This science backed guide to intermittent fasting 101 will help you decide if you should give it a try for healthy aging or weight loss. Get a free printable intermittent fasting PDF meal plan too!
Intermittent Fasting 101
There are so many different diets out there that claim to help with disease prevention and weight loss: low-fat, low-carb, ketogenic, paleo, vegetarian, vegan, Mediterranean, DASH, MIND, etc. But I want to talk about one of the latest trends: intermittent fasting.
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 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. Sound interesting? Let’s do a deep dive into intermittent fasting 101.
Different types of intermittent fasting
Most weight loss diets work by reducing the number of calories consumed. Intermittent fasting does the same thing, but in a different way. This way of eating limits calories (requiring fasting) for certain durations of time (intermittently), while allowing few or no restrictions the rest of the time.
Thus, in a nut shell, intermittent fasting essentially means skipping meals on a regular basis, sometimes daily, weekly, or even monthly. Here are a few popular approaches to intermittent fasting 101:
16:8 or Time-Restricted Feeding
Probably the most popular method of intermittent fasting. With 16:8 IF, you fast for 16 hours then eat for 8 hours in a day. Many people choose this method as it is one of the easiest and simplest to follow.
Most people who do the 16:8 split skip breakfast and break their fast around lunch time. For example, they would have their first meal at around 11 am and eat up until 7 pm that night, creating an 8 hour eating window. Other people may choose any earlier eating window, such as 8am-4pm.
5:2 Eating Pattern
Consuming regular meals for five days per week, then restricting to no more than 600 calories per day for the other two days (typically non-consecutive). This happens by eating very little and drinking only water or non-caloric beverages on those two fasting days.
Alternate Day Fasting
Eating normally one day but only a minimal amount of calories the next; alternating between “feast” days and “fast” days every other day.
With periodic fasting, you restrict calories for several consecutive days and eat unrestricted meals on all other days. For example, fasting for five straight days per month.
In addition to these types of intermittent fasting, there is another approach, The Prolon Fasting Mimicking Diet (FMD), that’s gaining popularity for helping to prevent disease and promote weight loss. I’ll discuss FMD in a separate post.
Benefits of intermittent fasting
Studies show that intermittent fasting can achieve weight loss. The success is similar to other diets. Yes, similar—not necessarily better.
Overall, research on the effect of intermittent fasting 101 on people’s health is still emerging as to whether, in addition to some weight loss for some people, it can also 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 people) 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 resistance), fat burning/fat loss, and help 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. With a smaller appetite, you simply eat less and that is going to help you lose weight.
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.
What’s the science behind intermittent fasting?
When we don’t eat enough calories, we use up stored carbohydrates called glycogen. The liver stores enough glycogen to last 12 to 16 hours before running out of fuel. Beyond 16 hours, the body switches fuels and begins using fat as an energy source.
At this time, metabolism shifts from a carbohydrate-burning to a fat-burning state. Some of the fat is used directly as fuel, while some is metabolized into biochemicals called ketones. The state of ketosis brings other changes throughout the body that are thought to underlie some of the health benefits seen with some types of fasting.
Ketones are a more efficient source of energy for our bodies than glucose, so they keep many of our cells working, even during periods of fasting. This is particularly true for brain cells and may be part of the reason some animal studies show protection against age-related declines like Alzheimer’s.
Ketones may also help 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.
On a molecular level, intermittent fasting may extend lifespan in animals because of its effect on genetic DNA. It appears that, in animals, restricting calories may slow down these age-related changes and help them live longer.
All of this being said, most people who do 16:8 fasting don’t get into ketosis. Thus, weight loss benefits are mostly attributed to a decrease in calorie intake, and overall health benefits may be attributed to a decrease in blood sugar and insulin levels.
Bottom line: Stay tuned for more research to better understand the effect of intermittent fasting on health and aging.
Pros & Cons of Intermittent Fasting
Potential Pros of Intermittent Fasting
- Easy to follow. One of the biggest pros most people find with IF, is that it’s much easier to stick with than a traditional diet, because, technically, no foods are being restricted.
