What You Should Know About Intermittent Fasting

Intermittent fasting is everywhere in the health and fitness community. It has been a nice contrast to the once very popular dogma of eating multiple small meals throughout the day to “keep a fast metabolism” and prevent muscle loss, an idea that has been debunked already on multiple occasions. (Side note: I don’t recommend taking nutritional advice from anyone that still believes in quick post workout meals/shakes and eating 6 small meals per day.) So as with most extreme ideas, the pendulum has now swung in the other direction. Different methods of fasting have gained popularity, including 12-16 hour fasts, 24 hour fasts, alternate day fasts, and even extended fasts which can last several days. Scientific studies and personal anecdotes have also been suggesting that intermittent fasting/time-restricted feeding has some therapeutic health benefits on obese individuals, including improvements on CVD markers, fat loss, insulin resistance, and other metabolic markers. It has also been a useful tool for symptom management in individuals with rheumatoid arthritis and multiple sclerosis. But what about relatively healthy individuals who perhaps have some performance, aesthetic, or health goals? Is there a place for intermittent fasting for someone that is simply looking for general health improvements? After all, it seems to me that most people I encounter that are interested in or are already implementing fasting fall in this category. As with most new trends, the human-based evidence is limited, but there is some.

How Fasting Can Be Beneficial

Digestion is a stressor. And this is especially true if you have intestinal hyper-permeability (i.e. leaky gut), which most people do, especially those who consume or have consumed a standard American diet for an extended period of time. In the presence of a leaky gut, an inflammatory response is stimulated whenever food is consumed, so fasting allows the GI tract time to heal and recover from digestion. Fasting can also be beneficial by inducing autophagy, which is a process by which cells recycle cellular waste and repair themselves. Autophagy is important for building and maintaining muscle tissue, as well as slowing aging and protecting against neurodegeneration. And you can begin to reap these fasting benefits in as little as 16 hours of not eating. There are also plenty of people, men and women, who claim that intermittent fasting helps their exercise performance, cognitive function, energy stabilization, and satiety. Plus, it can be a very convenient for people that have less time on their hands to dedicate to meal-planning and eating.

Research and Genders

The biggest factor to consider when debating whether or not time-restricted feeding will be beneficial to you is your sex. Male and female bodies are extremely different, and therefore react differently to various diets. Although there is some evidence in the scientific literature that suggests that fasting can be beneficial for healthy individuals, this is unfortunately gender-biased in most cases, and this is true for most experimental studies. Generally speaking, most studies are either done on only men or mostly men, unless the study targets women in particular. This is not exactly done maliciously by researchers, but has more to do with the variability of female hormones. Researchers want to control for as many variables as possible to obtain more reliable data and come to more accurate conclusions, and since women of reproductive age typically operate on a 28-day cycle (give or take a few days), men are usually more optimal candidates for experimental studies. This becomes a problem when recommendations that are based on male-dominated research are given to the general population without the consideration of how such recommendations may affect an individual based on their gender.

What does the research say?

But more specifically, there is evidence that demonstrates how various methods of fasting can affect a person in different ways. When we look at some of the animal studies regarding intermittent fasting and calorie-restriction, we see that male and female rats respond differently. One study in particular demonstrated that male rats responded to alternate-day fasting as a mild stressor, while female rats responded much more drastically. While male rats slightly increased their overall activity level, female rats became much more active, particularly during the day (which is when they would typically be sleeping). So female rats on intermittent fasting swapped sleep for restlessness, and this effect seems to be directly related to the degree of food deprivation, as an instinctual attempt to find nourishment during the “famine”. Another study indicated that alternate-day fasting increased testicular size and testosterone-to-estrogen ratios on male rats, while female rats experienced a slight decrease in ovarian mass and a slight increase in testosterone-to-estrogen ratio. So basically, intermittent fasting made male rats much more masculine, and made female rats slightly more masculine.

But we’re not rats or mice or rodents, right? That’s correct. But since we have very limited research on this topic, we must make do with what we do have. Fortunately, we also have a bit of human-based evidence we can interpret. One small study tested for glucose tolerance on non-obese men and women after a 22-day alternate-day fasting regiment. The results after the intervention showed that women experienced higher blood glucose levels after a meal, which indicates that alternate-day fasting may negatively affect women’s glucose tolerance. Men on the other hand experienced no changes in glucose response post-meal, but had a significant reduction in insulin response. From this, we can infer that men required less insulin to clear blood sugar after the intervention, and therefore were more insulin sensitive (which is a good thing). Insulin sensitivity is an important factor in reducing the risk of type II diabetes.

Another study found that alternate-day fasting increased HDL-cholesterol (“good” cholesterol) in women only, and decreased triglycerides in men only, both of which are beneficial results for reduction of cardiovascular disease risk. But ghrelin, a hormone that is known to induce hunger and appetite, was elevated in women after the intervention. Both men and women experienced weight loss during this study, which may have been a result of an unintentional overall calorie-reduction, which may help explain the elevated ghrelin levels found in the women.

Another interesting study involving 10 women and 5 men compared the effects of consuming 1 meal/day vs. 3 meals/day over an 8-week period (calories and macronutrient ratios were controlled). The study found that the 3 meals/day diet resulted in lower cardiovascular markers–including heart rate, blood pressure, LDL-cholesterol–and lower liver enzymes compared to those of the 1 meal/day diet. In addition, those consuming 1 meal/day experienced more hunger and less fullness than those consuming 3 meals/day. But we should consider that this study was female-dominated, so perhaps the results would have looked more favorable for intermittent fasting if there had been more male subjects involved.

