We’ve all seen the article or social media post declaring the “proven” benefits of intermittent fasting.
Cellular repair. Improved insulin sensitivity. A longer life with reduced risk of chronic disease. Increased metabolism and fat burning.
Because of these strong claims, I’ve been wanting to do a deep dive on intermittent fasting for years now. So, in preparation for my latest book for midlife women, I finally did it.
And it revealed that the benefits of intermittent fasting are anything but a slam dunk.
But before we get into all the details, let’s do a quick review of the ins and outs of intermittent fasting.
What exactly is intermittent fasting?
Intermittent fasting is having periods during the 24-hour cycle when you don’t eat (fast).
There are two types of intermittent fasting, including time restricted eating (TRE) and alternate day fasting (ADF).
TRE includes an eating window between 4 to 12 hours. For instance, a 4-hour window would allow eating during that time followed by 20 hours of fasting while an 8-hour eating window allows 8 hours of eating and 16-hour of fasting.
ADF is eating as you normally would one day and fasting or nearly fasting (0-25%) the following day. Or a person chooses two days to fast like 5:2, which is five days of normal eating and two days fasting.
To give you an idea, data suggests most people eat within a 14–15-hour window, meaning they eat more hours of the day than they fast.
What happens when you fast (and “theory benefits”)
Once the human body goes more than 12 hours without food, there’s a decrease in blood sugar, insulin, and leptin as the body uses glycogen stores to make glucose (energy) along with amino acids and fatty acids.
When liver stores of glycogen run out, the body breaks down fat to make ketones as an alternate fuel source. Metabolic flexibility is one of the key theory benefits. I call them “theory benefits” because they are still theories needed to be proven in human studies.
The thinking goes that because fasting requires the body to switch between glucose and fat/ketones as fuel sources; it becomes more efficient at moving between these two states, boosting metabolic health.
A second thing that happens is a decline in nutrient sensing pathways, such as insulin and insulin growth factor. This “turning off” and deprivation of nutrients increases cellular repair and maintenance, referred to as autophagy.
It’s like being forced to leave your house to have it remodeled. When it’s done, you have a new and improved living space.
And of course, fasting is thought to enhance weight loss, especially fat loss. We touch on these theory benefits throughout this post.
1. The ancestor argument carries little weight
One of the most familiar arguments for intermittent fasting revolves around our ancestors. You hear about how they went days without eating, leading to helpful metabolic adaptions that helped them survive.
An article in Inverse interviewed historians to fact-check these claims. Paul Freedman, Ph.D., a food historian from Yale University, argues that our ancestors had more opportunities to eat than we have led us to believe:
We exaggerate the poverty of hunter gatherers. There were good days and bad days but for the most part, people had more choice and security than we think.
Much of this comes from animal research showing improved longevity with dietary restriction (DR). The “evolutionary theory of DR” posits that during food scarcity, organisms invest in themselves and are better prepared (and fit) when food is plentiful again.
But a 2020 study threw a wrench in this evolution theory. The researchers found that fruit flies who switch between fasting and feeding had increased mortality and laid fewer eggs compared to those who stayed on a rich diet. The researchers conclude:
Our results question the current explanation of DR’s evolutionary origins and, thereby, its relevance in interpreting DR’s mechanistic origins.
2. Most of the health claims rely on animal studies
In a 2021 review, Halpern and Mendes assert that most of the health benefits associated with intermittent fasting comes from animal models. This is a problem because rodents react differently to fasting than humans do.
For instance, in most of the studies, rodents eat during the active phase of their day for a few hours, then fast the rest of the time. And they go into ketosis sooner than humans do.
In humans, after 8-12 hours of fasting, ketones reach detectable levels of .2-.5 mM but this jumps to 1 and 2 mM by 48 hours. But rodents have detectable levels after 4-8 hours and are in ketosis by 16-24 hours. In their review, Halpern and Mendes write:
Many animal models demonstrate increased longevity and reduction in chronic diseases with caloric restriction and IF, but data in humans are lacking. Many benefits are alleged to be associated with ketogenesis, but it is unclear whether the level of ketone bodies achieved in popular IF patterns would be enough to elicit the same responses observed in rodents.
