For example, in this 3 month-long study, 30% CR was compared to an increased unsaturated fatty acid group and a control group (Witte et al., 2009). CR group lost weight whereas the other two groups actually gained weight (unintended). In this study, 30% CR resulted in an apparently pretty impressive improvement in memory:
HOWEVER: The actual dietary changes in each group were… interesting (see Table S1) -- the control group ended up doing dietarily almost exactly what the CR group did… so why didn’t their memory score improve? Why didn’t they lose weight? I don’t know, but something seems fishy.
CR garners zero points for this.
Next on the docket is a systematic review (Benau et al., 2014), which concluded: “The combined results are equivocal: several studies report no observable differences as a result of fasting and others show specific deficits on tasks designed to test psychomotor speed, executive function, and mental rotation.” That was based on 10 studies. Given the nature of human biology, “no effects of IF” would not have been unexpected. Kinda surprised at all of those negatives.
The ice age fairy tales go something like this: no food for a day or two improves your cognitive capacity, makes you sharper so you can get your next meal (be it via hunting, gathering, or whatever).
I'm not anti-IF; in fact, I think everyone in weight maintenance should be able to go through the night without the need to wake-up to eat. I'm just commenting on the available studies by people who aren't selling books on the topic.
This next study tested a full 2-day fast in young male weight lifters (Solianik et al., 2016) and came to a relatively similar conclusion (ie, mixed effects): “Fasting improved mental flexibility and shifting set, whereas no changes were observed in working memory, visuospatial discrimination, and spatial orientation ability. Fasting also increased anger, whereas other mood states were not affected by it.” … “48 h fasting resulted in higher parasympathetic activity and decreased resting frontal brain activity, increased anger, and improved prefrontal-cortex-related cognitive functions, such as mental flexibility and set shifting, in amateur weight lifters. In contrast, hippocampus-related cognitive functions were not affected by it.”
Lastly, this study on young overweight women undergoing a similar intervention -- a 2-day fast (Solianik et al., 2016). “Fasting did not affect mood, brain activity, and cognitive, motor, and psychomotor performance. Thus, 2-day total fasting evoked moderate stress with a shift of the autonomic nervous system balance toward sympathetic activity in overweight women.”
The Solianik studies confirm what seems to be one of the main problems caused by IF: it’s stressful. In the male weight lifters, this manifested as increased anger; in the overweight women, it was sympathetic nervous system-induced stress which can be agitating. In either case, no bueno.
It’s not really appropriate to compare across studies like this, but the 2-day fast increased parasympathetic nervous system activation & anger in men and sympathetic nervous system & agitation in women. Of course, this is confounded by lean vs. overweight, etc., but still, something to keep an eye on.
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