- Meal planning made simple. It makes meal planning simpler and easier as less food has to prepared and cleaned up making it ideal for busy schedules.
- Boost metabolism. Some studies have shown that lean muscle mass is more easily preserved with IF, than with calorie restricted diets, and does not negatively affect metabolism like low calorie diets do.
- Potential money savings. It can help you save money by eating out less and buying fewer groceries.
- Peace of mind. It is a great alternative for those who do not want to count calories and track their food intake.
- Improved Brain Health. Animal studies suggest fasting may enhance memory and learning. In addition, it may reduce the risk of neurological disorders including Alzheimer’s and Parkinson’s. More research is needed to see if these same benefits apply to humans.
Potential Cons of Intermittent Fasting
- Overeating at other meals. Some people may feel the need to reward themselves after fasting, and may overindulge during the eating window, or eat “less healthy” foods than usual.
- Long term compliance. While many people do well with intermittent fasting because they can keep it up for the long term, some people find it difficult to stick with.
- Food obsession. Depending on your fasting and eating split, food may be the only thing you think about on fasting days or restricted days.
- Potential lifestyle issues. It may be hard to fit social events, such as brunch or happy hour, into your intermittent fasting eating window.
- Hormonal imbalances. This seems to occur more often than women, but intermittent fasting may lead to hormonal imbalances depending on age and other factors.
How to get started with intermittent fasting
As with all major dietary changes, be sure to discuss intermittent fasting with your healthcare professional, or book a consultation with me. I’d love to work with you!
If you want to give intermittent fasting a try, the first step is determining if you’re a good candidate. Please consult with your physician before trying intermittent fasting if any of these conditions apply to you: Pregnant or breastfeeding, diabetes or hypoglycemia, low blood pressure, eating disorder (or history of), underweight, amenorrhea or trying to conceive, take certain medications (i.e. diuretics for high blood pressure), and/or if you’re under 18.
Next, decide WHY you want to try intermittent fasting and write down your health goals. Some reasons may include improved blood sugar levels, manage insulin resistance, weight loss, or healthy aging.
Monitor for potential side effects when starting intermittent fasting. These may include: fatigue, weakness, headache, reductions in sexual interest, and a reduced ability to maintain body temperature in cold environments. Many of these side effects are temporary, but if they last longer than a week or two, consider stopping intermittent fasting for awhile and see if your symptoms improve.
It’s also important to assess whether intermittent fasting is helping you achieve your health goals or not. Consider starting a journal, at least initially, to use on your intermittent fasting journey. Things to monitor might include: mood, energy level, blood sugar, blood pressure, and weight (I recommend waist measurements instead of weighing yourself, as a way to monitor fat loss). This will help you decide if intermittent fasting is a good health strategy for you in the long run.
Recap & Final Thoughts
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 macro and micro nutrients, appropriate amounts of food and fiber for a healthy gut microbiome, 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.
If you’re looking to get started with intermittent fasting, book an appointment with me to see if my program/service can help you. And, as a bonus, I’m offering a free 1 week 16:8 Intermittent Fasting Meal Plan Printable PDF to my newsletter subscribers!
Intermittent Fasting: FAQ
While some research suggests an earlier eating window (i.e. 8a-4p) is best for weight loss, you can still improve health parameters and lose weight with a later eating window, such as 11-7. Ideally, you should eat your last meal 2-3 hours before bed. Bottom line: Choose an eating window that best suits your lifestyle and that you can stick with in the long term. You may need to do a little experimentation to see what that is.
As mentioned above, if you have diabetes (type 1 or type 2), you should consult with your physician before trying intermittent fasting. One very small study looked at 3 men who had type 2 diabetes for 10-25 years. They did alternate day or periodic fasting under medical supervision, and were able to stop taking insulin after 1 month. In addition, they were able to reduce or stop medications after 1 year. Again, this is a very small study. Please consult with your physician before trying intermittent fasting if you are a diabetic.
