Metabolic Effects Of Intermittent Fasting: Are There Any?

Intermittent fasting remains one of the hottest and most intriguing topics in contemporary nutrition.

It’s something that comes up often in both my daily  and professional life, while also happening to be something I’ve already written about more than once in the past.

In case you missed them, I’ve previously discussed fasting in the context of weight loss and my own personal experience trying it, but never looked specifically at it’s potential “metabolic” benefits.

That’s the buzz word isn’t it? Metabolic…

Very often you hear proponents of any given dietary approach suggesting that it offers some form of metabolically advantageous effect beyond standard means of calorie management/restriction.

As of now, much of the data that supports a beneficial “metabolic” effect of fasting is limited to animal studies.

But what about humans?

Now, technically speaking, we know that human fasting, even for a short period of time, has a relevant effect on markers such as blood lipids and sugars ( think about those fasted blood tests you do…) but does that really mean anything in the long term?

Metabolic Benefits Of Intermittent Fasting: Are There Any?

Excluding weight loss, which I’ve discussed previously, I took a look at a sampling of the research in the area of fasting that explored other potential health effects.

Let’s see what I found…

Gene Expression

A study out of the Rejuvination Research journal in 2015 found that groups of individuals who engaged in intermittent fasting over a  period several weeks had a slight increase in the expression of the SIRT3 gene.

SIRT3 happens to be a mitochondrial protein. For those that may not know, the mitochondria help a cell that fuel metabolic activities by producing energy ( kind of like an engine).

It’s hard to make any sort of conclusion as to the long-term relevance of this finding, but it is a fascinating one.

Metabolic Rate/Hunger Hormones

A 2005 study out of the American Journal Of Clinical Nutrition found that alternate day fasting over a 3 week period yielded no changes to subjects resting metabolic rate or hunger hormone ( ghrelin) levels,  but did decrease fasting insulin levels.

This was not a double blind control study and just looked at a single group of fasting subjects at baseline and after the intervention period.

A systematic review of clinical trials out of the Molecular And Cellular Endocrinology Journal in 2015 found that subjects who participated in intermittent fasting tended to experience a decrease in the hunger suppressing hormone leptin, even though reported appetite did not increase.

Once again, interesting findings but nothing particularly decisive or mind blowing.

Broader Effects

Another article, this time out of the Journal Of The Academy of Nutrition and Dietetics in 2015, admitted that there is limited data to support the beneficial metabolic effects of fasting that animals experience, in humans.

Interestingly enough, a long-term 2017 trial out of JAMA comparing fasting to daily calorie restriction actually found that the fasting group ended up with increased LDL ( “bad”) cholesterol levels at the end of a 12 month period. There was no significant difference in any other metabolic indicators of health or weight, besides the fact that drop out rates were higher in the fasting group ( unsurprising as fasting simply isn’t for everyone!).

An even more recent 2018 study out of the British Journal of Nutrition suggests that fasting may improve metabolic indicators of health in “overweight/obese” subjects.

To add a further layer of intrigue, a 2017 review of the evidence out of the Annual Review of Nutrition journal  unearthed some observational evidence suggesting that longer periods of nighttime fasting ( the period of time from your last meal at night to your first meal in the morning) was associated with lower A1C levels ( an indicator of blood sugar control) and a reduced risk of recurrence in breast cancer survivors.

There is, at the very least, a limited but intriguing body of evidence looking at the metabolic and health effects of fasting in humans.

Final Thoughts

Although today’s article probably left you with more questions than answers, it’s fair to say there is little high quality human evidence to suggest that engaging in fasting of various types offers any profound health benefits.

There is certainly some intriguing evidence out there that warrants further examination but it appears, for now, that we are still a ways away from any concrete determination of the practical health benefit of fasting.

Guess what though? If you are otherwise healthy and enjoy fasting, why not continue?

It is an approach that may be impractical for some, but enjoyable, manageable and even preferable for others.

What you choose to eat, rather than when you choose to eat it, appears to remain the most important predictor of long-term health.

Personally I find fasting a useful activity to engage in once in a while, and if nothing else, it really allows you to appreciate that first post-fast meal.

Until next time,

Andy De Santis RD MPH