top of page

INTERMITTENT FASTING: The Evidence-Based Spark-Notes

Updated: Nov 10, 2020

Somewhere between the Insta-spam, Dr. Oz's science fair demonstrations, and your friendly neighborhood mirror-enthusiast in the've heard of it: Intermittent fasting.

First breakfast was the most importantly meal of the day. Now we're dropping it like a clingy ex and opting for a hot helping of fasted cardio instead. So what happened? What is this "intermittent fasting" business and is it really worth forgoing a steaming stack of flapjacks for?

The term "intermittent fasting" refers to a type of fasting where you alternate between short periods of eating and fasting. The most common types of intermittent fasting include:

1. Alternate-day fasting - Consuming little to no food one day, and eating as normal the next

2. The 5:2 diet - Eating as usual for five days, and then fasting for two

3. Time-restricted feeding - Narrowing your food intake to a set time frame - usually 8-10 hours per day - allowing your body to be in a fasted state for a slightly longer duration.

Like most fads within the industry, the origins of this hot-topic stem back to presumptions of how our cave-dwelling ancestors would have eaten.

Its true. Our ancestors could not eat three meals every day. If fact, they likely consumed meals much less frequently, often having just one large meal per day and sometimes fasting for days in between any meals at all. Thus, they were in fact adapted to some form of intermittent fasting. But just because our ancestors did something out of necessity, doesn't necessarily indicate its gold-standard practice for human health and longevity today.

First, lets be clear. Fewer than 1 in 10 Americans even make it 12 hours without eating. Smartphone apps that track hundreds of thousands of eating events reveal (UHM OKAY MY FITNESS PAL, SAVAGE OF YOU) that most people tend to eat every 3-ish hours over an average span of about 15 hours per day. Christ on a cracker, people. We could definitely afford to retire our forks for a minute. But how much of a break is warranted, exactly? And are any of these fasting trends sustainable - or for that matter - risky, in the long-term?

Many diet and exercise trends have origins in legitimate science, but overtime the facts can get distorted, benefits exaggerated and risks downplayed. So, stellar marketing aside - let's separate the facts from the fiction here.

Research supports fasting of any kind as a means to weight loss, improved insulin sensitivity, reduced blood glucose levels, lower blood pressure, improved lipid profiles, and reduced markers of inflammation and oxidative stress. However, weight loss of any kind tends to have these effects.

So the real question here is - does fasting have any unique benefits when compared to weight loss via traditional methods (such as calorie cutting)?

The answer is complex.

Fasting is believed to upregulate certain processes in the body like DNA repair and the recycling of damaged or dysfunctional cell components. This has led researchers to hypothesize that it may play a role in disease prevention and extended lifespan.

On the fat-loss front, fasting has gained press due to our bodies' tendency to switch from burning glucose (in the form of glycogen) to instead burning through our fat stores between 12-36 hours after cessation of food consumption. But if it is fat loss you're after, skipping breakfast may not be the fastest route to washboard abs after all.

Where the Research Stands:

Alternate Day Fasting: Some folks consume absolutely nothing on their fasting days, while others allow themselves to gorge on unlimited quantities of calorie-free beverages and above-ground vegetables (lettuce, cucumber, celery, etc.). Either way you slice it, all types of alternate day fasting have resulted in significant reductions in body fat over 3-12 week durations.

There are, however, a few caveats. This body-fat reduction is only seen if patients are able to refrain from eating two full days' worth of calories on their feeding day. While some participants in the clinical trials only eat about 120% of a typical day's worth of calories following their 24-hour fasts, others go all 'Old-Country Buffet' and end up consuming upwards of 210% their normal daily intake - negating any potential benefits of fasting the day prior.

Additionally, alternate day fasting results in decreased metabolism and increased appetite (just as traditional caloric-restriction does), and has been reported by participants as being difficult to sustain in the long run. And while it can potentially aid in the reduction of blood pressure, triglyceride levels, and asthma symptoms (all dependent upon weight loss), alternate day fasting actually results in worse-off LDL cholesterol levels (a prime causal risk factor for heart disease) when compared to traditional calorie-cutting weight-loss efforts.

All in all - its a no for me, dawg.

The 5:2 Diet: I'll keep it brief here.

The largest study to date on the 5:2 diet found that while participants did lose weight, they actually lost on average two pounds less than they would have, had they just employed traditional calorie-cutting methods. Not only that, but the 5:2 diet group was hungrier and complained of headaches, low energy and difficulty sticking with the diet. The silver-lining being that the 5:2 diet did result in slightly lower LDL cholesterol levels than both traditional calorie-cutting and alternate day fasting.

But any diet that doesn't let me eat for 2 days straight and doesn't transform me into a mother-freaking UNICORN in the process is, quite frankly, dead to me.

Time-restricted feeding: We know from the research (and narc spouses) that people tend to consume more foods and higher fat foods later in the evening. So what happens when we strategically eliminate the prime-time binging hours from our day?

Studies show that when obese individuals limit their eating window to 8 hours (in this case, from 10am-6pm) for 12 weeks, they lose an average of 12 pounds.

