of large buoyant LDL (pattern A) or a larger percentage of small dense LDL (pattern B).
Despite the attention total cholesterol and total LDL has received over the last 60 years, they were never good indicators of risk from cardiovascular disease. There were frequent cases of serious cardiovascular disease in individuals with low total cholesterol.
It turns out the concentration of small dense LDL (pattern B) and the potential of small dense LDL particles to get trapped in arterial walls and then oxidize is a much stronger predictor of cardiovascular disease risk.
The U.S. Government Position on Fat
The Institute of Medicine (IOM) establishes Estimated Average Requirement (EAR), Recommended Dietary Allowance (RDA), Adequate Intake (AI), and Tolerable Upper Limits (UL) for nutrients. A consolidated list of their values was published in a 2006 Reference Manual (link below).
The IOM found insufficient data to establish an EAR for total fat, so instead estimated an Acceptable Macronutrient Distribution Range (AMDR) for total fat of 20-35% of calories.
The 2006 IOM guidance on saturated fat is a good synopsis of what the U.S. Government position has been since the 1960’s:
“Saturated fatty acids can be synthesized by the body, where they perform structural and metabolic functions. Neither an EAR (and thus an RDA) nor an AI was set for saturated fatty acids because they are not essential (meaning that they can be synthesized by the body) and have no known role in preventing chronic disease. There is a positive linear trend between saturated fatty acid intake and total and low density lipoprotein (LDL) cholesterol levels and an increased risk of coronary heart disease (CHD). However, a UL was not set for saturated fatty acids because any incremental increase in intake increases the risk of CHD. It is recommended that individuals maintain their saturated fatty acid consumption as low as possible, while consuming a nutritionally adequate diet.”
There are no studies linking saturated fat consumption to CVD or CHD. The link between saturated fat and cardiovascular disease was a hypothesis which was never proven.
Credit Suisse Research Institute published an independent report on fat this year and stated:
“Plenty of research funding has been earmarked to study and back this hypothesis, yet we cannot find a single research paper written in the last ten years that supports this conclusion. On the contrary, we can find at least 20 studies that dismiss this hypothesis.”
The other maximal standards the DGAC based their findings on are the 2013 American Heart Association (AHA) Lifestyle Guidelines and a research review conducted by the DGAC looking specifically at fat consumption and hard outcomes (myocardial infarction, stroke, heart failure, and CVD related death).
Hard outcomes were not looked at by the AHA Lifestyle Work Group. The Work Group focused on the impact of dietary patterns on Blood Pressure and Blood Cholesterol (focusing mainly on the impact to total Cholesterol and LDL). From the guidelines:
“The Work Group focused on CVD risk factors to provide a free-standing Lifestyle document and to inform the Blood Cholesterol guideline and the hypertension panel. It also recognized that RCTs examining the effects on hard outcomes (myocardial infarction, stroke, heart failure, and CVD related death) are difficult if not impossible to conduct for a number of reasons (e.g., long-term adherence to dietary changes). However, the Work Group also supplemented this evidence on risk factors with observational data on hard outcomes for sodium. The Work Group prioritized topics for the evidence review and was unable to review the evidence on hard outcomes for dietary patterns or physical activity.”
The last line is significant. The 2013 AHA dietary review did not look at the relationship between dietary patterns and heart disease. It looked at the relationship between diet and LDL with the assumption that total LDL is an accurate predictor of heart disease risk (which it is not).
There are two significant recommendations for saturated fat in the AHA guidelines:
• “Aim for a dietary pattern that achieves 5% to 6% of calories from saturated fat.”
• “Reduce percent of calories from saturated fat.”
Those two recommendations are significant because there are no traditional diets which follow those guidelines and the evidence supporting the recommendations is astonishingly thin.
Thin Slice of Evidence
Of the 6084 possible citations revealed in the initial search by the AHA Lifestyle Work Group for the question of diet, 28 articles discussing 17 studies were selected.
