Sunday, July 14, 2013

Why We Should Question Dietary Advice: Part 2

Saturated Fat and Trans Fat 

Due to diet heart hypothesis (saturated fat >> cholesterol >> CVD), the first dietary guidelines for Americans recommended replacing animal fats and tropical oils with vegetable oils and margarine, regardless of the trans fat content of the vegetables oils and margarine.  And in the 1980’s the CSPI lobbied fast food to replace animal fats with partially hydrogenated vegetable oils.  There are two problems with this: 

1.      Saturated fat doesn’t increase CVD
2.      The artificial trans fats are toxic 

Concerns about artificial trans fats were raised in the 1970s by Mary Enig who was ignored and silenced for decades.  Artificial TFA only started to be taken seriously in the 90’s perhaps because in 1992 Walter Willett found an association between trans fats and CVD in the Nurse’s Health Study [1].  After more research in the 90’s, the FDA decided to have TFA on labels in 1999 [2].  (In the US food products can have up to 0.5g of TFA per serve.  Food manufactures can get around this by decreasing serving size and by having artificial TFA in mono- and diglycerides). 

We may have largely sorted out the artificial TFA problem now (not quite as it's still in the food supply), but before we get smug, I don’t think history will be kind on us either

Artificial TFA were introduced to the food supply in the 1910’s, decades past until their effects were researched and then two more decades past until those concerns were taken seriously and no longer ignored and silenced (although artificial TFA are still in the food supply).  True, regulations may be tighter now and we do have the internet, but I suspect history will repeat itself many times: 

  • Food manufactures create product
  • Industry promotes their product to health authorities using industry funded studies
  • Health authorities endorse product based on the results of biased studies and financial ties to industry
  • Sometime later, independent studies find the product is harmful 
  • More time passes until the independent studies are taken seriously rather than being inappropriately ignored, criticised or silenced, or perhaps this never happens 

Remember, half the reason for using artificial TFA was that replacing animal fats with vegetable oils, specifically SFA with linoleic acid, was supposed to reduce CVD.  Instead swapping SFA for linoleic acid seems to increase LDL oxidation and CVD.  To avoid writing the same stuff again see , The Diet Heart Hypothesis (most important) and Fats and LDL Oxidation.  Also see DGA2011 - Total Fat and Saturated Fat for

Health experts and the media often group artificial TFA (elaidic acid) and natural TFA (vaccenic acid and CLA) together as if they’re the same thing.  Elaidic acid has many negative effects beyond increasing the TC:HDL-C ratio, whereas natural TFA doesn’t share those negative effects and has some positive effects.  This shouldn’t be too surprising as natural TFA have probably been part of our diet for 2 million years and is also found in breast milk.  Breast milk also contains SFA, apparently your mother was trying to kill you

Also, on a somewhat related note, most dietary advice suggests reducing fat for weight loss and CVD, which is the reason why low-fat dairy and lean meat is specifically recommended.  Never mind that high fat dairy is either inversely associated with, or not related to obesity and CVD [3] (if you're going to use diet epidemiology to make recommendations, you could at least be consistent).    Anyway, low carb diets just as, if not more effective (at 6 months) than low fat diets for weight loss, and also improve cardiovascular risk factors like the Total:HDL-C ratio and triglycerides better than low fat diets.

See DGA 2011 - Total Fat and Saturated Fat: Part 1 and 2

Salt 

Reducing salt is a fairly universal dietary recommendation that has its basis in the assumption that high sodium intakes cause hypertension.  A Cochrane review found that while sodium intake has an effect on blood pressure, the effect is small and is therefore unlikely that high sodium intakes are the cause of hypertension. 

[3]
Systolic
Diastolic
Mainly Caucasians – High Blood Pressure
-4.18
-1.98
Mainly Caucasians – Normal Blood Pressure
-1.27
-0.54
African Americans – Normal and High Blood Pressure
-6.44
-1.98

However, salt restriction increases the activity of the renin-angiotensin-aldosterone system (the RAAS)* by more than 300%, increases noradrenaline by 30% and adrenaline by 12% [3].  Salt restriction can lead to an increase in insulin resistance through noradrenaline [4] and an increase in oxidative stress though the RAAS [5] 

Most importantly a Cochrane review found that salt restriction didn’t reduce all-cause mortality (there was a non-significant reduction in mortality) [6].  Some studies find salt restriction may increase all-cause mortality in some diseases such as T1D [7], heart failure [6] and hypertension [8] and may increase the risk of developing end-stage renal disease [7].  It's very interesting that the diseases blamed on salt and/or hypertension, heart failure, hypertension and end-stage renal disease, are the ones where reducing salt may be harmful, which is probably due to up regulation of the RAAS (the therapeutic target of some anti-hypertensives).  We shouldn’t generalise from disease states to other contexts

Further Reading:
(1) The Oiling of America
(2) Stop Trans Fat
(3) Salt and Blood Pressure
(4) Sodium Intake in Populations: Assessment of Evidence

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