Want To Be Running Marathons at 100? Then Try Intermittent Fasting!

Written by Cassi O'Brien
Reviewed by Lamia A Kader, MD

The thought of fasting is usually associated with religious rituals or unhealthy eating practices. It’s far from the first thing that would pop into someone’s head when thinking of a healthy diet. Yet, intermittent fasting has slowly begun to rise in popularity among weight losers and gym goers and now has multiple variations of planned fasting schedules available. The main point of logic behind intermittent fasting is a single concept:

Fasting is normal for humans.

Yeah, that idea made me go “What?!” too but it actually makes sense when you look at the lifestyles of early humans. In our modern world, food is abundant. We don’t have to hunt or forage for our meals, because we can simply go to the store and buy what we need. Our early ancestors didn’t have that same luxury, though. At times, they had to go days without food because it wasn’t readily available to them. Fasting was done by necessity rather than choice and it was a regular part of life before the rise of agriculture and the building of a food surplus that appeared after the formation of settled civilizations. According to the earliest human fossils, modern humans evolved roughly 200,000 years ago. The earliest evidence of agriculture dates back to 11,500 years ago. That means more of human history involved the cycle of hunting, gathering, and fasting than it involved living in settlements with a steady food supply. When taking that into account, it’s not hard to conclude that humans are fully capable of frequent periods of fasting and that our bodies may be designed for that type of lifestyle. If an intermittent fasting style of diet allowed our species to live for 188,500 years without the high rates of obesity, heart disease, cancer, diabetes, and depression that is common with our three-meal-a-day modern diets, then it’s at least worth taking a look at.

We’re constantly told that we need three meals a day and that skipping a meal will utterly screw up our metabolism. It’s a staple idea taught for every health class and one of the first things printed on almost every nutrition brochure. The logic behind intermittent fasting goes against that and is based on the concept that the body runs through two different physiological phases.

Phase I: Anabolic (Building)

This phase happens during a period of eating and is led by insulin, which stores nutrients for building new cells and tissue, as well as storing for times of scarcity or fasting.

Phase II: Catabolic (Cleansing)

Beginning roughly six hours into fasting, this phase is led by the Human Growth Hormone (HGH). Dead cells are torn down, the brain recycles waste material, the body regulates waste and self-repairs.

The main factor behind the reported benefits of intermittent fasting is due to the altering cycles of insulin and HGH. These two hormones have an opposite effect on the body:

Insulin stores energy, HGH uses energy. Insulin builds new tissue, HGH repairs already existing tissue. Insulin is pro-inflammatory, HGH is anti-inflammatory.

Both the insulin-led Phase I and the HGH-led Phase II are beneficial to the human body but the health benefits are optimal when the phases are balanced. The reported benefits of a balance between fasting and eating periods are numerous: decreasing risk for cardiovascular disease, decreasing hypertension, weight loss, muscle gain, increasing insulin sensitivity, regulating blood sugar levels, reducing inflammation, improving lipid profile, slowing down signs of aging, and leading to an increase in longevity (1).

It’s a long list and no one could be blamed for being a little skeptic at the claims. With so many trendy eating plans that cut out entire food groups, crazy health regiments that involve eating nothing but carrots and coconut oil for a week to ‘cleanse’ the body of toxins, and gimmick diets that require spending money to receive basic health information that is readily available on Google, being promoted everywhere we look, it’s hard to not be wary when hearing about an idea like intermittent fasting. Considering that there are programs that make money off fasting schedules, intermittent fasting would be easy to write off as another money-making scheme from the diet industry…or at least, it would be if it didn’t have scientific reasoning and evidence backing up the claims of health benefits.

One of the more impressive benefits is a possible decrease in cancer formation and this is directly linked to an increase of HGH, which reaches its highest point towards the end of a 24 hour fast. Cancer forms when the DNA of a cell becomes damaged. During the cycle of cell death and reproduction, the cell replicates itself and passes on the damaged DNA to the new cell, which then replicates out of control because the part of the DNA sequence that tells it when to die and how often to replicate is no longer fully intact. At that point, the cells are considered cancerous and a tumor often forms. With higher level of HGH, cell death is delayed because older cells get rejuvenated and repaired. This lowers the chances of both DNA damage and of damaged DNA getting passed along to new cells, which lowers the chances of cancer formation in the process (1).

The process of cell rejuvenation also slows down the aging process. And, that’s always a desired benefit. I mean, who doesn’t want to stay looking young? There is an entire industry that specifically caters to that desire. HGH increase just may be the less expensive way to retain our youthful looks. The DNA in our cells gets progressively weaker and incomplete every time it replicates so as we get older, our cells change and that leads to the hallmark signs of aging – lower skin elasticity, gray hair, more health problems, etc. Delaying cell death by repairing the cell leads to less frequent cell replication, slowing down the aging alterations to our cells. HGH has been seen to reduce wrinkles, repair collagen, and strengthen bones, muscles, tendons, and ligaments. The anti-aging effects associated with intermittent fasting are also seen in normal low-calorie diets but the low-calorie diet does have a few drawbacks that intermittent fasting is missing (2).

