Have you ever noticed that every month seems to bring a trendy new diet? And yet obesity rates continue to rise and with it a growing number of health problems. That’s why I wrote my new book How Not to Diet. Check it out at your local public library. Welcome to the Nutrition Facts Podcast. I’m your host Dr. Michael Greger.
Today we conclude our series on fasting with a cautionary tale about intermittent fasting.
Are there any downsides to fasting every other day? For example, might going all day without eating impair your ability to think clearly? Surprisingly, the results appear to be equivocal. Some studies show no measurable effects, and the ones that do fail to agree on which cognitive domains are affected. Might the fasting/feasting cycles cause eating disorder-type behavior like bingeing? So far, no harmful psychological effects have been found. In fact, there may actually be some benefit, though the studies that have put it to the test specifically excluded those with a documented history of eating disorders for whom the effects may differ.
What about bone health? No change in bone mineral density was noted after six months of alternate-day fasting, despite about 16 pounds of weight loss, which would typically result in a dip in bone mass. There were no skeletal changes noted in the control group either, however, that lost a similar amount of weight using continuous calorie restriction. The researchers suggest that this is because both groups tended to be more physically active than the average obese individuals by one or two thousand steps a day.
Proponents of intermittent fasting suggest it can better protect lean body mass. But most of the intermittent trials have employed less accurate methods of body composition analysis, whereas the majority of continuous caloric restriction trials used vastly more accurate technologies. So, to date, it’s not clear if there’s a difference in lean mass preservation.
Improvements in blood pressure and triglycerides have been noted on intermittent fasting regimens, though this is presumed to be due to the reduction in body fat, since the effect appears to be weight-loss dependent. Alternate-day fasting can improve artery function too, though it does depend on what you’re eating on the non-fasting day. Randomized to an alternate-day diet high in saturated fat, artery function worsened, despite a 15-pound weight loss (whereas it improved, as expected, in the lower-fat group). The decline in artery function was presumed to be because of the pro-inflammatory nature of saturated fat.
A concern has been raised about the effects of alternate-day fasting on cholesterol. After 24 hours without food, LDL cholesterol may temporarily bump up, but this is presumably just because so much fat is being released into the system by the fast. An immediate negative effect on carbohydrate tolerance may stem from the same phenomenon: the repeated elevations of free fat floating around in the blood stream. After a few weeks, though, LDL levels start to drop as the weight comes off. However, results from the largest and longest trial of alternate-day fasting have given me pause.
A hundred obese men and women were randomized into one of three groups: alternate-day modified fasting (25 percent of their baseline calories on fasting days, 125 percent calories on eating days), continuous daily calorie restriction (75 percent of baseline), or a control group instructed to maintain their regular diet. So, for those going into the trial eating 2,000 calories a day, in the control group they would have continued to eat 2,000 calories a day. The calorie restriction group would have started at 1,500 a day every day, and the intermittent-restriction group would alternate between 500 calories a day and 2,500 calories a day.
With the same overall average prescribed calorie cutting in both weight-loss groups, they both lost about the same amount of weight, but, surprisingly, the cholesterol effects were different. In the continuous calorie-restriction group, the LDL dropped as expected compared to the control group as the pounds came off. But in the alternate-day modified fasting group, they didn’t. At the end of the year, the LDL cholesterol in the intermittent fasting group ended up 10 percent higher than the constant calorie-restriction group despite the exact same loss of body fat. Given that LDL cholesterol is a prime causal risk factor for our number one killer, heart disease or even the prime risk factor for our number one killer. This strikes a significant blow against alternate-day fasting. If you do want to try it anyway, I would advise you to have your cholesterol monitored to make sure it comes down with your weight.
If you’re a diabetic, it’s critical you talk to your physician about medication adjustment for any changes in diet, including fasting of any duration. Even with proactive medication reduction, advice to immediately break the fast should sugars drop too low, and weekly medical supervision, type 2 diabetics fasting even just two days a week were twice as likely to suffer from hypoglycemic episodes compared to an unfasted control group. We still don’t know the best way to tweak blood sugar meds to prevent blood sugars dropping too low on fasting days.
Even just fasting for a day can significantly slow the clearance of some drugs (like the blood-thinning drug coumadin) or increase the clearance of others (like caffeine). Fasting for 36 hours can cut your caffeine buzz by 20 percent. So, consultation with your medical professional before fasting is an especially good idea for anyone on any kind of medication.
In our next story, we put alternate day modified fasting to the test for lifespan extension.
Is it true that alternate-day calorie restriction prolongs life? Doctors have anecdotally attributed improvements in a variety of disease states to alternate-day fasting including asthma, seasonal allergies, autoimmune diseases like rheumatoid arthritis, osteoarthritis, infectious diseases like toenail fungus, periodontal disease, and viral upper respiratory tract infections, neurological conditions like Tourette’s syndrome and Meniere’s disease, atrial fibrillation, and menopause-related hot flashes. The actual effect on chronic disease, however, remains unclear.