These were the most relevant and helpful studies I could gather on regarding how IF affects non-obese individuals.

So who should try intermittent fasting?

Intermittent fasting may be beneficial for obese individuals in reducing CVD and type II diabetes risk markers, although obese women should practice a bit more caution in this area by applying more moderate and less frequent fasts. Men who are experiencing low-testosterone, fertility issues, have fat loss or other aesthetic goals may find fasting to be beneficial. Individuals who desire more energy and cognitive boosts may experience positive results from intermittent fasting. But because women’s bodies are much more sensitive to stress (because we’re made to bear children), women should lower stress as much as possible before implementing fasting. Women with sleep problems, hormonal and menstrual problems, low body fat percentages (lower than 20% body fat), chronic stress environments, and/or women who regularly exercise at high intensities may react negatively to intermittent fasting.

How To Implement fasting

If you have decided that you want to start fasting in some way, I recommend starting slowly. You can begin by trying a 16 hour fast a few times per week, and then gradually increase the fasting window and its frequency based on how you feel. You can stop there, or you can make this a daily habit if you feel like it is manageable and beneficial for you. Or you can incorporate a few 24-hour fasts per month or even an extended fast (24-72 hours) once every month or every other month. But it is important that you pay attention to how you feel on these different methods and adjust accordingly. You may find that you reach a point with fasting at which you stop experiencing benefits and start to have negative reactions. These can include trouble sleeping, irritability, fatigue, ravenous cravings, fertility issues, thyroid issues, fat gain, decline in exercise strength/endurance, and muscle loss. If you experience any of these, then I’d recommend reducing your fasting window and/or frequency until things normalize and you start seeing benefits again. And if you find that fasting is detrimental for you from the start, then that’s okay! Fasting is not for everyone. It’s important that you feed your body in a way that makes it thrive, and fasting may not be a part of that equation.

A Note On Calorie-reduction

It’s important to consider that when fasting, you might unintentionally (or perhaps intentionally) reduce your calorie-consumption. For example, let’s say you normally consume about 2000 calories per day as follows: 650 calories at 7am, 650 calories at 12pm, and 700 calories at 6pm. Then one day, you decide to fast until noon but consume your usual meals at lunch and dinner. You would then only consume a total of 1350 calories on that day. This isn’t particularly problematic if it happens every once in a while, but if it becomes a frequent habitual practice, you might find yourself excessively tired and hungry within a few days due to the sudden drastic calorie reduction. A mild reduction may not be a bad thing. It may even be beneficial for some people, but fasting can lead to an excessive reduction, especially if you’re not paying attention. My point is that if you are implementing fasting, you might notice that you require larger-than-usual meals during your feeding window in order to feel satiated, and that is okay. The point of fasting is NOT overall calorie reduction but a shortened eating window that thus shortens the digestion window.

If you found this helpful or have anything to add, feel free to leave your question or comment in the comments section below!

Du, L., Hickey, R. W., Bayir, H., Watkins, S. C., Tyurin, V. A., Guo, F., . . . Clark, R. S. (2009). Starving Neurons Show Sex Difference in Autophagy. The Journal of Biological Chemistry, 284(4), 2383-2396. doi:10.1074/jbc.M804396200
Heilbronn, L. K., Civitarese, A. E., Bogacka, I., Smith, S. R., Hulver, M., & Ravussin, E. (2005). Glucose Tolerance and Skeletal Muscle Gene Expression in Response to Alternate Day Fasting. Obesity Research, 13(3), 1071-7323, 574-581. doi:10.1038/oby.2005.61
Heilbronn, L. K., Smith, S. R., Martin, C. K., Anton, S. D., & Ravussin, E. (2005). Alternate-day fasting in nonobese subjects: Effects on body weight, body composition, and energy metabolism. The American Journal of Clinical Nutrition, 81(1), 0002-9165, 69-73. doi:10.1093/ajcn/81.1.69
Longo, V. D., & Mattson, M. P. (2014). Fasting: Molecular Mechanisms and Clinical Applications. Cell Metabolism, 19(2), 1550-4131, 181-192. doi:https://doi.org/10.1016/j.cmet.2013.12.008
Martin, B., Pearson, M., Brenneman, R., Golden, E., Keselman, A., Iyun, T., . . . Mattson, M. P. (2008). Conserved and Differential Effects of Dietary Energy Intake on the Hippocampal Transcriptomes of Females and Males. PLOS ONE, 3(6), 1-18. doi:10.1371/journal.pone.0002398
Martin, B., Pearson, M., Brenneman, R., Golden, E., Wood, W. I., Prabhu, V., . . . Maudsley, S. (2009). Gonadal Transcriptome Alterations in Response to Dietary Energy Intake: Sensing the Reproductive Environment. PLOS ONE, 4(1), E4146. doi:10.1371/journal.pone.0004146
Stote, K. S., Baer, D. J., Spears, K., Paul, D. R., Harris, G. K., Rumpler, W. V., . . . Mattson, M. P. (2007). A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults. The American Journal of Clinical Nutrition, 85(4), 0002-9165, 981-988. doi:10.1093/ajcn/85.4.981

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