By Joseph E. Scherger, MD, MPH

Vice President, Primary Care, Eisenhower Medical Center; Clinical Professor, Keck School of Medicine, University of Southern California

Dr. Scherger reports no financial relationships relevant to this field of study.

SYNOPSIS: The NIH has been conducting systematic review and meta-analysis of randomized controlled trials show that replacing saturated fat with unsaturated vegetable oils rich in linoleic acid does not result in a reduction in atherosclerosis, cardiovascular morbidity, and death.

SOURCE: Ramsden CE, Daisy EZ, Majchirzak-Hong S, et al. Re-evaluation of the traditional diet-heart hypothesis: Analysis of recovered data from the Minnesota Coronary Experiment (1968-73). BMJ 2016;353:i1246.

A team of investigators from the National Institutes of Health has been conducting systematic reviews and meta-analyses of previously published and unpublished data from experiments testing the traditional diet-heart hypothesis that saturated fat contributes to cardiovascular disease and death. This hypothesis and subsequent dietary recommendations were based on evidence that replacing saturated fat with unsaturated vegetable oils, particularly with linoleic acid, lowered serum cholesterol.1,2 At the same time, observational studies showed that lower serum cholesterol was associated with lower cardiovascular morbidity and mortality.3 The authors reported that no randomized, controlled trials of using unsaturated fats to lower cardiovascular morbidity have been conducted to prove the hypothesis.

The Minnesota Coronary Experiment was conducted between 1968 and 1973 in one nursing home and six state mental hospitals. Researchers randomized 9,423 women and men ranging in age from 20 to 97 years, with a mean age of 52 years. One-quarter of the subjects were ≥ 65 years of age. Feeding subjects in a cafeteria allowed for more precise assurance of diet adherence than most population diet studies. Both groups ate a similar amount of fat and the average body mass index was 24.5 kg/m2. In the intervention group, saturated fat in cooking oils, salad dressings, and butter was replaced with corn oil products and corn oil polyunsaturated margarine. Autopsy data were available on 149 subjects.

The intervention group with more unsaturated fat intake experienced significant reductions in both total and low-density lipoprotein (LDL) cholesterol, an average of 13.8%. However, there was no difference in mortality among any age or sex cohort. The intervention group actually experienced a higher number of cardiovascular infarcts. Autopsy reports showed no difference in the degree of atherosclerosis among the two groups.

The same authors previously reported a re-analysis of unpublished data from another large randomized, controlled trial, the Sydney Diet Heart Study, conducted during the same period. In this study, the intervention group on the unsaturated fat diet (linoleic acid from vegetable oils) actually showed a higher cardiovascular morbidity and mortality.4

COMMENTARY

It is becoming clear that saturated fats from natural food sources are not a cause of cardiovascular disease. Indeed, the opposite may be true as discussed by Dr. Mark Hyman in his new book, Eat Fat, Get Thin.5 The authors in this study noted any food consumed is a substrate that is biochemically processed in the human body. Man has consumed saturated fats for thousands of years, and most commercially available unsaturated fats are processed foods foreign to the body. The fact that LDL particles in the unsaturated fat group may have greater LDL particle oxidation and may be more inflammatory to the cardiovascular system may explain the paradox that lowering total and LDL cholesterol did not result in health benefits.

A recent randomized, controlled trial showed patients on a low-carbohydrate/high-fat diet had increased LDL particle size (less inflammatory) than patients eating a traditional American Heart Association low-fat diet with higher complex carbohydrates, whose LDL particles actually shrank and became more inflammatory.6 Hence, measuring only total and LDL cholesterol has limitations in that such measurements do not show how inflammatory these cholesterol particles are.

It has become apparent that clinicians have been giving wrong dietary advice to patients for many decades. The low-fat and non-fat food industry was a mistake because it resulted in higher carbohydrate intake and coincided with the epidemic of overweight, obesity, and type 2 diabetes, major risk factors for cardiovascular disease.

Science has a way of breaking paradigms and keeping medical professionals humble. Greater understanding of the biochemistry of nutrition and the human body gives healthcare providers an opportunity to provide better nutrition recommendations and, hopefully, improved food science. The relative lack of nutrition education in American medicine is most unfortunate, as nutrition is emerging as a major therapeutic tool to prevent and reverse disease.

REFERENCES

  1. Executive Committee on Diet and Heart Disease. National Diet-Heart Study Report. American Heart Association, 1968.
  2. Getz GS, Vesselinovitch D, Wissler RW. A dynamic pathology of arteriosclerosis. Am J Med 1969;46:657-673.
  3. Oh K, Hu FB, Manson JE, et al. Dietary fat intake and risk of coronary heart disease in women: 20 years of follow-up of the nurses’ health study. Am J Epidemiol 2005;161:672-679.
  4. Ramsden CE, Zamora D, Leelarthaepin B, et al. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: Evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis. BMJ 2013;346:e8707.
  5. Hyman M. Eat Fat, Get Thin: Why the Fat We Eat is the Key to Sustained Weight Loss and Vibrant Health. New York: Little, Brown and Company; 2016.
  6. Bazzano LA, Hu T, Reynolds K, et al. Effects of low-carbohydrate and low-fat diets: A randomized trial. Ann Intern Med 2014;161:309-318.