The Latest on the Science of Intermittent Fasting
Welcome Spring! The weather is warming up, and with all seasons of spring we start stripping off the layers. So naturally, people begin their‘Spring Break Bod’ plans [#SpringBreak2020]. There are so many different diets out there that claim to help with weight loss and disease prevention: low-fat, low-carb, ketogenic, paleo, whole 30, vegetarian, vegan, DASH, Mediterranean, MIND, etc. But I want to talk to you about one of the latest trends: the science of intermittent fasting, otherwise known as IF in the world of food and nutrition.
You may be used to eating three meals every day, plus snacks. That’s pretty common. With intermittent fasting you can essentially eat how much of whatever you want—but here’s the catch: you have to stay on schedule. With intermittent fasting there are scheduled periods of time when you can eat and others when you have to fast. Unlike most other diets, intermittent fasting tells you when to eat, not what to eat.
And, many people say that it can help lead you to better health and a longer life.
Let’s dive into some of the pros and cons of intermittent fasting.
How to intermittently fast
Most of the diets that help achieve weight loss work by reducing the number of calories consumed. Intermittent fasting does the same thing, but in a different way. This way of eating significantly limits calories (requiring fasting) for certain durations of time (intermittently), while allowing little or no restrictions the rest of the time.
Intermittent fasting essentially means skipping meals on a regular basis, sometimes daily, weekly, or monthly. Here are a few different approaches:
- Time-restricted feeding—Having all of your meals during an 8 to 12 hour window each day, drinking only water the rest of the day.
- Alternate day fasting—Eating normally one day but only a minimal number of calories the next; alternating between “feast” and “famine”.
- 5:2 eating pattern—Consuming meals regularly for five days per week, then restricting to no more than 600 calories per day for the other two. This happens by eating very little and drinking only water on those two fasting days.
- Periodic fasting—Caloric intake is restricted for several consecutive days and unrestricted on all other days. For example, fasting for five straight days per month.
What’s the research say on the benefits of intermittent fasting?
Studies show that intermittent fasting can help you to achieve weight loss. The success is similar to other diets; in my ten plus years of working as a dietitian, all conventionally, though of diets work initially, and then for various reasons, don’t, either you stop seeing weight loss (hit a plateau), or regain the weight altogether. Intermittent fasting is no different, results are not typically sustained if the diet is not a lifestyle.
Overall, research on the health effects of IF is still emerging as to whether the benefits go beyond weight loss for some people, and may actually help to prevent disease or slow down aging.
Most of the research on calorie restriction and intermittent fasting have been conducted in cells (e.g., yeasts), rodents, and even monkeys. Some, but not all of these studies show it may help to build exercise endurance, immune function, and live longer. It also seems to help resist some diseases like diabetes, heart disease, certain cancers, and Alzheimer’s.
When it comes to clinical studies (those done in humans) on intermittent fasting, most have been pretty short—a few months or less. But, what we know so far is that it may help with markers of inflammation (C-reactive protein), diabetes (blood glucose levels and insulin sensitivity), and help to reduce high blood pressure and cholesterol.
When it comes to weight loss, intermittently fasting seems to work just as well—not better—than other diets. Researchers think that eating this way decreases appetite for some people by slowing down the body’s metabolism. With a smaller appetite, you simply eat less and that is going to help you lose weight. Other people who intermittently fast struggle with, and are much more uncomfortable during the fasting days, and some animal studies show that when they were allowed to eat as much as they wanted, many overate. This is one of my biggest concerns with IF. Disclaimer: if you have a history of disordered eating (diagnosed or otherwise), I do not recommend intermittent fasting. In my experience with these clients, IF can exacerbate those food phobias and the disordered relationship with food. Remember, for many, undereating almost always leads to overeating, eventually… (be it tomorrow, next week or six months from now).
What about extending the lifespan of humans? Those studies haven’t been done yet, so we simply don’t know the effects of intermittent fasting on our lifespan.
How does intermittent fasting affect our health?
Naturally, our bodies have survival mechanisms allowing us to adjust to periods of fasting. This has been necessary, as throughout history, humans have endured many periods where food was scarce.
What happens when we don’t take in sufficient calories is that our body starts using up stored carbohydrates called glycogen. The liver stores enough glycogen to last about 12 to 16 hours before it runs out of fuel. Beyond 16 hours, the body switches fuels and begins to use fat as an energy source.
At this time, our metabolism shifts from a carbohydrate-burning state to a fat-burning state. Some of the fat is used directly as fuel, while some is metabolized into biochemicals called ketones. This new fat-burning metabolic state is called ketosis. The state of ketosis brings on other changes throughout the body. It’s these changes that are thought to underlie some of the health benefits seen with intermittent fasting.
Ketones are a more efficient source of energy for our bodies than glucose and so they can help keep many of our cells working well even during periods of fasting. This is particularly true for brain cells and this may be part of the reason some animal studies show protection against age-related declines like Alzheimer’s.
