I am so excited to share with you the myriad benefits of Intermittent Fasting, all backed by extensive scientific research.
This is post #2 in a series on IF.
This post will go into the incredible health benefits of IF.
Note on terminology
Scores of research studies have been done on what is popularly known as “Intermittent Fasting” or what researchers like to call “Time-Restricted Feeding.” These terms, as well as their abbreviations (IF and TRF), I will use interchangeably to cover all forms of choosing to eat only during a specific time window.
Review on How IF Works
The two main types of energy your body and brain use to function are glucose/carbs and fat/ketones.
Your body uses glucose when you are in a “fed state” or several hours after eating. Your body uses some glucose that is absorbed immediately after eating but then stores the rest in your liver and muscles for use over the next several hours.
Your body uses ketones when you are in a “fasting state” or after your body has used up all its glucose stores. Ketones come from broken-down fat stores. Basically all of the fat you eat and even some excess carbs are stored as fat to be used if needed in this fasting state.
Your body uses up all of its glucose stores and switches into a fasting state approximately 12 hours after your last meal, sooner if your last meal is smaller.
The many different versions of IF that I discussed in the previous post all put your body into a fasting state for some period of time regularly every day or every week.
Even a small amount of time in a fasting state every day or every week adds up. As it turns out, this fasting time offers your body many great health benefits.
Now on to what the research shows:
Weight loss, in particular fat loss, has been a primary focus of IF research with lots of compelling, positive results.
Often studies will compare IF to traditional dieting (restricting daily calories overall but not restricting eating time).
Three well-done studies showed that daily IF (eating during only certain hours every day) caused people to lose more weight than traditional calorie-restricting diets. Even when both groups (IF and traditional dieters) were eating the same number of calories, the IF group lost more weight, most notably when the eating window was earlier in the day compared to later. (Gill, 2015; Gabel, 2018; Antoni, 2018)
Interestingly, daily IF has only recently come into the research focus over the last few years but weekly IF has been researched for a very, very long time.
SEVENTEEN different studies compared to weekly IF (alternate day or 5:2 method).
Five of them did show MORE weight loss than traditional dieting. (Harvie, 2013; Williams, 1998; Wing, 1994; Schubel, 2018; Hutchinson, 2019)
The other twelve studies showed that weekly intermittent fasting had EQUAL amounts of weight loss compared to non-time-restricted calorie-deficit diet.
While the difference in weight loss wasn’t statistically significant, almost all trended towards the IF group losing more weight, suggesting that if the study was done for a longer period of time, it likely would have shown more weight loss in IF, even when the calorie deficit is the same.
Another interesting thing about these studies is how many people dropped out. A certain number of participants always drop out of studies for various reasons. In these studies, the number of people that dropped out was almost twice as high in the continuous restricted diets compared to the intermittent fasting diets. This suggests that IF is much easier to stick to than traditional dieting. (Catenacci, 2016; Harvie, 2011; Varady, Bhuntani, et al 2011; Hill, 1989; Ash, 2003; Keogh, 2104; de Groot, 1989; Carter, 2016; Zuo, 2016; Trepanowski, 2017; Carter, 2018; Sundfor, 2018)
In summary, IF causes similar or MORE weight loss than traditional dieting AND was easier to stick to.
Diabetes and Blood Sugar Control
High blood sugar over a long period of time is what leads to prediabetes, type 2 diabetes, and what is called “metabolic disease.”
Many studies have shown that IF helps with many different measures of blood sugar control.
Hemoglobin A1c (HbA1c) is a blood test level that tells roughly how high your blood sugar has been over the last couple months.
Many studies have shown that women who followed a daily IF routine had lower HbA1c levels and lower blood sugar spikes 2 hours after meals even when they ate the same number of calories per day as the group that didn’t restrict the time of meals. For every 3 hours extra that women fasted per day, their HbA1c dropped by 20%! (Marinac, 2015, Williams, 1998)
Similarly, when people ate the same number of calories they ate before to maintain their weight, eating only during 6 hours of the day improved their insulin sensitivity, improved the insulin production from the pancreas, and decreased insulin spikes after meals even though they didn’t lose weight. (Sutton, 2018)
Many, many other studies have shown:
- Lower insulin levels (Harvie, 2011; Harvie, 2013; Jamshed 2019)
- Improved insulin sensitivity (Harvie, 2011)
- Better function of cells (β-cells) that produce insulin in pancreas (Harvie, 2013)
- Lower overall blood sugar and smaller blood sugar spikes (Antoni, 2018; Jamshed, 2019)
Even in athletes who had no problems with diabetes or metabolic disease to begin with, daily IF showed significant decrease in insulin levels, decrease in fasting blood sugar, and decrease in insulin resistance compared to those athletes maintaining a normal diet, even when the calories were the same (Moro, 2016).
