New Analysis of Old Data: A Paradox to the Diet-Heart Hypothesis?

In All, General, Nutrition, Research Brief by Keri Szejda


A research brief on “Re-evaluation of the traditional diet-heart hypothesis: Analysis of recovered data from Minnesota Coronary Experiment (1968-73),” a peer-reviewed article published in the journal, The BMJ.

Background

The well-accepted “diet-heart hypothesis” predicts that replacing dietary saturated fat with unsaturated fat (specifically vegetable oils high in linoleic acid) reduces heart disease by decreasing serum cholesterol. Randomized-controlled trials have established that this dietary substitution reduces serum cholesterol, but only observational evidence has linked low serum cholesterol to heart disease outcomes.

Purpose

Key data from the Minnesota Coronary Experiment (MCE), a landmark randomized controlled trial from 1968-1973, remained unpublished. The researchers analyzed previously unpublished data from the MCE trial.

Methods

The MCE was a double blind randomized controlled trial set in a nursing home and six mental hospitals. The control diet was high in saturated fat; the intervention diet appeared the same but corn oil (a vegetable oil rich in linoleic acid) replaced the saturated fat. A total of 2355 participants were exposed to the study diets for one year or more. Observed outcomes included plaque build up, heart attacks, and deaths.

Secondly, the researchers conducted a meta-analysis of five randomized controlled cholesterol-lowering trials.

Key Results

The intervention resulted in lower serum cholesterol level, but did not improve coronary heart disease outcomes, including plaque build up, heart attacks, and deaths. Moreover, participants with a greater reduction in serum cholesterol had a higher risk of death (though this finding was driven by the 65 and older subgroup). The meta-analysis of five studies showed no mortality benefit of cholesterol-lowering interventions.

Editor’s Note

Understanding risk factors for heart disease is important. In the United States, heart disease is the leading cause of death for both men and women, accounting for 25% of all deaths.

The Seven Countries Study, led by Ancel Keys from 1958 -1970, was the first to make the connection between saturated fat and cholesterol, and cholesterol with heart disease risk. This study found support for the “diet-heart hypothesis,” which predicts that replacing dietary saturated fat with unsaturated fat reduces heart disease by decreasing serum cholesterol.

Every five years, the Dietary Guidelines Advisory Committee conducts a systematic review of the body of nutrition literature, and publishes a Scientific Report with nutrition recommendations. In accordance with the diet-heart hypothesis, the 2015 US Dietary Guidelines recommend limiting saturated fat to 10% of total calories per day.

To reduce the risk of developing heart disease, the Centers for Disease Control and Prevention (CDC) recommends a diet low in saturated fat, trans fat, and salt. Other recommended lifestyle factors include physical activity, maintaining a healthy body weight, limiting alcohol consumption, and avoiding tobacco use.

One of the fascinating things about this new study is that the researchers were able to recover previously unpublished data – collected by Ancel Keys and his colleagues – from nearly fifty years ago. And the methods were strong – a double blind randomized controlled trial allows for interpretation of cause and effect, not just associations.

Of course, this single publication does not offer sufficient evidence to call into question the diet-heart hypothesis. There are always limitations to research studies. One of the biggest limitations for this study is that we don’t know what the effects might be for longer-term dietary interventions; participants remained on the intervention diet between 12-56 months. Causality claims are valid, but can only be ascribed to the intervention diet as a whole. The intervention diet was indeed low in saturated fat – but was also high in corn oil and linoleic acid. The authors noted that “Increasing dietary linoleic acid has been shown to increase oxidized linoleic acid derivatives”… which have “been implicated in the pathogenesis of many diseases including coronary heart disease” (p.13). Finally, the participants were residents of mental hospitals and a nursing home – which means the findings may not be applicable to a more general population.

Still, the study’s findings should not be discounted – they are an important addition to a body of literature. And when a study contradicts current scientific knowledge, it certainly offers food for thought.


Source

Ramsden, C. E, Zamora, D., Majchrzak-Hong, S., Faurot, K. R, Broste, S. K, Frantz, R. P, Davis, J. M, Ringel, A., Suchindran, C. M, and Hibbeln, J. R. (2006). Re-evaluation of the traditional diet-heart hypothesis: Analysis of recovered data from Minnesota Coronary Experiment (1968-73). BMJ, 353, i1246. doi:10.1136/bmj.i1246