Not so fast…

empty plateInterest in fasting as a method of dieting, rather than as religious observance, seems to be the rise. Certainly lots of people are starting the new year with the 5:2 diet (five days of regular eating followed by a 2 day fast). More radically, others are trying the ADF version, alternate day feeding, of intermittent fasting. Our post on intermittent fasting and sex differences in response continues to be one of the most read posts on the blog.

Fasting didn’t work for me (in the sense of fat or weight loss) though I still think fasting is a useful exercise in getting comfortable with hunger. Making peace with hunger, treating it neutrally, rather than as an emergency was one the things that the lean eating program (reviewed here) taught me. I still avoid hunger but I’m now I’m able to respond without panic, drastic measures, or eating in advance in order to avoid it.

I was interested then to see the research summary published by the Dietitians of Canada 2014 (Practice-based Evidence in Nutrition): December 2013, “Is Fasting Healthy?.

I’m going to quote it in length.

They write that fasting and juice-only fasting are being broadly advocated as health-promoting measures. Fasting is promoted as a way to remove toxins. Fasting is also promoted as a way to improve cardiovascular health. It’s also promoted as a route to weight loss.

But are these claims borne out by the evidence?

In the section of their report, Evidence Analysis, they write:

“In spite of the promotion of fasting to remove accumulated toxins through liver enzyme activation, liver enzyme activation is much more complex than first assumed. Further animal and human research does not support the suggestion that activation of these enzymes is health promoting. The apparent increase of cytochrome P-450 enzymes in rodent livers is actually due to the reduction of liver size, as fasting shrinks total liver size and so the actual liver content of this enzyme does not increase

Of additional concern, fasting markedly reduces liver glutathione which facilitates drug excretion and is an important endogenous antioxidant….

Thus, fasting and very low calorie diets, which are promoted to enhance the liver’s ability to remove toxins, are in fact more likely to reduce the liver’s detoxifying abilities by reducing both cytochrome P-450 and glutathione.”

What are the overall benefits and risks of fasting?

“Some trials of alternate day fasting for eight weeks in obese people did find improvements in coronary heart disease risk markers however, several studies revealed increases in insulin resistance within 24 to 72 hours of initiation of fasting (11-13). Even skipping breakfast for 14 days was found to raise LDL cholesterol and produce insulin resistance in non-overweight women (mean BMI=23.2±1.6)”

There are also additional concerns about fasting raised the evidence they summarize.

They write that human studies have illustrated various
concerns about fasting including:

• The loss of muscle mass was noted in one study as being significant within 24 hours and progressed to further losses as the fast proceeded to 72 hours. This loss occurred even when the eight healthy young male athlete participants exercised (60 minutes of running and
strength training including sit-ups, push-ups, and squat repetitions) during the fast. The fast also resulted in a progressive loss in physical work.

• Fasting resulted in significant declines in functional reach and how long healthy, young women (n=22) were able to balance on one leg after even a one-day.

• Fasting for 24 hours in men (aged 18-35) decreased mononuclear cell
mitochondrial activity, an important aspect of immune function .

• Fasting headaches occur at rates of 4% after 16 hours of fasting (18), and in up to 50% of people with a single day of fasting. These headaches were not due to dehydration were not related to caffeine or nicotine withdrawal, and were found to resolve within 72 hours after the resumption of food intake.

• At the point of 5% weight loss, a six- day fast produced less fat mobilization and more intracellular water loss compared with very low calorie (600 kcal) and low calorie (1240 kcal) diets in three groups of six obese men (21).

• Results of studies of long-term juice- only fasting for up to eight weeks
indicate a loss of 20% body protein; a loss of 20-25% fat-free mass;
biochemical evidence of deficiencies in thiamine, riboflavin, and vitamin K; a loss of 40% of whole body zinc (22), and osmotic myelinolysis (23). Once a long-term fast (44 days) was ended, people who fasted for these durations developed complications including refeeding syndrome with marked hypophosphatemia; hemodilution and neurological symptoms with difficulty breathing, speaking, andswallowing, and requiring ventilation support.”

What’s the bottom line?

“A close examination of the evidence regarding the health effects of fasting does not support the denial of eating as a health-promoting activity. In fact, fasting may actually be deleterious to health by:

-increasing insulin resistance while decreasing important liver detoxifying enzyme functions
– decreasing muscle mass
– decreasing nutrients
– decreasing ones ability to work and exercise”

Sounds to me like, just don’t do it. For the full report with lots and lots of references and notes, see here.

7 thoughts on “Not so fast…

  1. Thanks for posting about the drawbacks of fasting. I’m a yoga teacher in California, and I see so many people fasting and juice-fasting like crazy in the belief that it is “cleansing” and detoxifying. My own experience with a short juice fast is terrible gas and overall sluggishness. I am firmly in the Michael Pollen camp: Eat real food; not too much; mostly vegetables and fruits. I love fresh juices and smoothies, but not as replacements for the full range of foods we were designed to eat. On a positive not, maybe people are turning away from artificial powders and supplements that promise outlandish health benefits. Also juicing seems to promote greater awareness of the importance of organic produce and eating locally:-)

  2. I came to your blog via the WordPress blog highlighted healthy eating and living, and I’m so glad I clicked on that link!

    I believe I always knew that fasting was a bad idea (I mean, nothing about it sounds fun or healthy), but it’s so nice to have scientific research to back it up. In my mind, eating mostly healthy meals most of the time is better than vacillating from one extreme to the other. I’ll take my sauteed kale with a glass of red wine, please!

  3. Try the Clean Program. This is currently my second attempt and I love the way I feel. Yes, I am looking forward to a meal for dinner in one week to complete my 21 day Clean…but it’s still nutrition that blended up and I make sure it’s yummy or I won’t eat it.

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