We also know that night-time calories are significantly more fattening than those same calories during the day. A large dinner is more fattening than a large breakfast. A piece of cake causes more weight gain in the evening than if eaten at breakfast time. This is all thanks (but no thanks, douche lord) to our circadian rhythm. That's right, that thing that you and I actively combated throughout all four years of college is unfortunately kind of imperative.

Metabolic slowing, hunger, carbohydrate intolerance, triglyceride levels, and a propensity for weight gain all increase during the night. *Internally sobs*

In fact, research shows that shift workers who work and eat the bulk of their meals in the night time and sleep throughout the day are at higher risk for cardiovascular disease, diabetes, and certain forms of cancer. I agree. Its no way to treat our 3am Uber drivers with the hard candy, aux cord and barf bag triple threat. You absolute legends.

When men and women are randomized into two groups - one eating 3 meals throughout the day as normal and the other squishing their 3 meals into a 4 hour window in the evening (from 5-9pm), the time-restricted group will lose some weight, but will also incur a number of poor metabolic effects due to the late timing of their eating window.

So what about the best of both worlds? What about time-restricted feeding with a strategically early eating window?

Sure enough, the best intermittent fasting results can be seen from a 2018 study on early time-restricted feeding. Those eating a day's worth of food between the hours of 8am-3pm experienced significant improvements in blood pressure, oxidative stress and insulin resistance...even if they didn’t lose weight. They were also more energetic, slept better, had lower inflammation markers and better blood sugar control.

In breast cancer survivors, those who avoided eating after 8pm and fasted for at least 13 hours overnight experienced significantly reduced rates of breast cancer recurrence.

While early time-restricted feeding appears to be great for certain disease-related biomarkers, its relevance for athletic performance and weight loss benefits unique from traditional calorie-cutting methods are still areas where further research is warranted.

One study of time-restricted feeding in healthy men showed improvements in strength and endurance as well as an increase in lean body tissue (despite decreased food intake) – but this was with a mere four hour eating window, making it hard to extrapolate those same benefits to the more commonly employed feeding windows of 8-10 hours.

There has been, however, research to support a much more lenient lifestyle able to provide the same benefits as these strict fasting regimens, sans the hunger.

Researcher Valter Longo, a professor of gerontology and biological sciences a USC sought to design a meal-plan that would simulate the effects of fasting, but without patients having to starve. His meal-plan was called the "fasting-mimicking diet" (FMD) and consisted of patients eating as normal for 25 days, and then consuming his "fasting-mimicking diet" for 5 days consecutively. The diet was low in protein, sugar and calories, with zero animal protein or animal fat – comprised of plant-based soups, energy bars, kale chips, algae-based DHA supplements, a multivitamin, and a variety of plant-based drinks.

By making the diet plant-based, Longo only sought to lower the cancer-promoting growth hormone IGF-1 - which he successfully achieved. However upon completion of the study he discovered a number of other benefits as well. After 3 months, the FMD group (eating as normal for 25 days and then a plant-based diet for 5 days, for 3 month-long cycles) was down 6 pounds compared to the control group, with reductions in body fat, waist circumference, and blood pressure. What's more, 3 months after discontinuing the study, the FMD group maintained these significantly improved biomarkers, suggesting this plant-based diet - even for just 5 days per month - had lasting effects.

I see dollar signs, people. Nothing but gold and glory.

"If you could choose between semi-starvation every other day, eating absolutely nothing twice a week, or forgoing late-night dinners and dessert for the rest of your life, which would you choose?" sounds like some cheap play-ground talk to me.

If intermittent fasting, and more specifically time-restricted feeding works for you - wonderful. It is likely a great option for improving disease-related biomarkers and may even help you lose some weight in the process.

But if you could get those same reductions in body fat, waist circumference, blood pressure, blood glucose and serum LDL cholesterol levels without having to go hungry for hours on end, wouldn't you? If you could reap lasting benefits from skipping the starvation and instead eating an abundance of whole plant foods for just five or more days out of every month, doesn't that seem like a sexier B-line to vitality? Who has two thumbs and likes dessert? Who enjoys the occasional late evening dinner date, taking the family out for a sweet treat, or buying snacks at the 7pm movie showing without a cloud of guilt and failure lurking overhead?

Because if that person's you - this fasting business is not your gig.

Bottom line: You have to decide what works for you. If early time-restricted feeding suits your lifestyle perfectly, then get after it! You sexy, circadian slinging, son of a gun!

But between you and zero-starvation, all-the-benefits-with-none-of-the-downsides, unlimited-quantity, fruits, vegetables, whole grains, beans/legumes, nuts & seeds, all day.

Or, more preferably - until about 8pm :)

Until next time, loves.



*Intermittent fasting is NOT recommended for the following individuals: Pregnant and breastfeeding women, those with type 2 diabetes, those with a history of disordered eating, or those needing to eat along with taking certain medications.

**Always consult your medical professional before doing any kind of fasting.

This post was written & medically reviewed by a Registered Dietitian Nutritionist.

Grace Pascale, MS, RDN. Grace Pascale Nutrition.


bottom of page