Friday, October 31, 2014

Summary of the Low Carb Diet Trials: Part 2

This is just a reminder of the results in the previous post
 
Y = Low carb had significantly better result than low fat
- = Not significantly different regarding low carb vs. low fat
N = Low carb had significantly worse result than low fat
 
Low Carb Diet Myths
 
These results may not be particularly exciting but these trials debunk myths about low carb diets and weight loss
 
1)      Low carb, high fat diets adversely affect blood lipids
 
As you can see in the table above there was often no difference in the changes in blood lipids between the low fat and low carb groups and in some trials the changes in blood lipids were better in low carb group compared to the low fat group
 
This would be expected as other clinical trials have found that replacing carbohydrate with fat decreases both the total cholesterol:HDL-C ratio and triglycerides [1].  Articles that criticise low carb diets based on supposed adverse effects on blood lipids may be forgetting that even though SFA (except stearic acid) increases the total cholesterol:HDL-C ratio, fats in foods are a combination of SFA, MUFA and PUFA and it’s the overall effect is a reduction in the ratio.  Alternatively they may be basing their assessment of CHD risk exclusively on LDL-C or even worse, total cholesterol, despite the total cholesterol:HDL-C ratio being a better predictor of CHD risk than total cholesterol and LDL-C [2]; and non-HDL-C being a better measure of LDL-P than LDL-C (see my rant here). 
 
2)      Low carb, high fat diets adversely affect body composition
 
Often when critics of low carb diets concede that low carb diets can cause weight loss their next argument is essentially ‘yes, you can lose weight on low carb diets, but this just glycogen and/or muscle loss (muscle wasting because of the idea that low carb >> gluconeogenesis from amino acids in muscle >> muscle loss).  Some trials measured body composition (Gardner, Brinkworth and Bazzano) and found nothing that would suggest that the proportion of weight loss from muscle was higher in the low carb group, whereas the low fat diet had an adverse effect on body composition in Bazzano.
 
Also, in case anyone was concerned about bone mineral density, the Foster trial found no difference between the groups regarding the change in bone mineral density
 
3)      Weight loss requires deliberate calorie restriction and induces ‘the starvation response’
 
One of the most unfortunate beliefs regarding weight loss it that it requires deliberate calorie restriction.  However, in many instances with these trials (both low carb and low fat, but mainly low carb) significant weight loss occurred without advice to deliberately reduce calories.  Although whether this statistically significant weight loss is clinically significant is debatable and depends on the trial.
 
 
Low Carb
Low Fat
Calorie Restriction
Significant Weight Loss
Calorie Restriction
Significant Weight Loss
Stern
 
Y
Y
Y
Dansinger
 
Y
Y
Y
Gardner
 
Y
Y
Y
Shai
 
Y
Y
Y
Davis
 
Y
 
Y
Brinkworth
Y
Y
Y
Y
Lim
Y
Y
Y
Y
Foster
 
Y
Y
Y
Iqbal
 
Y
Y
 
Bazzano
 
Y
 
Y
 
Another common belief regarding weight loss is that it induces ‘the starvation response’ (hunger, low metabolic rate, etc).  Dansinger measured the metabolic rate and found it didn’t decrease in any diet group at any time point

Adherence 

Most of the trials followed a design whereby the participants received an intensive intervention for the first 2-3 months then were largely left alone with a few follow-ups afterwards.  Not surprisingly, adherence to the diets was much better during the first 2-3 months and seemed to fall immediately after the intensive support was no longer provided.  In most trials the participants in the low carb diet ended up eating ~30-40% carbs and the low fat diet ~30-35% fat. 

Generally adherence to the diet was equally poor at the final time point, though there were some exceptions.  The Brinkworth trial was a high intensity intervention for the whole period and the participants adhered very well to the diets, which is reflected by their weight loss and metabolic markers improving to a far greater degree than participants in the other trials.  Another exception is the Stern trial, where the low carb group adhered better to the diet (251g to 120g carbs at 12 months) compared to other trials, whereas the low fat group really didn’t change at all (74g to 69g fat at 12 months).
 
Considering that the participants were overweight or obese and some of them also had type 2 diabetes, you would think that one of the best times to start a serious attempt at weight loss would be while participating in a diet trial with all the support you’re given.  So the big question is: why was adherence so poor?  This should be studied with the aim to find a solution to this problem.
 
Diet Quality
 
The focus on these trials was macronutrients, but this doesn’t give much indication of diet quality (with the possible exception of protein).  Most of the trials measured fibre intake which is perhaps not a bad measure of whole plant foods (and therefore diet quality).  The participants had an average intake of ~15g of fibre at baseline, which is similar to the US average (~15g) [3].  Fibre declined in the low carb group in some of the studies, but often only at 2-3 months when carbohydrate intake was more like 20-30% as opposed to 30-40%.  The real surprise was that the low fat group often didn’t increase fibre intake.  The Bazzano trial also measured folate and vitamin C intake and found that baseline intakes were just above the RDI (considering how easy the RDI for vitamin C is to meet, this is pretty bad), which didn’t really change during the trial (even though you would expect both to increase in the low fat group).  Altogether this suggests that baseline diet quality of the participants was poor and didn’t improve in these trials. 

Change in Fibre Intake
Low Carb
Low Fat
Stern
-
Dansinger
↓ (transient)
-
Gardner
↓ (transient)
-
Shai
Davis
↓ (transient)
↑ (transient)
Brinkworth
?
?
Lim
↓ (transient)
↑ (transient)
Foster
?
?
Iqbal
-
-
Bazzano
-
-

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