Weight loss may be the reason you are reading this, but intermittent fasting advocates believe IF can offer health benefits beyond ripped abs. Very low calorie diets (VLCs) have scientific support for extending life in animals and humans. But have you ever seen the meals those folks eat? Caloric restriction may the best way to live longer, but if you feel the way I do about food, you may not want to live longer if you have to eat like that. You have probably heard people say, “You may not live longer, but it will sure feel like it.” After all the quality of life needs to be considered in addition to quantity of years.
Intermittent fasting (IF) may be the answer. There is evidence it has similar health benefits to caloric restriction, but still allows you to eat a lot some times, and many have an easier time adhering to this kind of diet. Some biohackers do make it sound like a panacea. If you have been in the blogosphere in the last few years, you have seen claims that intermittent fasting extends your life, prevents chronic disease, increases muscle mass, melts fat, improves concentration, and does your dishes (well it does decrease the number of dishes that you have to wash). Let’s see how much that picture matches its current scientific status and what intermittent fasting protocols look like in practice. (Photo Credit © Pascu Gheorghe)
What is Intermittent fasting?
Intermittent fasting refers to a temporal pattern of eating and non-eating episodes. Simply put, there are specific feeding windows, times during which you can eat, and fasting windows, times during which you do not eat- and yes, sleeping counts. While some IF protocols specify how many calories you can have during a feeding window, most allow you to eat ad libitum, i.e., as much as you want during your feeding windows. Yes, that is right, after fasting, you can eat as much as you want. Many encourage you to eat high quality nutrient dense foods, but the amount is typically up to you. Interestingly, and this surprised researchers, most people do not go crazy and make up for all of the calories they missed during the fast. In addition in one study, it also appears that hunger levels decrease after two weeks and people get used to fasting, which makes compliance more likely.
If moving between fasting and feeding is what it is, what are the benefits?
What are the benefits of Intermittent Fasting?
If you are a rodent, this is definitely the way to go. Similar to very low calorie diets, intermittent fasting extended the lives of some strains of mice, (effects varied by genetics and age at which IF was initiated). Similarly, body weight significantly decreased as well, but the genetic strain of mouse and the age at which it received IF mattered for weight loss too- so not all mice lost weight and lived longer. But IF has even more specific health benefits for mice than just weight loss and longer life expectancies.
Intermittent fasting appears to protect mice from developing certain cancers, e.g., lymphoma and liver cancer. In one trial 33% of mice eating at regular times injected with a carcinogen developed cancer, while none of the mice on the intermittent fasting schedule developed cancer. This is quite interesting, especially considering that the two groups ate roughly the same number of calories- just at different times. So the timing was the critical component, not a reduction in calories.
Diabetes risk also appears to be diminished in animal studies and cardiovascular risk factors are reduced as well. So if you want your hamster to live longer and look great, you know what to do. But what about the rest of us?
Is Intermittent Fasting Healthy for Humans?
Intermittent fasting studies have been conducted in humans. Both normal weight and obese populations have been studied, and studies have included men and women. This is important because there have been some gender differences. The majority of trials have lasted from two to 24 weeks and have involved a particular type of intermittent fasting called alternate day fasting (ADF). ADF involves alternating a feed day with a day of fasting. So basically you eat every other day. That typically means there is a 36-hour fast. On a feeding day (Monday) you may eat between 8:00 am and 8:00 pm, and then fast the entire next day (Tuesday) until 8:00 am two days later (Wednesday). One study utilized an alternate day modified fasting (ADMF) protocol where the fast day allowed for one meal between the hours of 12 pm and 2 pm. So the fasting window here would be shortened considerably.
Will Intermittent Fasting increase Life Expectancy?
In terms of living forever, we don’t have studies following subjects to the grave yet (for IF or extreme calorie restriction). But based on animal studies for calorie restriction, some advocates of the practice hope to live to be 120 years of age. While some experts, believe 5% to be a more reasonable estimate of how much longer humans might live on caloric restriction. ADF has shown to increase life expectancy in mice. But I have not yet seen any computations extrapolating those findings to humans, but I am sure they are coming. But there are plenty of findings for disease in humans.
Fasting is good for Heart Health
High-density Lipoproteins (HDL), the good cholesterol, was increased by intermittent fasting and triglycerides and Low-density lipoproteins (LDL) decreased in some studies. Many physicians use these as predictors of cardiovascular disease. So it appears that intermittent fasting may improve heart health.
Weight Loss from Intermittent Fasting
There is also evidence that overall fat oxidation is increased and the majority of people in studies lost significant weight in a short amount of time. One study of normal weight men did not find weight loss, but the trial was only two weeks- and they were normal weight. One trial of obese women that lasted 24 weeks had an overall weight loss of 7% of total body weight, a fat mass decrease of 13%, and a reduction of waist circumference of 6%. Other studies had ranges of 4-9% weight loss from their initial weight. Another important finding in one trial was that, unlike in caloric restriction (CR) diets where clients lose both fat and muscle, obese participants losing weight with intermittent fasting, lost fat while preserving muscle mass. This may be one of the most important benefits of intermittent fasting and hopefully many other studies will replicate this finding in both obese and overweight populations.