While low-calorie diets are considered the best option for weight loss, they lead to a decrease of physical performance. This happens because a daily calorie deficit not only burns fat but also burns some muscle in the process, making it difficult to either build or retain muscle mass. While intermittent fasting can also produce a calorie deficit, the calorie and nutrition amount isn’t low every single day, which allows time for muscle growth and maintenance. After a period of fasting, the anabolic response is increased. This stimulates muscle protein synthesis because carbs and protein are digested and can be used to build muscle tissue more quickly. Having a higher level of fasting-induced HGH also promotes faster healing and less inflammation, which can make the few days after an intense workout a little less painful (3).

This muscle building perk is not only effective for gaining muscle but also for losing fat. In essence, it is a version of ‘calories in, calories out’. However, intermittent fasting was found to be an easier weight loss plan than the standard method of restricting calories on a daily basis. Rather than trying to reach a daily calorie goal, the focus shifts to a weekly goal. For example, if it was recommended that you stick to a 1,500 cal/day for weight loss, you would aim for 10,500 cal/week instead. You’d still be creating enough of a deficit to burn fat but due to having fasting days where you consumed little or no calories, your calorie restriction on the non-fasting days wouldn’t be as drastic. When maintaining a diet of healthy whole foods, obese study participates were found to have less trouble adjusting to and maintaining a routine of intermittent fasting than they did to daily calorie restrictions. Additionally, unlike with a standard low-calorie diet, intermittent fasting can help you burn more fat during your workout. When working out on a fasting day, the body doesn’t have any recently consumed carbs to use as fuel and glycogen breakdown is stunted, leading to the body using the fat reserves for energy instead.

The routine has also been seen to help prevent a couple of the illnesses associated with obesity. Diabetes is a common illness and getting diagnosed with prediabetes is one of the main motivators that leads people to get started on a weight loss journey. In the case of someone trying to lose weight for this reason, intermittent fasting offers an additional benefit. While eating a three-meal-a-day diet, the body is prone to becoming less insulin sensitive to avoid a high calorie stress. The body stores fat but doesn’t break it down as easily for fuel and that’s when you start getting problems such as excess insulin production, inflammation, high blood sugar, and the other common symptoms associated with diabetes. A routine of fasting does the opposite. It raises insulin sensitivity, making cells more sensitive to insulin during a period of fasting so that all nutrients are used in their entirety. As a consequence, the body avoids making an abundant amount of insulin and stays within the healthy levels (4).

The effect on insulin was also seen with metabolic syndrome in a study involving overweight women. In the study, the symptoms of metabolic syndrome were improved by intermittent fasting. Metabolic syndrome is a precursor to diabetes. Depending on which health organization or medical encyclopedia, it can be considered the same thing as prediabetes or a separate but similar condition. Both illnesses are caused by developing insulin resistance and can lead to Type II diabetes over time. By preventing or lessening the severity of both prediabetes and metabolic syndrome, intermittent fasting reduces the risk for diabetes (5).

Studies also showed an improvement in lipid profiles after starting a fasting schedule. Lower triglycerides and LDL (the ‘bad’ cholesterol) were observed during a study conducted on obese adults. Furthermore, the present LDL particles were larger in size. Small, dense particles of LDL are more likely to contribute to plaque build up and cardiovascular disease while larger particles are much less harmful to the cardiovascular system. The conclusion of the study indicates that with a health diet, intermittent fasting may decrease the chances of experiencing a heart attack (6).

All-in-all, the claims of intermittent fasting do have a leg to stand on from a scientific perspective. As is usual with newer health regiments, more research is needed to solidly confirm the health benefits but what is currently available does shed a positive light on intermittent fasting.

However, this routine is not for everyone and may pose some health risks to certain individuals. Those with Type II diabetes, kidney issues, and those on certain medications may experience negative effects. If you have any health issues, it’s highly advised that you speak to your doctor before starting any of the intermittent fasting regiments available. Women may also experience some issues with a few types of intermittent fasting. The 24 hour fast is not advised for women that are trying to conceive due to a few cases of menstruation stopping or becoming irregular after a few weeks on the fasting schedule. Breastfeeding women should also avoid any fasting schedule that involves more than a 16 hour fast because longer fasts may disrupt milk production.

There are many ways to participate in intermittent fasting and many programs available. Here are some of the most popular programs.


This schedule involves fasting for 14 to 16 hours then eating for the next 8 to 10 hours every day. During the period of eating, a high protein diet with high carbs on workout days and high fats on non-work out days is required. While no calories are supposed to be consumed during the fasting time, coffee with a small bit of milk is still allowed. This is considered to be an easy fasting routine to get into because you basically just have to skip breakfast and eat a healthy diet.

The Warrior Diet

Very similar to the every day fasting approach of LeanGains, the Warrior diet consists of fasting for 20 hours a day then eating one large meal. Instead of just skipping breakfast, you skip breakfast and lunch then eat a large nutrient-laden dinner. No junk food is allowed at any point. However, you are allowed to cheat a little bit during the fasting period by drinking coffee and having small portions of raw fruits, vegetables, and nuts. This diet is supposed to make you feel more relaxed by stimulating the sympathetic nervous system and it may also boost the production of hormones. Sadly, there isn’t much scientific evidence to back up that claim of the diet so it can’t be confirmed 100%.

