The potential health benefits associated with intermittent fasting continue to be a topic of great interest to myself, my clients and the public at large.
Brought to the forefront once again via a familiar celebrity’s latest pseudo-scientific book on “Intuitive Fasting” – it’s time to come back down to earth and have an honest conversation about what we know, and what we don’t, about IF.
In previous blog posts I’ve explored the popularized notion of working out in a “fasted state” and more even explored the limited evidence around fasting and gene expression.
Intermittent fasting is a popular and polarizing subject area but, as with almost all topics in the world of nutrition, is fair game for an honest scientific discussion.
That’s exactly what I try to accomplish with today’s article, which I start by dividing into theoretical and findings-based categories.
Let’s take a closer look at both.
Intermittent fasting, like anything else, comes stocked with a list of theoretical benefits that may be partially supported by some level of literature evidence in either animals or humans.
1) Altered Lipid Metabolism: In theory, because the human body may tend to use more fat for fuel in a fasted state, it has been suggested that intermittent fasting may be offer benefits as it relates to “burning body fat”.
I’ve not found any high quality human studies which suggest this to be the case but when it comes to general weight loss using intermittent fasting there are some things to keep in mind:
A systematic review and meta-analysis of human intervention studies using intermittent fasting as a weight loss treatment found it no more effective than continuous energy restriction.
There is also little evidence that intermittent fasting is superior at modulating resting energy expenditure(REE). REE, in essence, is the amount of energy a body expends at rest and not including other activities that would also require energy.
Some have suggested that fasting “boosts” your metabolism and one potential way to assess that is by measuring changes in resting energy expenditure in people doing intermittent fasting vs more conventional caloric reduction strategies.
No high quality evidence so far to show a compellingly differential effect.
2) Metabolic Flexibility: Because your body flips to fat metabolism in a fasted state, alterations occur on the cellular level ( including to your mitochondria) which some scientists suggest can promote what is known as metabolic flexibility, ultimately improving the health of your metabolism and functioning of your body on a metabolic level. Some believe, for example, that lacking metabolic flexibility contributes to insulin resistance in the long-term. Much more human evidence on the effects of fasting on this phenomenon will be required before firm conclusions can be drawn in this area.
3) Cellular & Metabolic Revitalization: This one is interesting and quite hard to prove with the current state of evidence, but some scientist suggest that the stress induced by fasting actually helps your bodily systems in the long-term. You can think about this along the same lines of taking a cold shower or an ice bath. It may not be particularly pleasant in the moment, but there are benefits to be had after it’s over.
Obviously further exploration and evidence will be required in this area but, to say the very least, the first meal one enjoys post-fast is highly enjoyable and reward on a number of levels ( speaking from limited experience!).
Does Really High Quality Human Evidence On Intermittent Fasting Exist?
A very salient quote from a 2015 paper:
[W]hether fasting actually causes improvements in metabolic health, cognitive performance, and cardiovascular outcomes over the long term; how much fasting is actually beneficial; and where the threshold of hormesis resides (i.e., a balance between long-term benefit from fasting compared with harm from insufficient caloric intake) remain open questions. Unfortunately, the vast majority of human studies of a fasting intervention were weight-loss studies using single-arm, nonrandomized approaches or multiple intervention arms with no control.
For fasting to be more than a weight-loss fad, greater scientific rigor is needed from interventional trials than is found in the literature. Whereas enthusiasm for fasting is increasing, clinical relevance remains low because of insufficient human data, including almost nonexistent controlled trials.
Intermittent Fasting And Diabetes-related Indicators
Despite similarities in body fat loss between intermittent fasting and continuous energy restriction, there is some evidence to suggest that those who engage in fasting experience greater reductions in insulin resistance, which is a very fascinating consideration.
A 2018 randomized trial published in JAMA following patients with T2DM over 1 year period showed that intermittent fasting is no more effective than continuous energy restriction at reducing A1C ( a marker of blood sugar control).
Benefits Independent Of Weight Loss
A 2018 study out of Cell Metabolism explored, for the first time through a controlled trial, if fasting had metabolic benefits independent of weight loss.
They used a specific time of fasting ( early time-restricted feeding – eating early in day to align with circadian rhythm – finished eating by 3pm) and found it improved insulin sensitivity and blood pressure and reduced oxidative stress.
I actually discuss this particular topic, known as ETRF, at much greater length in another article.
Today’s article is another step towards a better understanding of the potential benefits of intermittent fasting on human health.
There are certainly areas of interest that exist in this subject area, but higher quality evidence will be required before more firm claims can be made.
Please do be wary of those who speak about intermittent fasting in a dogmatic fashion, I’m very careful with my clients to be honest about the pros and cons of this approach to food.
Until next time,
Andy De Santis RD MPH