Autophagy likely happens earlier in mice too, as some studies suggest. Very few studies even look at autophagy in humans, but the ones that have suggested it can take anywhere from 18 hours to four days to kick in.
3. Intermittent fasting is not superior to calorie restriction for weight loss
When compared to people eating their regular diet, those who intermittently fast lose an average of six and half pounds, which varies depending on the type of fasting.
The weight loss is comparable to calorie restricted diets, but some people prefer to fast because they don’t have to count calories.
There isn’t much long-term data to see if people maintain their weight. One of the longest studies to date compared people on a calorie-restricted diet or ADF for a year. The first six months was the weight loss phase and the second six months maintenance.
There were 100 participants (86 women and 14 men; mean age 44). Weight loss and metabolic biomarkers were similar between the groups.
ADF participants had higher HDL at six months, but not 12 months. And at 12 months, ADF participants had increases in LDL cholesterol. A 2022 review in the Journal of the Academy of Nutrition and Dietetics examined 13 studies comparing fasting and continuous calorie-restricted groups with identical calories. The weight loss and metabolic biomarkers were similar between groups.
READ: Trying to lose weight at midlife. Don’t. Do THIS Instead.
4. Health benefits derived from intermittent fasting alone are inconclusive in humans
To help sort out the benefits of fasting alone, I found a handful of studies that controlled for calorie intake (isocaloric) but did not induce a calorie deficit for weight loss.
In one study, middle-aged men and women were assigned to eating three meals a day or one meal (consumed between 5 and 9pm) of isocaloric meals.
Even though the one-meal-a-day group fasted for longer, the subjects had higher fasting glucose levels, and impaired morning glucose during the 2-month diet period compared to those consuming three meals spread throughout the day.
In another study in Cell Metabolism, eight pre-diabetic men consumed meals of identical calories. Yet one group ate between 6:30-8:30 am and 12-2 pm (6-hour window) while the other had a longer window, starting at the same time but ending at 8pm.
There was no weight loss in either group, yet the early feeding group had large reductions in blood pressure and insulin levels and improved insulin sensitivity.
What these isocaloric studies suggest is eating windows that start earlier and end earlier improve metabolic health more than later eating windows. But it has more to do with the timing of eating.
5. The time you eat may be more important than hours fasted
In just three days of eating and sleeping 12 hours out of a regular pattern, people experience decreased leptin, increased glucose and insulin, increased blood pressure, and reduced sleep quality.
It’s like a retail store that has fewer workers later in the day and at night. If they get an unexpected surge of customers, people will have to wait, and the process is inefficient.
Well, with eating, our bodies are more “ready” to utilize glucose earlier and throughout the day than at night. Needing less inulin to get the job done.
Seventeen people with closely controlled calories had their energy measured in a respiration chamber. While eating the same calorie load, they had days of skipping breakfast or dinner.
On the breakfast-skipping day, glucose was 46% higher after lunch than on the dinner skipping day. Additionally, breakfast skipping increased “the inflammatory potential of peripheral blood cells” after lunch by 45%.
Another respiratory chamber study found that eating 700 calorie meal at 10pm instead of 8am reduced fat burning significantly. The researchers conclude:
We found that the daily timing of nutrient availability coupled with daily/circadian control of metabolism drives a switch in substrate preference such that the late-evening Snack Session resulted in significantly lower lipid oxidation (LO) compared to the Breakfast Session.
6. Having a later eating window may have null or negative effects
Many people push off the morning meal to extend their fast. Plus, it allows them to have dinner with their family or to go out with friends.
