Sunday, July 28, 2013

Why We Should Question Dietary Advice: Summary and Implications

Summary 

Most of the evidence that forms the basis of national dietary guidelines and position statements of health authorities comes from observational studies where they find correlations between diet and disease.  While you shouldn’t draw conclusions from observational studies anyway, observational studies of this type are notoriously unreliable because of the large number of confounding variables and inaccurate self-reporting.  Just because something has a positive relationship in observational studies doesn’t mean it will hold up in clinical trials as evidenced by hormone replacement therapy, grain fibre, etc. 

Many health authorities have financial ties to the food industry.  While they say that the need to work with industry, the reality is that the food industry funds education programs that spread propaganda to protect their interests.  The financial ties to the food industry are also an implicit endorsement of their products, intentional or not 

Most dietary recommendations include the reduction of saturated fat and trans fats, and often replacing them with polyunsaturated fats, to reduce cholesterol, thereby lowering the risk of cardiovascular disease.  However, replacing saturated fat with linoleic acid (an omega 6 polyunsaturated fat) increases cardiovascular disease and total mortality in controlled clinical trials.  The disaster with artificial trans fats should be a warning against other new additions to the food supply 

One of the most common and unquestioned bits of dietary advice is to reduce salt to lower pressure.  However, reducing salt has a very mild effect on blood pressure and doesn’t reduce mortality, suggesting it is far from being the primary cause of hypertension.  Salt reduction has some adverse effects: it increases the renin-angiotensin-aldosterone system and noradrenaline, and actually increases mortality in a few diseases. 

One would think that a perquisite of good diet advice is that the recommended diet is at least adequate in all nutrients (depending on the criteria for adequacy).  However, most dietary advice suggests reducing (or at least not promoting) choline rich foods such as eggs and liver (due to unfounded dietary cholesterol concerns).  The result is that most people are choline deficient 

The dietary guidelines and other sources of diet information regularly reduce foods or food groups to a few nutrients such as protein, iron and fibre.  They then group foods into groups based on similarities in only a few nutrients such as the dairy and meat alternatives, even though the dairy and meat alternatives have a very different nutrient profile to dairy and meat, even if they are rich in calcium or protein.  This may result in poor, unbalanced nutrient intakes 

The dietary guidelines and other sources diet information often have a biased ranking of nutrient density they mention the benefits of fibre and phytonutrients, but don’t mention the benefits of non-essential nutrients in animal foods such as carnitine, carnosine, creatine, taurine and coenzyme Q10 


Implications 

  • Be sceptical when observational studies are used to claim causality
  • Be especially sceptical of diet epidemiology and advice that relies on it heavily
  • Be sceptical of advice coming from people/groups with conflicts of interest
  • Replacing saturated fats with vegetable oils won’t reduce the risk of cardiovascular disease
  • Don’t eat artificial trans fats (natural trans fats are fine) and be sceptical of new additions to the food supply
  • Low salt diets won’t reduce blood pressure by much and may be harmful
  • Get enough choline (eggs, liver)
  • Don’t be a reductionist, there is more to food than a few nutrients here and there
  • Omnivorous diets are best for meeting our nutrient needs
  • Appreciate the non-essential nutrients and beneficial compounds in both plant and animal foods

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