Juice fasting has become pretty popular, but there’s surprisingly little concrete information about what a juice fast does for you and why. 

They just seem to happen because lots of people have a friend of a friend who’s tried them and they seemed to say good things about it.

So let’s take a closer look at what you’re actually doing to your body when you go on a juice fast.

Juice Fasting Is Nutrient Dense Caloric Restriction

Just to be clear, let’s properly define a juice fast. It’s when you buy a juicer and then use it to liquefy different fresh vegetables, and then only eat the liquid vegetables for a certain period of time. People have done juice fasts for anywhere from 3 days to 6 months. (I’m not sure the latter idea is best for most people).

So when you go on a juice fast you are eliminating everything from your diet except a highly absorbable form of condensed food that’s very nutritious.

So on a per-calorie basis your body is all of a sudden getting way more nutrients than it’s used to that are getting injected and absorbed into your body much more quickly than usual.

Your body’s unusually good at doing a lot with a very little, and with a lot of nutritional bullets in its holster, it can shoot the different nutrients to where they need to go and do their thing.

Caloric Restriction

There is not a lot of formal research done on juice fasting. However, there is a lot of research done on the benefits of caloric restriction.

Caloric restriction is when you deprive your body of as many calories as possible but keep nutrients very high. 

Caloric restriction rocks for your health.

For the 4 billion years leading up to the 20th century, caloric scarcity was the name of the game. Any living thing that consistently passed on its genes had to get very good at getting by with hardly anything at all.

Scores of research have verified the same thing. Practically every living organism undergoes beneficial physiological change when it’s forced into deprivation mode. From C. elegans to fruit flies, to rats to rhesus monkeys.  I’m not aware of any type of animal which hasn’t shown an improvement in stress markers.

Your body needs a certain level of nutrients to survive, and the less calories it takes to attain the nutrients it needs the healthier it becomes.

It’s no accident that the longest living people in the world tend to have caloric intake of about 1200 calories a day. The difference in caloric intake might be the only reason women live longer than men.

It’s why fasting became such a widespread practice in ancient times, and it’s why restrictive diets like veganism, raw foods, or a strict paleo style of eating have beneficial effects.

There’s probably nothing especially beneficial about veganism or paleo per se, it’s just that eating that way requires you to cut out about 95% of the junk you shouldn’t be eating and all of sudden you’re maximizing the nutrients/calorie ratio.

And that’s why juice fasting is so good for you. (If done properly of course).  You’ve stripped away every possible type of dietary filler and are giving your body straight shots of all the different micro nutrients that it needs.

Those are physiological conditions that it responds very well to.

Juice Fasting: Not An Outgrowth of Modern Medicine

When I begin to dig into the available research on a topic the first thing I go to is google scholar.

The results that populate the front page is a geeky delight that I always look forward to.

About 60% of the time I more or less get the results I expect. Different studies of varying quality that were designed to test such and such effect. Blah, blah, blah, blah, blah.

About 30% of the time I don’t find as much useful information as I’d expect, either because the research is unusually dense, or it just covers aspects of the subject that are peripheral to what I’m looking for.

And 10% of the time the results just make me cock an eyebrow.

This was the case when I looked into the health effects of carrageenan. I was expecting a harmless food additive but was very surprised to find out that it’s a dangerous carcinogen.

Juice fasting was a similar story, but for very different reasons.

What caught me by surprise?

There was only 1 paper that studied juice fasting in any way shape or form!

When I typed into google scholar, here’s the results page I was brought to:

For a practice that’s been around for a long time and is frequently talked about in the media that’s remarkably thin.

Of course how you interpret this information depends entirely on your own scruples.

Depending on your priors I’d guess you’ll probably feel two different ways about this:

1). “Modern medicine is a sham! It’s always prescribing hazardous drugs that kill us while they cure us, but has nothing to say about drinking our vegetables! What a crock!”

2). “Do you need any more evidence that juice fasting is a fad!? It’s little more than superstitious quack dieting for people who are desperate to lose weight but don’t want to put in the work! Get real people!”

I’ll leave it up to you to decide which one is the best interpretation.

References

1). Ellers J, Ruhe B, Visser B. Discriminating between energetic content and dietary composition as an explanation for dietary restriction effects. J Insect Physiol. 2011 Sep 5

2). Cruzen C, Colman RJ. Effects of caloric restriction on cardiovascular aging in non-human primates and humans. Clin Geriatr Med. 2009 Nov;25(4):733-43

3). Willcox BJ, Willcox DC, Todoriki H, Fujiyoshi A, Yano K, He Q, Curb JD, Suzuki M. Caloric restriction, the traditional Okinawan diet, and healthy aging: the diet of the world’s longest-lived people and its potential impact on morbidity and life span. Ann N Y Acad Sci. 2007 Oct;1114:434-55

4). Everitt AV, Le Couteur DG. Life extension by calorie restriction in humans.Ann N Y Acad Sci. 2007 Oct;1114:428-33. Epub 2007 Aug 23. Review

5). Houthoofd K, Vanfleteren JR. The longevity effect of dietary restriction in Caenorhabditis elegans. Exp Gerontol. 2006 Oct;41(10):1026-31.

6). Partridge L, Piper MD, Mair W. Dietary restriction in Drosophila. Mech Ageing Dev. 2005 Sep;126(9):938-50