Intermittent fasting can help decrease your blood pressure whether you are decreasing your calorie intake or not.
When people ate the same number of calories to maintain their weight, eating only during 6 hours of the day decreased their blood pressure even though they didn’t lose weight! (Sutton, 2018) The people in this study ate early in the day which means that most of their salt intake was earlier in the day and their kidneys could remove the sodium more easily without being hindered by melatonin (your kidneys don’t make as much urine at night so that you can sleep uninterrupted).
Other studies have shown that both weekly and daily IF decreased subjects blood pressure (Gabel, 2018; Harvie, 2011) and maintained long term decrease in blood pressure even after they stopped the IF trial (Zuo, 2016).
Both when subjects were using IF to restrict calories and lose weight as well as when subjects were only limiting time of eating but did not restrict calories and did not lose weight, IF led to improved cholesterol and triglyceride levels. Many of these showed higher level of improvement in IF subjects compared to normal calorie-restricted diets.
- Decreased bad cholesterol (LDL) (Moro, 2016; Harvie, 2011; Zuo, 2016; Hutchinson, 2019; Antoni, 2018)
- Decreased triglyceride levels (Moro, 2016; Harvie, 2011; Hutchinson, 2019; Antoni, 2018)
- Increased good cholesterol (HDL) (Trepanowski, 2017; Antoni, 2018)
Suppresses Inflammation & Increase Stress Resistance
Inflammation is a good thing to have at least some of the time to repair physical injuries, fight infections, and also build muscle. This kind of good inflammation is referred to as acute inflammation. But inflammation can easily get out of control, damaging more than only what it intended to fix. This usually results in chronic inflammation which leads to arthritis, heart disease, stroke, and even Alzheimer’s disease.
During the fed state, when your body has excess energy and protein sources available, it focuses on growth and energy storage. This is a pro-inflammatory state which means your body is ready and primed to throw full resources towards any inflammatory spark.
During the fasting state, the body does what it can to conserve resources and therefore is an anti-inflammatory state. During the fasting state, the body also removes damaged proteins and intracellular organelles to recycle for fuel. These partially damaged components are often what causes inflammation to become out of control in a chronic fed state.
It’s easy to see why IF helps fight chronic inflammation while still providing resources for acute inflammation when needed. But more than just theory, many research studies provide actual evidence of this.
In one study, when overweight people ate the same number of calories they previously ate to maintain their weight, eating only during 6 hours of the day decreased their oxidative stress levels (a molecule called 8-isoprostane) even though they didn’t lose weight! (Sutton, 2018)
Another study in athletes showed decreased inflammatory markers (called IL-6, IL-1β, and TNF-alpha) in the IF group but no change (and even some increase) in the normal diet group. (Moro, 2016)
Another study looked and what genes were activated in IF and found that IF increased expression of a gene (SIRT1) that protects against inflammation, oxidative stress, and DNA damage. (Jamshed, 2019)
Interestingly, all the research showing decrease in inflammation comes from daily forms of IF, such as the 16:8 method. This is likely because the daily methods allow the body a good balance of resources to regularly fix what needs to be fixed while still sufficiently interfering with the chronic inflammation cycle.
Improved Mental Performance
Personally, one of my favorite effects of IF and one of the main reasons I follow this eating pattern is the feeling of liveliness it brings me. Scientific studies have corroborated this effect beyond my personal anecdotal evidence.
One study had subjects track how they felt on their regular diet and then when they switched to daily IF, they reported better energy and improved mental performance. (Gill, 2015)
One possible explanation why comes from a study that showed that daily IF (especially early in the day) increased a signaling chemical in the blood called BDNF which allows the brain to maintain and regrow nerve cells. (Jamshed, 2019)
Notably, the studies that show improved mental performance are also the daily methods (16:8) whereas a few studies showed some subjective decrease when doing the weekly (5:2) method.
