Those of you familiar with my nutritional beliefs will know that I don’t have too many good things to say about the ‘low-fat’ paradigm that has pervaded nutritional advice for some 30 years. Not only do I believe that there is not much to be feared in naturally-occurring fats in the diet, but the fat-phobia engendered in us has also tended to give the impression that carbohydrate-rich foods can be eaten with impunity. Actually, there is abundant evidence, I think, to suggest that a glut of carb in the diet might speed our path to excess weight, and the attendant problems that may result from this including type 2 diabetes.
The fact that conventional nutritional guidelines may have actually contributed to the obesity epidemic was recently explored in an article published on-line in the American Journal of Preventive Medicine [1]. Its lead author was Dr Paul Marantz, associate dean and professor of epidemiology and population health at the Albert Einstein College of Medicine at Yeshiva University, New York, USA. The crux of the article is that nutritional guidelines, particularly the party-line on fat, has been unleashed into the public domain despite the fact that there is little, if any, evidence to support its benefits for health.
They also draw attention to the fact that the rush to de-emphasise fat in the diet has been accompanied by more prominence coming from carbs. As the authors of this article point out, as carbohydrate intake has increased, so has as obesity. They also draw to our attention that this may relate to the fact that people might tend to eat more as a result of believing that low-fat foods are ‘safe’. Also, they suggest that fat can help sate the appetite, which may put a natural lid of food intake. To these mechanisms, I think it’s worthwhile noting that the principle hormone that drives fat storage in the body is insulin ” and the primary driver for this is the consumption of carbohydrate.
Dr Marantz and his colleagues do not make the case that any of this proves that the recommendations to eat a low-fat, high carb diet has caused the obesity epidemic seen in the States. However, they state there is a realistic possibility that they may have. And I have to say I agree with them.
The authors go further, in reminding us that there is a dictum in medicine which states ‘First, do no harm.’ In light of this, and in the absence of convincing evidence that a particular dietary approach does good overall, they suggest that maybe the best thing to do is not to issue a guideline at all. If its evidence-based advice we’re interested in, then I don’t think you can fault the logic here.
However, not everyone agrees with some of the sentiments expressed by Dr Marantz and his co-authors. The piece is followed by a commentary written by Dr Steven Woolf and Dr Marion Nestle of Virginia Commonwealth University and New York University respectively. In their commentary, they concede that nutritional research is far from perfect and can be a sometimes blunt tool with which to discern the truth about the relationship between food and health.
However, they go on to make a strong case that Dr Marantz and his colleagues are wrong on the idea that the evidence used to support the low-fat paradigm is inadequate. They say, for instance, that: “Dozens of randomized trials have now documented the ability of low-fat diets to reduce the incidence of cardiovascular events and sometimes to reverse atherosclerotic disease.” The study they quote to support the idea that low-fat diets reduce cardiovascular events (like heart attacks and strokes) is a meta-analysis of 27 relevant studies [3]. Here’s the results of this study:
There was no significant effect on total mortality (rate ratio 0.98, 95% CI 0.86 to 1.12), a trend towards protection form cardiovascular mortality (rate ratio 0.91, 95% CI 0.77 to 1.07), and significant protection from cardiovascular events (rate ratio 0.84, 95% CI 0.72 to 0.99). The latter became non ” significant on sensitivity analysis.
Trials where participants were involved for more than 2 years showed significant reductions in the rate of cardiovascular events and a suggestion of protection from total mortality. The degree of protection from cardiovascular events appeared similar in high and low risk groups, but was statistically significant only in the former.
So, what this means is that people at high risk of cardiovascular events appeared to be less likely to suffer a heart attack or stroke. However, those who are essentially healthy (that’s most people, by the way) did not benefit. And overall, NO LIVES WERE SAVED. Not exactly a ringing endorsement for the effectiveness of low-fat eating, unless of course you’re only concerned with focusing on subsets of the population (those at high risk of cardiovascular disease) and only one disease process (cardiovascular events) rather than focusing on broad, meaningful measures of health like overall risk of death.
The other study Drs Woolf and Nestle quote to support the idea that low-fat eating can cause regression of atherosclerosis (furring up of the arteries) is a peach. It took individuals with heart disease and subjected them not just a low fat diet, but a aerobic exercise, stress management training, smoking cessation and group support (a clue to the methodology used in the study is in the title) [4]. It is a truly fundamental principle of science that a ‘multiple intervention’ study of this nature cannot be used to prove the effectiveness of any single approach employed in the study. How do we know that the regression in atherosclerosis seen in this study was due to the low-fat diet? We don’t, because it could have actually been due to one or more of the other interventions. To quote this study as supportive of the supposed benefits of low-fat eating are, quite honestly, laughable (and shocking).
Later on in their commentary, Drs Woolf and Nestle claim that fat is ‘causally linked’ with cancer. They quote a recent review of the literature that, although voluminous in nature, relies on epidemiological research that can only find associations between things and can never prove ‘causality’ [5]. And they neglect to mention that in the biggest study to date to assess the effects of healthy eating (including lower fat consumption) on health, the result was NO REDUCED RISK is any of the 20 plus types of cancer that were assessed [6].
Dr Marantz and his colleagues don’t pretend to have the answers. However, they seem to me to have the intelligence to know this and the humility to admit it. I think they make a persuasive argument that the traditional ‘low-fat’ paradigm is based on inadequate science. The ultimate irony, of course, is that it seems that it’s just this sort of inadequate science that has been cited by Drs Woolf and Nestle in an attempt to ‘prove’ them wrong.
References:
1. Marantz PR, et al. A call for higher standards of evidence for dietary guidelines. American Journal of Preventive Medicine 2008 [epub 22 Jan 2008]
2. Woolf SH, et al. Do dietary guidelines explain the obesity epidemic. American Journal of Preventive Medicine 2008 [epub 22 Jan 2008]
3. Hooper L, et al. Reduced or modified dietary fat for preventing cardiovascular disease. Cochrane Database Systematic Review 2000.
4. Ornish D, et al. Intensive lifestyle changes for reversal of coronary heart disease. JAMA 1998;280:2001-2007
5. World Cancer Research Fund and American Institute for Cancer Research. Food, nutrition, physical activity, and the prevention of cancer: a global perspective. 2007
6. Prentice RL, et al. Low-fat dietary pattern and cancer incidence in the Women’s Health Initiative Dietary Modification Randomized Controlled Trial.
J Natl Cancer Inst. 2007;99(20):1534-43 [Epub Oct 9 2007]
