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
-
-

Sunday, October 19, 2014

Summary of the Low Carb Diet Trials: Part 1

Over the past several weeks I looked at the low carb trials, specifically those with at least 100 participants and running for at least 12 months.  The links to those blog posts are below:


Overall, the trials had a good balance between men (796) and women (1064).  All of the studies set ambitious targets for the low carb group as a daily carbohydrate limit of 20-50g or even 100g is a major difference to what most people regularly eat.  In almost all the trials the low carb group returned to eating ~30-40% carbohydrate at the final time point (except in Stern and Brinkworth).  The targets for the low fat group were not too different from what people usually eat, but almost always featured deliberate calorie restriction (CR).


There wasn’t much difference between the low carb and low fat groups, although the low carb group far more often had better outcomes than the low fat group with one exception (more adverse symptoms during first 6 months in the Foster trial).

* In the Gardner trial the low carb group often had a trend for better results compared to the other groups but didn’t reach statistical significance.  This may have been at least partially due to there being 4 groups in the trial, which makes it more difficult for one group to reach statistical significance.



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

Some trials (Stern, Shai, Bazzano) fairly consistently found a better outcome with the low carb diet while others (Dansinger, Brinkworth, Lim, Iqbal) fairly consistently found no difference.  This doesn’t seem to be related to gender or the intensity of the intervention.  I’ve speculated on the factors responsible below:

Low Carb > Low Fat Trials
Low Carb = Low Fat Trials
Trial
Possible Reasons Why
Trial
Possible Reasons Why
Stern
Fairly good adherence in LC group.  Very poor adherence in LF group
Protein intake higher in LC group
Dansinger
Self-selected adherence for last 10 months, therefore likely poor adherence
Shai
Protein intake higher in LC group
Brinkworth
Diets were isocaloric
Bazzano
Protein intake much higher in LC group
Lim
?


Iqbal
Very poor adherence to diets in both groups (no difference between groups regarding diet)

Protein intake may be a major factor in explaining this.  In 3/4 of the trials in the first column the low carb group had a higher protein intake (we aren’t given any dietary intake information in Foster).  Whereas only 1, maybe 2 (but not a big difference), of the trials in the second column had a higher protein intake in the low carb group and the diets in Brinkworth were isocaloric which explains the lack of difference between the groups anyway.


* When considering these results it’s important to remember that the carbohydrate intake in the low carb group was ~30-40% of total calories (rather than 20-50g) in all trials except Stern (251g to 120g carbs at 12 months) and Brinkworth (9% carbs at 12 months).  This means that generally these trials didn’t really test a true low carb diet (which I define as ~25% of total calories or ~100-150g) or a very low carb or ketogenic diet (where carbohydrate intake is ~10% of total calories/~50g or less).

Saturday, October 18, 2014

Low Carb Diet Trials: Bazzano, et al (2014)

 
Participants and Diets
 
148 people (mean age, 46.8 years; 88% female; 51% black) with a BMI of 30-45 and without CVD and T2D were randomised to a low carb diet or a low fat diet.  The groups were similar at baseline (Table 1)
 
Low Carb
Low Fat
<40g Carbs (excluding fibre)
Fibre 25g, prefer MUFA, limit/eliminate TFA
<30% Fat, <7% SFA, 55% carb
Fibre 25g, prefer MUFA, limit/eliminate TFA
 
“We also provided 1 low-carbohydrate or low-fat meal replacement (bar or shake) per day to participants in each group for the duration of the study”
 
“Participants met with a dietitian in weekly individual counseling sessions for the first 4 weeks, followed by small group counseling sessions every other week for the next 5 months (a total of 10 sessions) and monthly for the last 6 months of the intervention.”
 
 
Low Carb
Low Fat
Pro:Fat:Carb
Δ Calories
Pro:Fat:Carb
Δ Calories
Baseline
17:33:48
0
18:35:46
0
3 Months
26:43:29
-740
19:28:53
-616
6 Months
26:43:28
-674
18:28:52
-553
12 Months
24:41:34
-550
19:30:54
-507
 
Physical activity levels were similar throughout the study.  For more detailed info on nutrient intakes see table 2 below
 

Results
 
Both groups maintained significant weight loss at 12 months and the low carb group lost significantly more weight at all time points.  The weight loss in the low carb group came largely from fat loss as the % of lean body mass increased.  Whereas in the low fat group the % of lean and fat mass didn’t change.
 
 
 
At 12 months the low carb group had greater improvements in HDL-C, triglycerides, the TC:HDL-C ratio and the Framingham Risk Score.  In addition
 
  • There was no significant change in TC or LDL-C
  • Systolic blood pressure was significantly lower in both groups at 3 and 6 months, but was not maintained at 12 months
  • Diastolic blood pressure was significantly lower in the low carb group at 3 and 6 months
  • CRP slightly increased in the low fat group (NS) and slightly decreased in the low carb group (NS).  The difference between the groups was significant
  • Creatinine was significantly decreased in both groups at 12 months, perhaps indicating improved kidney function
 
 
75% completed the trial in the low carb group and 73% completed in the low fat group.  There were no differences between the groups regarding ‘symptoms’ except more participants in the low fat group reported headaches at 3 months