Alternate-day fasting has been put to the test for asthma in overweight adults. Asthma-related symptoms and control significantly improved, as did their quality of life, including objective measurements of lung function and inflammation, significant improvements in peak airflow, significant improvements in mood and energy. But, their weight improved too—about a 19-pound drop in 8 weeks—so, it’s hard to tease out effects specific to the fasting beyond the benefits we might expect from weight loss by any means. For the most remarkable study on alternate-day fasting, you have to go back more than a half century.
The 2017 cholesterol findings were the most concerning data I could find on alternate-day fasting. The most enticing was published in Spain 61 years earlier, in 1956. The title of the study translates as “The hunger diet on alternate days in the nutrition of the aged.” Inspired by the data being published on life extension with calorie restriction on lab rats, researchers split 120 residents of an old-age home in Madrid into two groups. Sixty residents continued to eat their regular diet, and the other sixty were put on an alternate-day modified fast. On the odd days of the month, they ate a 2,300-calorie regular diet, and on the even days were given only a pound of fresh fruit and a liter of milk, estimated to add up to about 900 calories. This continued for three years. So, what happened?
Over the duration of the study, 13 died in the control group, compared to only 6 in the intermittent fasting group. But those numbers were too small to be statistically significant. What was highly significant was the number of days spent hospitalized. Residents in the control group spent a total of 219 days in the infirmary, whereas the alternate-day fasting group only chalked up 123 days. This is held up as solid evidence that alternate-day fasting may improve one’s health span and potentially even one’s lifespan, but a few caveats must be considered. It’s not clear how the residents were allocated to their respective groups. If instead of being randomized, healthier individuals were inadvertently placed in the intermittent fasting group, that could skew the results in their favor. Also, it appears the director of the study was also in charge of medical decisions at the home. In that role he could have unconsciously been biased towards hospitalizing more folks in the control group. Given the progress that has been made regulating human experimentation, it’s hard to imagine such a trial being run today; and so, we actually may never know if such impressive findings can be replicated.
Finally today, we look at the effects of eating only 5 days a week or a fasting-mimicking diet 5 days a month.
What about instead of eating every other day, you ate five days a week, and fasted for the other two? The available data is actually similar to that of alternate-day fasting. About a dozen pounds of weight loss was reported in overweight men and women over a 6-month period with no difference found between those on the 5:2 intermittent fasting regimen and those on a continuous 500-calories-a-day restriction. The largest trial to date found an 18-pound weight loss within six months in the 5:2 group, not significantly different from the 20 pounds lost in the continuous calorie-restriction group. Weight maintenance over a subsequent six months was also found to be no different.
Though feelings of hunger may be more pronounced on the 5:2 pattern than an equivalent level of daily calorie cutting, it does not seem to lead to overeating on the non-fasting days. One might expect going two days without food might negatively impact mood, but no such adverse impact was noted for those fully fasting on zero calories, or sticking to just two packets of oatmeal on each of the “fasting” days (which provided about 500 calories a day). Like alternate-day fasting, the 5:2 fasting pattern appeared to have inconsistent effects on cognition and lean mass preservation, and failed to live up to the popular notion that intermittent fasting would prove to be easier to adhere to than daily calorie restriction.
In fact, fewer subjects on the 5:2 pattern expressed interest in continuing the diet after the study was over compared to the continuous restriction control group, attributed to quality of life issues, citing “headaches, lack of energy and the difficulty of fitting the fasting days into their weekly routine.” However, there has yet to be a single 5:2 diet study showing elevated LDL cholesterol compared to continuous calorie restriction at six months or a year, offering a potential advantage over alternate-day regimens.
Instead of 5:2, what about 25:5, spending five days a month on a “fasting-mimicking diet”? Longevity researcher Valter Longo designed a five-day meal plan to try to simulate the metabolic effects of fasting by being low in protein, sugars, and calories with zero animal protein or animal fat. By making it plant-based, he was hoping to lower the level of the cancer-promoting growth hormone IGF-1, which he indeed accomplished, along with a drop in markers of inflammation, after three cycles of his five-days-a-month program.
One hundred men and women were randomized to consume his fasting-mimicking diet for five consecutive days per month, or to maintain their regular diet the whole time. After three months, the FMD group (the fasting-mimicking diet group) was down about six pounds compared to control, with significant drops in body fat and waist circumference, accompanied by a drop in blood pressures. Those who were the worse off accrued the most dramatic benefits. What’s even crazier is that three further months after completion, some of the benefit appeared to persist, suggesting the effects may last for several months. It’s unclear, though, if those randomized to the fasting-mimicking diet group used it as an opportunity to make positive lifestyle changes that helped maintain some of the weight loss.
Dr. Longo created a company to commercially market his meal plan, but to his credit says he donates 100 percent of the profits he receives from it to charity. The whole diet appears to mostly just be a few dehydrated soup mixes, herbal teas like hibiscus and chamomile, kale chips, nut-based energy bars, an algae-based DHA supplement, and a multivitamin dusted with vegetable powder. So, I figure, why spend 50 dollars a day on a few processed snacks when you could instead eat a few hundred calories a day of real vegetables?
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