Inducing weight gain and glucose intolerance in mice using a high fat diet (for my honours project) has got me interested in the low carb diet trials. I don’t think fat per se is promotes obesity or glucose intolerance, at least not in humans. After all, low carb diets are reputed to be equal to if not more effective than low fat diets in clinical trials. That being said, I also don’t buy into the carbohydrate insulin hypothesis of obesity, nor do I think that carbohydrates are poisonous or toxic (within reason).
I searched for low carb diet trials by going through meta-analyses and this Wikipedia page (which was quite thorough). I was surprised how many of them there were, but many of them were quite small and/or short, which is to be expected.
This meta-analysis (published 2012) was particularly helpful. It included 23 trials based on the following criteria: the low carb diet was defined as ≤45% of energy from carbohydrates*; there being a low fat comparison where ≤30% of energy came from fat; at least 6 months long; random allocation; participants at least 18 years old; and no “differences other than macronutrient and energy intake between the 2 diets”***
They arrived at the conclusion that:
“Reductions in body weight, waist circumference and other metabolic risk factors were not significantly different between the 2 diets. These findings suggest that low-carbohydrate diets are at least as effective as low-fat diets at reducing weight and improving metabolic risk factors. Low-carbohydrate diets could be recommended to obese persons with abnormal metabolic risk factors for the purpose of weight loss. Studies demonstrating long-term effects of low-carbohydrate diets on cardiovascular events were warranted.”**
I want to look at several of the main low carb diet trials more closely. So after compiling a list I decided to look at trials that had at least 100 participants, lasted at least a year and were proper low carb or very low carb diets (~ ≤25% energy from carbohydrates). This left me with 9 trials which I’ll look at in the following weeks
1. Stern, et al (2004). The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults: one-year follow-up of a randomized trial.
2. Dansinger, et al (2005). Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction: a randomized trial
3. Gardner, et al (2007). Comparison of the Atkins, Zone, Ornish, and LEARN diets for change in weight and related risk factors among overweight premenopausal women: the A to Z Weight Loss Study: a randomized trial
4. Shai, et al (2008). Weight loss with a low-carbohydrate, Mediterranean, or lowfat diet
5. Brinkworth, et al (2009). Long-term effects of a very-low-carbohydrate weight loss diet compared with an isocaloric low-fat diet after 12 mo.
6. Davis, et al (2009). Comparative Study of the Effects of a 1-Year Dietary Intervention of a Low-Carbohydrate Diet Versus a Low-Fat Diet on Weight and Glycemic Control in Type 2 Diabetes
7. Foster, et al (2010). Weight and metabolic outcomes after 2 years on a low-carbohydrate versus low-fat diet: a randomized trial.
8. Iqbal, et al (2010). Effects of a low-intensity intervention that prescribed a low-carbohydrate vs. a low-fat diet in obese, diabetic participants.
9. Lim, et al (2010). Long-term effects of a low carbohydrate, low fat or high unsaturated fat diet compared to a no-intervention control.
* In my opinion 45% of energy from carbohydrates should be considered moderate and ~25% of energy from carbohydrates should be considered low (see here). That meta-analysis included 8 trials where 30-45% of energy from carbohydrates. I’m also kind of interested in those because that’s kind of the level I eat, but I suspect there probably won’t be much, if any difference between the groups considering the macronutrient composition is so similar and food quality seems to be far more important than macronutrient ratios
** The wording in trials, meta-analysis, reviews, etc suggests that in the authors’ mind low carb, high fat (LCHF) diets are kind of guilty until proven innocent. This probably goes back to ideas that ‘fat clogs your arteries’ and fat is calorie dense therefore fattening. But then when LCHF diets don’t worsen the lipid profile and are at least as effective as low fat diets regarding weight loss they still remain hypervigilant, looking for the first warning signs to say ‘I told you so’ or ‘the LCHF diet is unsafe’. I wouldn’t mind so much if the same approach was also applied to conventional dietary advice (not to mention testing national dietary guidelines in clinical trials), but it isn’t
*** Even though this was one of their criteria, many of their included trials didn’t actually meet it as for example, many low carb diet trials include an ad libitum low carb diet and a calorie restricted low fat diet