24 Hour Diet

This one is simple: you eat nothing for 24 hours, once or twice a week, then go back to your regular eating schedule on the days you’re not fasting. For example, you’d eat dinner then not eat until dinner the next day. It can be a hard routine to get used to and you have to use self-control to keep from overeating at the end of the fast. Instead of jumping right into fasting for the whole 24 hours, you can start with 16 hours then work your way up (7).

Alternate-Day Diet

As the easiest fasting routine, the Alternate-Day diet doesn’t actually have an hour requirement for fasting. On one day, you eat 400-500 calories of nutritious food then eat a normal amount on the next day. The timing of the meals doesn’t seem to count as much as the amounts and you can control how long you fast by spreading out the 400-500 calories or getting them from one meal (8,9).

Fat Loss Forever

This diet can be extremely confusing because it’s a combination of Leangains, The Warrior Diet, and the 24 Hour diet. You do get one cheat day but it’s then followed by a 36 hours fast and the rest of the week is split up between the different fasting methods. There’s a lot of scheduling involved but the program does come with a fully structured schedule and meal plan.

Many of these programs do cost money because they come with program supplements such as guides, schedules, and meal plans. If you’re someone that likes having everything set up and laid out for them, then one of these programs may be ideal. However, you don’t need a program or structured schedule for intermittent fasting. You can get into a fasting routine without spending a penny. It can be as easy as simply skipping a meal because you’re busy or not feeling that hungry. Or, you can easily make your own schedule and healthy eating plan that works well for you.

Intermittent fasting sounds good from a health aspect but what is it like to go on a fasting routine? The idea of going a time period without eating, even 14 hours, can seem daunting to most people and the claim that the routine is easy to adjust to can be hard to believe for some. Even when something sounds logical and great in theory, the reality of it can be quite different.


(1) Aftab Ahmed, Farhan Saeed, Muhammad Umair Arshad, Muhammad Afzaal, Ali Imran, Shinawar Waseem Ali, Bushra Niaz, Awais Ahmad & Muhammad Imran (2018) Impact of intermittent fasting on human health: an extended review of metabolic cascades, International Journal of Food Properties, 21:1, 2700-2713,

(2) Anton, Stephen, and Christiaan Leeuwenburgh. “Fasting or Caloric Restriction for Healthy Aging.” Experimental Gerontology 48.10 (2013): 1003-005.

(3) Anson, R. M., Z. Guo, R. De Cabo, T. Iyun, M. Rios, A. Hagepanos, D. K. Ingram, M. A. Lane, and M. P. Mattson. “Intermittent Fasting Dissociates Beneficial Effects of Dietary Restriction on Glucose Metabolism and Neuronal Resistance to Injury from Calorie Intake.” Proceedings of the National Academy of Sciences 100.10 (2003): 6216-220.

(4) Heilbronn, Leonie K., Lilian De Jonge, Madlyn I. Frisard, James P. Delany, D. Enette Larson-Meyer, Jennifer Rood, Tuong Nguyen, Corby K. Martin, Julia Volaufova, Marlene M. Most, Frank L. Greenway, Steven R. Smith, Walter A. Deutsch, Donald A. Williamson, Eric Ravussin, and For The Pennington Calerie Team. “Effect of 6-Month Calorie Restriction on Biomarkers of Longevity, Metabolic Adaptation, and Oxidative Stress in Overweight Individuals.” Jama 295.13 (2006): 1539.

(5) Harvie, M. N., M. Pegington, M. P. Mattson, J. Frystyk, B. Dillon, G. Evans, J. Cuzick, S. A. Jebb, B. Martin, R. G. Cutler, T. G. Son, S. Maudsley, O. D. Carlson, J. M. Egan, A. Flyvbjerg, and A. Howell. “The Effects of Intermittent or Continuous Energy Restriction on Weight Loss and Metabolic Disease Risk Markers: A Randomized Trial in Young Overweight Women.” Int J Obes Relat Metab Disord International Journal of Obesity 35.5 (2010): 714-27.

(6) “Study Finds Routine Periodic Fasting Is Good for Your Health, and Your Heart.” EurekAlert! Web. 01 Mar. 2016.

(7) Varady, K. A. “Intermittent versus Daily Calorie Restriction: Which Diet Regimen Is More Effective for Weight Loss?” Obesity Reviews 12.7 (2011).

(8) Klempel, Monica C., Surabhi Bhutani, Marian Fitzgibbon, Sally Freels, and Krista A. Varady. “Dietary and Physical Activity Adaptations to Alternate Day Modified Fasting: Implications for Optimal Weight Loss.” Nutrition Journal Nutr J 9.1 (2010): 35.

(9) Varady, Krista A. “Alternate Day Fasting: Effects on Body Weight and Chronic Disease Risk in Humans and Animals.” Comparative Physiology of Fasting, Starvation, and Food Limitation (2012): 395-408.