What is intermittent fasting? And is it a good idea?

Intermittent fasting, or intermittent energy restriction, is an umbrella term for a diet which involves cycling between a period of significantly reduced caloric intake (fasting) and a period of regular eating.

“Diet” or “dietary restriction” is what most of us think of as “going on a diet”, that is, reducing caloric intake.

Eating fewer calories.

Losing weight.

Those of us with estrogen-positive breast cancer who reduce our risk of recurrence by blocking the body’s estrogen production (aromatase inhibitors) … often find that weight gain is a direct side effect.

For most folks, “going on a diet” is a challenge. And whether we diet because of concerns about health or appearance,  just about anyone who has “gone on a diet” can attest to how difficult long-term “calorie reduction” can be.

That’s one reason scientists began researching intermittent fasting as an alternative to 7-day-a-week calorie restriction. 

What is intermittent fasting?

There is no “one” definition for intermittent fasting. The more accurate term is intermittent energy restriction because, for most people, the fasting days aren’t true fasts. Some approach this systematically: they fast every-other-day. Others simply skip a meal or two on certain days of the week.

One example in popular culture is the 5:2 diet promoted in a 2012 BBC Horizon program and in The Fast Diet by Michael Mosley and Mimi Spencer.

For the purposes of this article, I’ll use the colloquial phrase ‘intermittent fasting’ or its abbreviation, IF, to include all dietary regimes featuring intermittent energy restriction, systematic or ad hoc.

Intermittent fasting and health/longevity, animal studies

Consuming fewer calories than normal leads to longer lives for laboratory animals like mice. It also seems to delay or prevent age-associated disease and improve cardiovascular risk factors. And prevent type 2 diabetes.

Alternate day fasting can also extend the life of some laboratory animals.

In 2004, Mark Mattson found that the health benefits for mice fed every other day was similar to those on a calorie-restricted diet: both populations lived 30 percent longer. “We see vast improvements in variables that indicate risk of disease,” Mattson, a researcher at the National Institute on Aging, said. Bonus: there was no calorie restriction on the “feed” days.

Mattson hypothesized that IF might reduce the number of free radicals in the cells.

In 2005, researchers found that IF coupled with a nominal calorie reduction achieved results similar to cutting calories 33% every day.

A year-long intermittent fasting diet “induced a robust decrease in body weight” in rats. In addition, rats with heart problems improved under this diet. This was the first study to show that IF “ameliorates cardiac function and inotropic reserve in an experimental model of heart failure.”

Laboratory animal research has also shown improved fasting glucose concentrations using alternate day fasts.

Much of the research on caloric restriction has been focused on weight, blood glucose, and the cardiovascular system. But what about the brain, which uses more energy than any other human organ?

In 2008, researchers reported that juvenile female rats on caloric-restricted diets “showed a considerably greater increase in hippocampal weight than the males.” The hippocampus is involved with memory, and size correlated positively with maze performance.

Intermittent fasting and gender, animal studies

In 2009, researchers showed that reproductive response to IF differed by gender. Male rats maintained or increased their reproductive ability. However female rats experienced reduced fertility. Hormonal changes reflected a “tendency towards ‘heightened’ masculinity.”

This genetic response is not illogical. In order to extend the life of the species, the male rat needs to mate with more females, because the females are less fertile. The more malnourished the female, the less likely she could both successfully birth and nurture a litter, so “nature” curtails her fertility.

Although research suggests that excessive caloric restriction may be an impairment for those attempting to become pregnant, other research suggests that caloric restriction may extend the age at which females are fertile.

The game is too young.

Intermittent fasting, human studies

There are fewer studies with human subjects than animals, but the numbers are growing.

Three small sample studies in 2005 suggested such a diet might help men. However, that was not the case for the women in the study, who experienced a decline in glucose clearance and no improvement in insulin sensitivity. (Delayed glucose clearance may lead to insulin resistance.)

Subsequent research suggests the 2005 study may have been an anomaly.

An April 2009 report of the experience of 54 pre-menopausal women showed that intermittent fasting was as effective as daily caloric restriction for both weight loss and health markers.

Research featuring 16 obese individuals (12 women/4 men) showed the subjects adapted “quickly” to an IF diet. There was a two-week control phase, where body weight remained stable, and an 8-week period of intermittent fasting. On fast days, participants consumed only 25% of their energy needs. On alternate days, they could eat as much as they liked.

In this study, average total weight loss was 5.6 kg as reported in 2010 in Nutrition Journal. Physical activity was constant (as measured by a pedometer) throughout the 10-week study, and there was no detectable difference in activity on fast days compared to feed days.