The rationale for the first recommendation is listed under Evidence Statement 11 (ES11). ES 11 is based on three studies, DASH, DASH-Sodium and DELTA and from the AHA Lifestyle Report:
“Of note, in the DASH trials, the effect of saturated fat on LDL-C could not be isolated because macronutrients and other nutrients such as dietary cholesterol were not held constant. In the DELTA trial, the dietary cholesterol and protein were held constant but other nutrients,including total fat and carbohydrates, differed in the comparison groups as shown in Table 4. The LDL-C lowering is consistent in the DASH trials with the lower saturated fat dietary pattern resulting in lower LDL-C. In DELTA, the greater reduction in saturated fat led to greater LDL-C lowering. “
Again, it is important to point out LDL is a poor indicator of heart disease risk. Lowering LDL-C does not mean the risk of cardiovascular disease is being lowered.
The second recommendation is also based on ES11 and adds ES12 and ES 13.
Rationale for ES12 & ES13 is based on two studies done 11 years apart by the same authors. The results of their studies were based on computer predictions of effects (emphasis added):
“We used two meta-analyses from the same authors published 11 years apart in which they used the same inclusion/exclusion criteria and generated predictive equations to estimate changes in plasma lipids when substituting dietary fat types with carbohydrates or other fat types.”
It is worth pointing out the two AHA recommendations on saturated fat both carry the highest “Level I / Class A”rating:
Level 1: Benefits >>>Risk. Procedure / Treatment SHOULD be performed / administered.
Class A: Multiple populations evaluated. Data derived from multiple randomized clinical trials or meta-analyses.
As stated above, “Aim for a dietary pattern that achieves 5% to 6% of calories from saturated fat.” is based on the result of one trial, DELTA and a lowering of total LDL. There was insufficient data to draw conclusions on the impact of saturated fat in the DASH trials.
“Reduce percent of calories from saturated fat.” Carries a top rating despite the fact it is only supported by one trial (DELTA) and one set of researchers who did two computer prediction studies.
Saturated Fat and CVD Risk
The 2015 DGAC also took a look at studies which examined links between saturated fat and cardiovascular disease endpoints. Seven meta-analyses were cited. None cited a link between saturated fat and cardiovascular disease.
Four meta-analyses (Hooper et al.’s 2012, Mozaffarian et al., 2010, Farvid et al., 2014, Jacobsen et al.’s 2009 ) predicted a reduction of cardiovascular disease risk by replacing saturated fatty acids with polyunsaturated fatty acids.
From the DGAC Report on Hooper:
“there was no clear evidence of reductions in any individual outcome (total or non-fatal myocardial infarction, stroke, cancer deaths or diagnoses, diabetes diagnoses), nor was there any evidence that trials of reduced or modified SFA reduced cardiovascular mortality.”
Despite the lack of evidence one sentence earlier, the DGAC still reached the conclusion:
“These results suggest that modifying dietary fat by replacing some saturated (animal) fats with plant oils and unsaturated spreads may reduce risk of heart and vascular disease.”
Shortfalls in the Mozaffarian, Farvid, Jacobsen analyses are all covered in the Credit Suisse Report ‘Fat: The New Health Paradigm’. (Pgs. 37-39).
References
2015 Scientific Report of the Dietary Guidelines Advisory Council
http://1.usa.gov/1MK8dQu
2015 DGAC Scientific Report Appendix E-2
http://1.usa.gov/1OuHklD
AHA – 2013 Lifestyle Guidelines
http://bit.ly/1KS1UeH
AHA – 2013 Lifestyle Guidelines Full Workgroup Report
http://bit.ly/1KS2EAA
Credit Suisse. Fat: The New Health Paradigm
http://bit.ly/1KwjnFD
Alice Lichtenstein March 2014 NY Times Op-Ed
http://nyti.ms/1KS1K75
Institute of Medicine 2006 Fat Guidance
http://1.usa.gov/1MK8TW1