With that said, it is my experience that bloggers tend to sensationalize research in an attempt to drum up followers. The excerpt you provided is fairly representative of this. I'm not going to address the animal studies cited, mostly because we would need to agree that rats and humans are equivalent, and I'm not prepared to do that. With respect to the human studies, though, as far as I can make out, the primary concern raised is with insulin sensitivity and glucose tolerance in women in response to IF:
Straight off the bat we have a problem because if you click through the link to get to the study, you'll notice immediately that PFW got some fairly basic things wrong, because that particular study had 16 total participants, 8 of each gender ( see the table below ), not 41 as the above quote would have it. Maybe PFW is confused and referring to another study, but it should make you raise an eyebrow, at least.Originally Posted by Paleo For Women(PFW)
Leaving that aside however, if we intend to make decisions based upon this study's results, the first thing we need to do is to make sure that the study participants were fairly representative of the general population. For instance, we wouldn't necessarily want to radically change our lifestyles based on studies where herbivorous animals were fed a carnivorous diet with resulting adverse effects.
Let's take a look at some of the characteristics of the subjects from the Heilbronn et al study:
A couple of things immediately strike me about this, and I've highlighted them above. First notice that the males in this study are pretty fat relative to the females, given that they have roughly the same body fat percentage, which is very unusual for the general population, as women tend to carry more body fat in interesting places ( for the most part )
The second thing that jumps out is that the males have double the fasting insulin levels of the females, and yet, they have about the same blood glucose levels. In other words, relative to the women in this study, the men are significantly hyperinsulinemic. Given that this is the case, would we not expect a different response among the men and women to an ADF intervention? Would the results have been the same if the researchers had studied hyperinsulinemic men compared to hyperinsulinemic women? How about hyperinsulinemic men vs. normoinsulinemic men? Hyperinsulinemic women vs. normoinsulinemic women? If we want those questions answered, we'll need to do the studies ourselves, unfortunately.
But setting all this aside, the research seems to suggest that the women cleared glucose less rapidly after the ADF regime. Why would this be the case? Well, one simple answer would be that they are adapted to oxidizing fat ( ketone adapted ) and are reserving glucose for tissues the depend on it for their survival. If this were the case, ketone oxidizing tissues would become physiologically insulin resistant, giving us the observed glucose tolerance results. This would have been a very simple thing to rule out by merely testing the subjects for ketones in the blood, but unfortunately these researchers did not do so.
Animal studies ( for what they are worth ), on the other hand, found that ADF, as expected, caused a doubling of ketone bodies ( aceto-acetate and beta-hydroxy butyrate ) due to increased fatty acid oxidation:
Heibronn et al also did not control for several confounding factors, including neither the subject levels of physical activity, nor their diets:Also noted by Anson et al was a doubling of the plasma concentrations of beta-hydroxybutyrate in the ADF group but no change in the control group.
It is well known that moderate activity is glycolytic in nature, which means that those subjects that were "quite active" would be glucose sponges. Since we are not told by the researchers who the "quite active" subjects were, if we were to assume that they were all male, that in and of itself would be enough to skew the glucose tolerance test.Subjects had different levels of physical activity, with seven being sedentary, three being moderately active (exercise one to two times per week), and six being quite active (exercise three or more times per week).
So we don't know what the subjects ate when they weren't fasting. We don't know the caloric intake, the macronutrient breakdown, nor the meal timing ... all of which potentially play a role in glucose clearance rates. Note, for example that a carbohydrate heavy diet predisposes one to rapid glucose clearance whereas a fat heavy diet does the opposite.On each feasting day, subjects were informed that they could eat whatever they wished.
When all is said and done, there are significant problems with this study, far too many for any conclusion to be drawn. I would be extremely leery of basing any decision about IF on this "evidence."
Lastly, an extremely important point that is glossed over by PFW is that this study employed an alternate day fasting protocol. So essentially, study participants would eat one day, but not the next. Even if we were to concede that this study's results were relevant, to claim that the ADF fasting protocol is equivalent and representative of the wide variety of approaches to intermittent fasting is disingenuous, if not downright intellectually dishonest. Consider that over the course of six days, participants would eat on three out of the six days. Were the ADF protocol to be replaced by three consecutive days of fasting , followed by three consecutive days of eating, it wouldn't surprise me if we would get different results. PFW, on the other hand, seems to believe that the two protocols would be equivalent.
PFW goes on to reference another Heilbronn study which found beneficial benefits for both men and women resulting from ADF such as beneficial changes in HDL ( women only ) and triglyceride levels( men only ). So, both genders experienced benefits, but in different ways.
I would certainly agree with PFW that it would be great if we had comparative studies of the effects of various fasting protocols on both men and women. I think the so called evidence that PFW cites in support of the assertion that fasting is unhealthy for women is drivel.