Saturday, December 27, 2014

Macronutrient Myths from the Mainstream

High Protein Diets are Bad for the Kidneys
 
A common objection to higher protein diets, particularly for weight loss and type 2 diabetes, is that high protein diets are bad for the kidneys.  The Brenner Hypothesis states that increasing the glomerular filtration rate (GFR) and glomerular pressure ultimately increases the risk for and progression of renal disease.  As dietary protein increases GFR and is associated with worse outcomes in people with renal disease, it’s thought that high protein is bad for the kidneys.  However, there is no association between high protein intake and renal disease in observational studies on healthy individuals, short term trials in athletes and people with type 2 diabetes and in animal models [1] [2].
 
We Eat Too Much Protein
 
The RDI* for protein is ~10% of total calories [3], and is often used to argue that the average Australian diet (16.5% [4]) is too high in protein.  However, the same document that sets the RDI also recommends 15-25% of calories should come from protein to reduce chronic disease risk.  As 16.5% is at the lower end of 15-25%, one could argue that if anything we should be eating more protein [3].  Disagreeing with the 15-25% range is fine, just don’t use the RDI as evidence of what we should be eating given that the RDI is the “usual intake at or above this level has a low probability of inadequacy” [3]
 
Too Much Fat Promotes Obesity, Type 2 Diabetes, Cardiovascular Disease and Cancer
 
This idea is on the decline, with many health authorities no longer saying that they recommend a low fat diet*, and instead recommend replacing saturated fat with unsaturated fat.  But the idea that fat is bad still gets reinforced a lot, usually when people lazily describe the problems of the standard western diet as being ‘high in fat’ or ‘high in fat, sugar and salt’ or when problems caused by excess calories are attributed to high fat diets, such as in research settings.  Simply put: is this were true then you would expect the low fat diet in the Women’s Health Initiative to be successful.  However, it only slightly reduced weight [5] and didn’t reduce type 2 diabetes [5], cardiovascular disease [6] or cancer [7] [8] (despite overall improvements in diet quality – more fruits, vegetables and whole grains)
 
* Whether you consider the recommendations to be low fat is another story.  I would classify a diet where fat provides ~30% of total calories to be low-moderate
 
Saturated Fat is Toxic and Promotes Cardiovascular Disease and Unsaturated Fats Are Heart Healthy
 
The idea that saturated fat (SFA) increases the risk of cardiovascular disease (CVD) and unsaturated fats (MUFA and PUFA) decrease the risk of CVD is largely based on their effects on blood lipids.  SFA increase LDL-C and the total:HDL-C ratio, while MUFA and PUFA decrease LDL-C and the total:HDL-C ratio relative to carbohydrates [9].
 
Since PUFAs reduce LDL-C and total:HDL-C ratio more so than other fats, there were a number of clinical trials (mainly in the 60s and 70s) that aimed to investigate whether replacing SFA with PUFA would reduce CVD.  Unfortunately those trials were often poorly controlled, such that the high PUFA group was often advised to eat less TFA and more fruit and vegetables, omega 3s, etc.  Consequently, in some of the trials the high PUFA group did better, and if you pool the trials together in a meta-analysis you can come to the conclusion that replacing SFA with PUFA reduces CHD.  However, in the clinical trials that were better controlled, but not perfect, there was either no difference or increase in CVD and total mortality in the high PUFA group.  See The Diet Heart Hypothesis.
 
As for MUFA, the main evidence from observational studies against SFA only found a benefit when SFA was replaced with PUFA, rather than MUFA or carbohydrate [10] and there doesn’t seem to be any clinical trials that measured CHD endpoints where SFA was replaced with MUFA, except for the Rose Corn Oil Trial (HT Zahc).  In the Rose Corn Oil Trial the MUFA group had more CVD events than the SFA group (9 vs. 6), which wasn't significant.  So if SFA is as toxic as some say it is, then the current evidence against saturated fat suggests MUFA and carbohydrate is equally as toxic (regarding CVD).
 
Vaccenic Acid and CLA are Trans Fats and therefore are Harmful
 
Trans fats can be found in partially hydrogenated oils (and foods containing them like some junk food) and animal fat from ruminants (beef, lamb, dairy).  TFA are pretty much universally accepted as bad due to the harmful effects of the TFA in partially hydrogenated oils.  But it’s a mistake to generalise the effects of the TFA in partially hydrogenated oils to those from ruminant fats as their respective health effects are very different.
 
 
Artificial TFA
(Elaidic acid, etc)
Natural TFA
(Vaccenic acid, CLA)
LDL-C
↔↓
HDL-C
↔↓
Triglycerides
↔↑
Inflammation
↔↓
Endothelial function
Reverse cholesterol transport
Arterial calcification
Association with CVD
[11] [12] [13] [14] [15] [16]
 
People giving nutrition information/advice should know this, but all too often there are articles on ‘butter vs. margarine’ where an argument against butter is that it contains TFA.  If anything, that’s an argument in favour of butter.
 
* The effects of CLA seems to depend on the isomer, where t10c12 appears to be harmful and t9c11 (the most common naturally occurring isomer) appears to be beneficial [14] [16]
 
The Glycemic Index is a Good Measure of Carbohydrate Quality and Low Glycemic Index Diets are Therapeutic
 
The glycemic index (GI) is a measure of how rapidly 50g of carbohydrate from various dietary carbohydrate sources increase blood glucose levels relative to ingesting 50g of pure glucose.  The glycemic load (GL) is equal to the GI multiplied by the total carbohydrate content of the food/meal.
 
The GI has been promoted as a way to distinguish between ‘good’ carbohydrates (those with a low GI) and ‘bad’ carbohydrates (those with a high GI).  High GI carbohydrates are thought to be beneficial by improving glucose control, increasing satiety and reducing the insulin response.  However, RCTs have only found either a modest or no benefit with low GI diets [17] [18] [19].
 
Since carbohydrate sources with low GI often seem to be healthier choices for other reasons than those with a higher GI (fruit and legumes vs. whole grains and refined grains)*, there’s always the issue of whether the results of the positive GI trials are due to low GI or improved food quality, but it’s also surprising that the results even from positive reviews is quite underwhelming.
 
* There are definitely exceptions to this rule.  For example, many starchy vegetables have a high GI while a lot of junk food has a low GI (because of fructose and fat)
 
** The concept of simple vs. complex carbohydrates is kind of similar, but different.  Simple carbohydrates are small sugars such as sucrose and lactose, while complex carbohydrates are large chains of sugars, such as starches and fibre.  While foods with simple carbohydrates often have a higher GI than foods with complex carbohydrates, there are plenty of exceptions.  For example, fruit contains mostly simple carbohydrates but most have a low GI because fructose has a very low GI, and some grains and potatoes contain complex carbohydrates but have a high GI.

5 comments:

  1. "and there doesn’t seem to be any clinical trials that replaced SFA with MUFA"

    Rose Corn Oil Trial?

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  2. Rumenic acid is a cis, trans FA with but one trans bond. Dunno why they call it a trans fat really.
    Seems to inhibit omega 6 elongation and promote omega 3 inclusion in membranes, maybe why there's so much in milk, it's conserved as a promoter of the rarer DHA.
    Probably an optimum intake of CLA that could vary.

    ReplyDelete
    Replies
    1. Perhaps for simplicity, that's why I group them together

      It's interesting that people suggest that both the inflammatory effects of elaidic acid and the anti-inflammatory effects of CLA are due to them both inhibiting AA synthesis and AA eicosanoid synthesis, just different AA eicosanoids.

      Delete
  3. There's also a series of CLA eicosanoids. They probably have activity in their own right.

    ReplyDelete