Diabetes and Cancer Risk
Diabetes risk factors appear to significantly decrease for men, but it is less clear if there are benefits for women. Some authors believe that conflicting studies don’t make it clear at this point, but longer trials will likely clarify what the effects might be- which will be positive if they are congruent with the animal studies. While IF appears to protect mice from some forms of cancer, human studies have not yet been conducted. Given that the majority of animal studies have found these effects, many scientists are hopeful that IF could be beneficial for humans regarding cancer, but further studies are certainly required to make those conclusions.
So, for humans – it does look like many people can expect intermittent fasting to result in weight loss (hopefully with preservation of muscle), reduced risks of heart disease, diabetes (if you are a male), and possible protection against carcinogens (the jury is still out, awaiting human trials).
Those are the benefits, what are the options?
As I mentioned earlier the majority of animal and human studies use the ADF version of intermittent fasting. But, many health advocates recommend different IF protocols. I will detail a few of the most popular.
An infinite number of IF protocols could be implemented. It is really as simple as picking specific feeding and fasting times. What you think the optimal amount of time for a fast is balanced with the likelihood you can maintain that time should be what guides your plan selection.
Full Day Fast
Similar to the ADF protocol in the scientific studies, you have a full day of fasting, followed by a feeding day. When there are only one or two fast days per week, this is often called Eat Stop Eat, a program promoted by Brad Pilon. He does recommend eating normal amounts on the feeding days.
Alternate Day Fasting (ADF)
To recap, this is an entire day of eating as much as you want, followed by a day where you do not eat at all. While ad libitum eating was allowed in the studies I cited, it is not typically recommended to overdo the amount or to eat unhealthy foods. Instead listening to your hunger cues and eating healthy quality foods is your best bet.
Alternate day modified fasting (ADMF)
This is very similar to ADF, except you are allowed to eat one meal during the fast day. The scientific trial that utilized this protocol specified the hours of 12 pm and 2 pm to be the feeding time. It also specified the caloric amount of that meal to be 25% of baseline caloric needs, e.g., 500 calories. The meals in that trial included an entrée, snack, and even a dessert, e.g., a cookie.
This is a term I use to refer to protocols in which the fast and feeding windows occur on the same day. In these protocols, the differences are simply how long each window is. Some popular ones also recommend when to exercise, how much protein to consume, and whether you can have coffee, tea, creamers, butter, etc. during fasting windows. I will list some of the most popular timings below.
IF-10 refers to a 10-hour feeding window and a 14-hour fast. This timing is promoted by Martin Berkhan in his Leangains program. This ratio is what he recommends for women.
IF-8 refers to an 8-hour feeding window and a 16-hour fast. This timing is also promoted by Martin Berkhan in his Leangains program, but is recommended for men.
IF-4 refers to a 4-hour feeding window and a 20-hour fast. This timing is promoted in the Warrior Diet by Ori Hofmekler.
Those are the intermittent fasting protocols I have seen promoted the most in the blogosphere. It is important to note that the scientific literature has not yet tested the majority of these. ADF and ADMF have been evaluated the most. There are countless anecdotes of those utilizing many IF protocols on the internet. Many of those blogging about these report incredible weight loss stories with beneficial indicators in blood panels that result in physician’s amazement. Based on the research findings, it seems very plausible that many of these reports are accurate. While the mechanisms are still debatable (something I may tackle in a future blog) as to why or how intermittent fasting works, there are some pretty incredible findings in both animal and human studies in terms of health outcomes. More studies need to be conducted. And I would very much like to see not only a wider variety of human subjects used, but longer trials, and protocols other than ADF.
Thus far, I think the scientific findings are pretty remarkable and I am incredibly optimistic that future research may help determine precisely who can benefit from particular protocols the most. But the research has demonstrated that IF can have significant impacts on various physiological indices without many side effects. Not many medications have these kinds of effects without creating other health problems.
With that said, there are not many human trials yet, and they certainly have not tracked participants across years. So it is unclear what the effects will be in the long-term. My hope is that longer trials will result in an increase the intensity and number of health benefits. But questions remain about potential adverse effects as well. Some authors and professionals believe IF may be detrimental to women, diabetics, or those at risk for adrenal fatigue. And I think given the lack of longer terms studies proceeding sensibly and cautiously makes sense.
I have educated a number of my patients about intermittent fasting. I have always recommended that they speak to their physician before implementing any protocol. I work to improve many lifestyle factors of my patients, but without question weight loss is a focus for many of my clients. Intermittent fasting not my first tool when it comes to weight loss. While IF may result in weight loss regardless of the types of food people eat, I make sure clients are making better food choices regarding macronutrients, micronutrients, and portions as well- and I typically emphasize food quality and quantity prior to IF protocols. But, given the scientific support for weight loss and decreases in cardiovascular disease risk factors, I believe intermittent fasting is a promising strategy to consider as long as you are working closely with an informed medical professional that will closely monitor any potential adverse effects.
Overall it appears that for many people, IF could have enormous potential. Fat loss, without losing muscle, reductions in heart disease risk, diabetes risk (at least for men), possibly protection against cancer and life extending benefits in animal studies, while being more palatable than caloric restriction make it an important strategy to consider. It may not be right for everyone, but if more studies demonstrate it is safe in the long-term, I believe health care practitioners will be recommending it more frequently to help us lead long, lean lives.