To test this later eating window, in 2020 researchers assigned 116 men and women to eat between 12-8pm or as they normally did (control group). At 12 weeks, there was no difference in weight and metabolic parameters between the groups.
But the fasting group also showed a decrease in lean mass.
“I went into this hoping to demonstrate that this thing I’ve been doing for years works,” lead author and UCSF cardiologist Dr. Ethan Weiss told CNBC. “But as soon as I saw the data, I stopped.”
Another study compared an early TRE (6-2pm) to a later window (11-7pm), and a control with 82 people for 5 weeks.
The early group had greater improvements in insulin sensitivity. And the early group, but not the late group, showed improved fasting glucose, reduced body fat and inflammation, and increased gut microbial diversity.
7. Not everyone responds to fasting in the same way
In every study, there are people who respond better than others. So, if you have a later eating window and your metabolic health is good, then that may simply work for you.
And how we respond to any fasting regimen may have more to do with our gut, according to a review in the Journal of Diabetes:.
Because the gut microbiome is involved in the personal response to food, it would be reasonable to assume that the personal response to IF can also be linked to the gut microbiome.
Also, study populations vary based on age, metabolic health, etc. For instance, the results for a population like the prediabetic men above may not apply to women who are not prediabetic.
And any restrictive eating plan is not beneficial for people with a history of disordered eating or eating disorders.
In a 2022 study in Eating Disorders, researchers assessed 44 women and 20 men who were currently intermittently fasted for 16 hours daily for eating disorder symptoms. Compared to controls, the fasting men and women not only had more eating disorder symptoms, but 31% were at or above eating disorder cut-offs.
“While I started to tune in more to my body and develop a more healthful relationship with food, I started to find the restriction in intermittent fasting more and more problematic,” writes Erin, The Incremental Mama, who shared her two-year experience with intermittent fasting and how it triggered disordered eating
8. Men and women react differently to intermittent fasting
A 2013 study on rodents showed that fasting can disrupt female reproduction. The rats did water fasts every other day for 12 weeks, resulting in changes in estrogen, LH, and the menstrual cycle.
Yet according to a review on intermittent fasting and hormones in humans, estrogen was not affected, but there were lower levels of testosterone. Studies also show decreases in insulin, T3, DHEA, and higher levels of cortisol.
What does this mean? No one really knows. Fasting stresses the body, so cortisol spikes are to be expected. Over the long haul, this may or may not be helpful depending on the population.
We know menstruating women who don’t consume adequate energy, experience stress, or have too low body fat may not ovulate. In one study, women with BMIs lower than 25 had increases in blood sugar with ADF, but the men did not.
Women over 40 need to be aware of a few things. First, they experience increases in cortisol during the menopause transition. And second, they are more likely to experience thyroid dysfunction. Could the hormonal fluctuations from fasting negatively affect them?
Again, we are far from decided on this, but it’d be nice to get some answers.
READ: The Telltale Signs You’re in Perimenopause
9. It may not be kind to muscle mass
People who lose weight without resistance training and high protein consumption typically lose about a quarter of lean body mass, yet some research shows fasting can induce even more of a loss.
One review in the International Journal of Obesity comparing fixed regimens of eating found that those with an intermittent approach lost significantly more lean body mass than those on calorie restriction. The researchers conclude:
Since it is ultimately fat mass loss that improves health indices and not the loss of muscle, the significantly greater loss of lean mass in the regular intermittent dieting group versus the continuous dieting group is concerning and needs to be further assessed.
Is this last word on fasting and muscle mass? No. But since it has been argued that fasting protects muscle, this is something people need to be aware of.
10. There is a high cost to benefit ratio
My biggest takeaway from this deep dive is the high cost to benefit ratio. Intermittent fasting in the modern world is challenging and the benefits are not as straightforward as we’ve been led to believe, as you can see the outstanding research questions below.
But there are other things we can do that there have clear benefits and less cost.
For one, exercise helps improve metabolic flexibility, autophagy, and insulin sensitivity.