Along with increased mental performance, IF, especially eating early in the day also brings me personally much better sleep.
This has also been shown in at least one study where participants recorded better sleep after switching to a daily IF diet (Gill, 2015).
Many, many studies in both animals and humans have shown IF decreases our risk for cancer. I’ll only focus on the studies in humans here but this has been shown multiple times over in rodents, pigs, and monkeys as well.
One study showed that women with type 2 diabetes have a higher rate of breast cancer and that treating the type 2 diabetes with IF as the only intervention decreased those women’s chance of getting breast cancer later in life (Marinac, 2015).
Another study showed that in women who were already treated for breast cancer, starting a daily IF eating plan decreased their risk of breast cancer recurrence (Marinac, 2016).
How could this possibly work?
One hormone called DHEAS is both an inflammatory hormone and high levels of this hormone have been linked with increased risk of breast cancer. Harvie, et al (2011) showed significant decrease in this DHEA hormone with weekly IF compared to regular dieting.
Jamshed (2019) similarly showed that daily IF increased expression of a gene called LC3A which produces an method of removing damaged genetic material and damaged genetic material is often what leads to cancer.
Along with being anti-inflammatory, by regularly “cleaning” your body of unsafe, possibly cancerous cells, it makes sense that IF would decrease your risk of getting cancer.
Please note that although this evidence does indicate a DECREASED risk of cancer, it does not eliminate your risk. Please discuss your individual risk factors with your personal physician and follow his/her recommendations for you for cancer screening.
Interestingly, the primary focus of IF research in the 1980s and 1990s was on effects of IF on aging and lifespan in mice and rats, with remarkably promising results.
Human research on the subject has only been started more recently but is equally as exciting.
Research by Anton, et al in 2019 showed daily IF in older adults improved their functional ability and walking speed.
Other research by Jamshed, et al in 2019 showed early-timed daily IF reversed low levels of BDNF and high levels of IGF-1 which are associated with aging and decreased mental performance.
Decreased Alzheimer’s Disease and Parkinson’s Disease
This research has looked extensively and found remarkably positive effects of IF on neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease.
Over 80 studies (detailed in review article by Longo in 2014) have shown that from mice to penguins, IF has prevented development of Alzheimer’s and Parkinson’s diseases in those animal populations.
There is no research on this directly that I could find in humans but one study in humans (Jamshed, 2019) showed that early-timed daily IF increased levels of a protein called BNDF and increased expression of a gene LC3A, of which low levels are well-associated with Alzheimer’s disease in humans.
Decreased Effects of Brain Injury
Another really cool effect of IF (that has only been studied on animals so far) is its ability to minimize damage from stroke and brain injuries.
Many different studies (detailed in Fann, 2017) have given animals IF diets both before and right after strokes or traumatic brain injuries. When compared to those animals with similar injuries on normal diets, the ones on the IF diets had better functional activity afterward and had less loss of brain tissue long-term.
Possible Increased Life Span
Again all of the research here has been done on animals as this would be too difficult time-wise to study in humans.
However, almost 100 studies on rodents and other (non-human) primates (detailed in review article by Longo in 2014) have consistently shown increases in lifespan with long-term IF diets.
With all the health benefits listed above, it’s no wonder any animal would live longer this way.
As no studies have been done on humans, and human lives are much more complex than lab animals, this can’t be directly translated to humans but it’s likely to have a meaningful impact.
Intermittent Fasting, whether daily or weekly, has shown so many incredible effects on health and life itself.
IF can help you lose weight and tackle many of life’s chronic diseases all while enjoying more energy, better sleep, and lower stress.
With all this scientific data behind all these benefits, what’s not to love?
It’s worth mentioning that while both daily (i.e. 16:8 method) and weekly (i.e. 5:2 method) IF showed substantial positive benefits, the daily IF seemed notably more effective at these outcomes, especially when the eating window was early in the day (i.e. 8a-4p).
I personally prefer the 16:8 method myself and after combing through all this research, I’d have to say that’s what I’d recommend to others. I realize it can be hard when an early eating window can interfere with typical social activities, but given the benefits, it may be worth eating earlier on days when possible and later on days with evening events.
If you haven’t read my previous article on how to do intermittent fasting, check it out here to get started.
Next week I’ll go into strength training more specifically and look at how Intermittent Fasting effects exercise and strength. Stay tuned!
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