A 2010 report noted success with a 2-day-per-week restriction. This six-month study involved 107 premenopausal women aged 30 to 45 years. It was the largest randomized study comparing IF with normal calorie reduction in “free living humans” (which suggests there is a prison study somewhere that I haven’t found). All women had experienced adult weight gain in excess of 10 kg.

We chose a pragmatic IER [intermittent energy restriction] regimen which … required a simple non-proprietary VLCD [very low calorie diet, 75% restriction] to be taken over 2 days/week. We believe this to be a more achievable than previously studied regimens of alternate days fasting… IER can be offered as an alternative to CER [continuous energy restriction] for reducing obesity and obesity-related disorders in some individuals.


IER [intermittent energy restriction] is as effective as CER [continuous energy restriction] in regards to weight loss, insulin sensitivity and other health biomarkers and may be offered as an alternative equivalent to CER for weight loss and reducing disease risk.

A 2011 report summarized the results of all animal and human trials.

By 2012, new research suggested that severely restricting calories only one day a week was effective. In this study, 54 obese women were divided into two random groups. Both had a calorie-restricted diet for six days with one day fasting (water consumption + 120 kcal of juice powder only). Both groups lost weight and improved coronary heart disease risk markers.

Intermittent fasting and cancer

A 2012 study showed that controlled short-term fasting in mice “improved the effectiveness of radiation and chemotherapy in treating” aggressive brain tumors.

This retrospective study from 2016 suggested that intermittent fasting (at least 13 hours/day) reduced the risk of breast cancer recurrence. Data were from 2413 women diagnosed with breast cancer but not diabetes; they were aged 27 to 70 years at diagnosis.  They had participated in the prospective Women’s Healthy Eating and Living study between March 1, 1995, and May 3, 2007.

Fasting may also be helpful as an adjunct to chemotherapy and radiation treatments for breast cancer as well (2012 study). Animal studies have demonstrated that IF  prolongs survival and decreases metastasis. When used prior to the administration of chemotherapy, IF research has shown that IF can also reduce toxicity.

A systematic review that included 8 studies of IF ranging from 24 to 72 hours found that 63% of the studies (5 of the 8) found that fasting inhibited tumor growth; the remaining 3 found no significant difference in tumor growth compared with animals that were fed a normal diet.

Many of the studies have been suggestive but small.

In another study, 13 patients with HER2-negative, stage II/III breast cancer who were planned to receive neoadjuvant or adjuvant docetaxel, doxorubicin, and cyclophosphamide were randomly assigned to undergo short-term fasting (STF) or normal eating before and after chemotherapy.7 Hematologic toxicities where significantly lower in the STF group, with higher mean erythrocyte and thrombocyte counts 7 days after chemotherapy (= .007 and = .00007, respectively). There were no differences in nonhematologic toxicities. The study also found that DNA damage recovery of peripheral blood mononuclear cells was increased or occurred more quickly from patients who fasted compared with those who did not.

Because these small and retrospective studies are suggestive of positive effects, more research is underway or being planned. If you are interested in the molecular background, see this May 2019 article: Effects of short-term fasting on cancer treatment (no paywall). 

Another fascinating mouse study from 2019 was designed to “find out if fasting could protect the intestines from high-dose radiation, which could allow for higher doses of radiation treatment in killing pancreatic tumor cells.” The answer appeared to be yes.

Is intermittent fasting a good idea?

Scientists have documented the benefits of eating less for more than a century. The anticancer effects of caloric restriction was first identified in animal studies in 1909. And epidemiological evidence argues that even a modest reduction in weight (i.e. 5% of body weight) reduces the incidence and progression of chronic heart disease in women.

A 2014 literature review concluded that intermittent fasting “shows promise” in overweight and obese populations as an alternative to classic caloric reduction.

It is unlikely that there is a dietary solution to sustained weight loss that works for everyone. But intermittent fasting is worth considering, if weight loss or improved health markers are your goals.

If you’re considering a comprehensive wellness system, consider Isagenix, which features a 5-2 or 6-1 system as part of its 30-day weight loss plan. A 10-week clinical study of the 30-day weight loss program, conducted at the University of Illinois at Chicago, showed superior results when compared with a “heart-healthy” dietary plan. A year-long clinical study also beat the traditional heart-healthy diet (pdf). See my Isagenix story.

Always consult with your doctor before beginning a weight loss program.

Photo credit: Wikipedia

Based on this 2015 essay.


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