And building muscle mass through resistance training helps improve insulin sensitivity due to its role in glucose disposal.
So is eating in line with your circadian rhythm by moving eating earlier in the day and less at night. In fact, closing the eating window to a doable 11-12 hours from 14 hours positively affects health.
A 2021 study in the Journal of the American Heart Association found crazy benefits to blood pressure, inflammation and endothelial health when middle-aged adults did just five minutes a day of high-intensity inspiratory muscle strength training (breathing with resistance).
I could go on, but it probably makes more sense to check out my Midlife Strong Newsletter for more easy, sustainable tips.
The research to date shows that intermittent fasting is an alternative to a calorie-restricted diet for weight loss, but is not superior.
Whether it boosts metabolic flexibility, insulin sensitivity, cellular repair, and fat loss beyond its role in calorie reduction and weight loss has yet to be shown in humans.
If intermittent fasting has worked for you, great! But if it hasn’t, there’s no need to feel guilty about it. There is a smorgasbord of tools available to improve your health.
What have your experiences been with intermittent fasting?
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Anne Greene says
As always, I really appreciate your research and thoughtful perspective. I tried “intermittent fasting” a year or so ago after reading that it had some benefits in Wayne Jonas’ excellent book, “How Healing Works.” I was hoping it might help me reduce abdominal fat and gain muscle. My casual method was just to take a mental note of when I finished dinner and try to wait at least 12 hours to have breakfast the next day. For me, this meant cutting out my habit of having late evening dessert with my husband. I didn’t notice any weight loss (abdominal or otherwise), and definitely didn’t gain muscle. I also noticed that deliberately extending my fasting period beyond 12-14 hours (while feeling hungry) led to muscle loss for me; or maybe it was just my perception as I felt weaker and less energetic. In any case, I decided strict IF wasn’t for me.
The good thing I took away from my IF experience (albeit a casual version) is that I no longer eat after dinner, which pretty easily and naturally leads to a nightly fast of about 12-13 hours. I figure it is probably a good thing to allow my digestive system that time to rest.
I’m 52 and post menopausal, so can very much relate to the gradual weight gain and body changes many women experience at this stage of life. After briefly accepting the muscle loss and extra belly fat as “part of life” and nothing to obsess over, I determined earlier this year that I needed to make some changes to my routine… Jogging and doing yoga regularly was no longer enough to keep me feeling healthy and fit at this stage of life.
Some changes that have made a very positive impact on my health and fitness this year include resistance training with free weights 2-3 days per week (~60-90 minutes/week) and eating a LOT more meat and animal fats (and far fewer carbs). I have never had the patience or inclination to count calories, carbs, fat grams, pounds, or anything else. But after about 7 months of adding more meat to my diet and consistent resistance training (along with a bit of jogging and yoga), my clothes are fitting far better than they have in years, I’ve gained significant muscle/strength, the belly fat is gone, and I feel great.
Something that helped motivate me was the book, “The Daniel Plan.” I didn’t follow every aspect of the plan to the letter, but it helped me set goals, stay focused, and be creative in finding what worked to keep my resistance workouts fun. I liked the wholistic approach of considering faith, food, fitness, focus, and friends (rather than just food and fitness). In addition to that, I read a couple of books by Tim Noakes (“Real Food on Trial” and “The Real Meal Revolution”). These motivated me to try increasing my meat and animal fat consumption and reduce the carbs in my diet. Simply eating more meat and fewer carbs gradually decreased my belly fat without any hunger, which has made it easy to stick to the changes. I still do a lot of baking for my family, but rarely partake in the breads and starches.
I agree with you that there’s no one-size-fits-all solution to excellent health for everyone, but just thought I’d share what has worked well for me in case someone else who is frustrated with menopausal weight gain and sarcopenia happens to read this and need a little motivation to get back to feeling great.
Maryann Tomovich Jacobsen, MS, RD says
Thanks for sharing Anne!