Ketones may also help to ward off some cancers and inflammatory diseases like arthritis. They are also thought to reduce the amount of insulin in the blood which may help protect against type 2 diabetes (Although, long term studies are limited, there have been individual experiences where people actually begin to see a rise in blood sugars as a result of the bodies defense mechanisms). On the other hand, too many ketones may be harmful, so more research is needed to better understand the links between fasting, ketones, and health.
On a molecular level, intermittent fasting may extend lifespan in animals because of its effect on the DNA in our genes. Over time as we age, the way our genes are switched on and off changes. It appears that, in animals, restricting calories may slow down these age-related changes and help them to live a bit longer.
More research is underway to better understand the effect of fasting on these biological processes.
Before you start intermittently fasting
Because everyone is individual you want to be sure to discuss any changes of your diet with your healthcare professional, or book a session with me.
Before considering intermittent fasting, know that there are certain conditions that can make it dangerous. For example, if you have diabetes you need to eat regularly to maintain your blood sugar levels, so fasting is not recommended. Also, if you’re taking certain medications like diuretics for high blood pressure or heart disease, intermittent fasting increases your risk for electrolyte abnormalities.
Intermittent fasting is also not recommended for anyone who is under 18, has a history of eating disorders or anyone who may be pregnant or breastfeeding.
Of course, whenever you change your diet you may experience side effects. Some side effects of people who restrict their calories or start intermittently fasting include fatigue, weakness, headache, reductions in sexual interest, and a reduced ability to maintain body temperature in cold environments.
Beyond the health risks and side effects, fasting is simply hard to do voluntarily—especially when it’s for two or more days. Some people may have a natural tendency overindulge on their “feast” days which can negate some of the benefits of fasting. As mentioned before, you know yourself the best, if this is your natural tendency I highly recommend seeking an alternate nutritional style.
According to the National Institutes of Health, “More research will be needed to determine the long-term impact of the diet on human health and provide information on when and how such a diet might be applied.”
Tips for intermittent fasting
Intermittent fasting can be hard. One thing that can help is having a social support network—especially for those days when you’re fasting.
Although the premise of intermittent fasting is to restrict when you eat, not what you eat, the quality of your food choices is still very important. Regardless of your eating style and preferences, you still need all of your essential nutrients and restricting the time in which you choose to eat can undermine those goals of meeting nutritional needs. Intermittent fasting is not an excuse to eat a lot of the high-calorie, nutrient-poor foods we all sometimes crave. I recommend eating adequate amounts of lean proteins, healthy fats, fruits, vegetables, legumes, and whole grains. Also, avoid too many sugars and refined grains.
Final thoughts on intermittent fasting
The main reason for any dietary change is to have a sustainable and healthy lifestyle that helps you meet your health goals. Whether you’re looking to lose weight or prevent disease, intermittent fasting is one eating style that may work for you. The most important thing with any diet is to get all of your essential nutrients, appropriate amounts of food, and enjoy your lifestyle in the long run.
Any diet or eating pattern that helps some people may not have the same effect on everyone. That’s why it’s important to not make any significant dietary changes without consulting your healthcare professional or dietitian.
Harvard Health Publishing. (2017, January). Any benefits to intermittent fasting diets? Retrieved from
Harvard Health Publishing (2018, June 29). Intermittent fasting: Surprising update. Retrieved from https://www.health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156
Harvard Health Publishing. (2019, July 31). Not so fast: Pros and cons of the newest diet trend. Retrieved from https://www.health.harvard.edu/heart-health/not-so-fast-pros-and-cons-of-the-newest-diet-trend
Mayo Clinic. (2019, January 9). Fasting diet: Can it improve my heart health? Retrieved from https://www.mayoclinic.org/diseases-conditions/heart-disease/expert-answers/fasting-diet/faq-20058334
Mayo Clinic. (2019, August 14). Mayo Clinic Minute: Intermittent fasting facts. Retrieved from https://newsnetwork.mayoclinic.org/discussion/mayo-clinic-minute-intermittent-fasting-facts/
National Institutes of Health National Institute on Aging. (2018, August 14). Calorie Restriction and Fasting Diets: What Do We Know? Retrieved from
National Institutes of Health NIH Research Matters (2015, July 13). Health Effects of a Diet that Mimics Fasting. Retrieved from https://www.nih.gov/news-events/nih-research-matters/health-effects-diet-mimics-fasting
National Institutes of Health NIH Research Matters. (2017, September 26). Calorie restriction slows age-related epigenetic changes. Retrieved from https://www.nih.gov/news-events/nih-research-matters/calorie-restriction-slows-age-related-epigenetic-changes
National Institutes of Health NIH Research Matters (2018, March 6). Intermittent dietary restriction may boost physical endurance. Retrieved from
National Institutes of Health NIH Research Matters (2018, September 18). Fasting increases health and lifespan in male mice. Retrieved from
NIH Intramural research program. (2018, March 13). Intermittent Fasting Boosts Endurance in Mouse Marathoners. Retrieved from
NIH National Center for Advancing Translational Sciences. (2018, August). NCATS-Supported Study Shows Eating Before 3 p.m. Can Improve Health. Retrieved from