Most people who do 5:2 intermittent fasting, consume about 500-600 calories 2 days per week, and eat normally the other 5 days. If one of your goals is weight loss, depending on your caloric needs, this should support 0.5-1 pound of weight loss per week. If you do 5:2 fasting, it’s very important to ensure you eat high fiber meals and adequate protein on non-fasting days.
Water, unsweetened seltzer, tea, and black coffee are all permitted while fasting. Some intermittent fasting practitioners allow small amounts of cream, half-and-half or milk in coffee. This is fine if you are using intermittent fasting as a way to decrease your overall calorie intake. If you are using intermittent fasting as a way to get into ketosis, keep in mind that even small amounts of caloric liquids may increase your blood sugar and insulin levels.
Like many people, you may have read somewhere that it’s better to eat several mini meals throughout the day to keep your metabolism up. While some people, especially those with GERD, gastroparesis, or other GI disorders, do better with frequent small meals, there is no good evidence supporting lots of mini meals to keep metabolism up. Indeed frequent eating throughout the day may result in over production of insulin to try and lower blood sugar. This may actually lead to weight gain in some individuals. As stated earlier, some studies have shown that lean muscle mass is more easily preserved with IF, than with calorie restricted diets, and does not negatively affect metabolism like low calorie diets do.
Focus your diet on eating mostly whole, minimally processed, high fiber foods including vegetables, fruit, beans/legumes, nuts, seeds, and unrefined grains. You may also include lean, high quality animal protein (fish, poultry, wild game, grass fed beef, eggs, & dairy), along with healthy fats (i.e. avocado, extra virgin olive oil, expeller pressed high oleic sunflower oil,& nuts/seeds). This will help keep you well fueled and feeling satisfied so you are less likely to eat after your eating window is over. If you are new to time restricted feeding, start slowly. Try decreasing your feeding window by 30 minutes -1 hour every day or two until you find your optimal eating window.
- Ageing Research Reviews (2017, October 39) Impact of intermittent fasting on health and disease processes. Retreived from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5411330/
- Harvard Health Publishing. (2017, January). Any benefits to intermittent fasting diets? Retrieved from https://www.health.harvard.edu/diet-and-weight-loss/any-benefits-to-intermittent-fasting-diets
- 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 https://www.nia.nih.gov/health/calorie-restriction-and-fasting-diets-what-do-we-know
- 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 https://www.nih.gov/news-events/nih-research-matters/intermittent-dietary-restriction-may-boost-physical-endurance
- National Institutes of Health NIH Research Matters (2018, September 18). Fasting increases health and lifespan in male mice. Retrieved from https://www.nih.gov/news-events/nih-research-matters/fasting-increases-health-lifespan-male-mice
- NIH Intramural research program. (2018, March 13). Intermittent Fasting Boosts Endurance in Mouse Marathoners. Retrieved from https://irp.nih.gov/blog/post/2018/03/intermittent-fasting-boosts-endurance-in-mouse-marathoners
- NIH National Center for Advancing Translational Sciences. (2018, August). NCATS-Supported Study Shows Eating Before 3 p.m. Can Improve Health. Retrieved from https://ncats.nih.gov/pubs/features/ctsa-kl2-fasting
Intermittent Fasting 101: 16:8 Meal Plan Printable PDF
I designed this 1 week Intermittent Fasting Meal Plan PDF to show you what a healthy 16:8 meal plan looks like. While you can adapt any “diet” to an intermittent fasting schedule, this plan features high fiber, low glycemic carbohydrates, lots of veggies and some fruit, healthy fats, and high quality proteins to help keep you full, energized, and satisfied.
You will eat 3 meals/day over a 8 hour window. I typically eat between 11a-7p, however you can pick the 8 hour eating window that best suits your lifestyle.
The plan averages 1550 calories/day to promote 0.5-1 pound of weight loss per week. If you do not need/want to lose weight, you may add in additional low glycemic, high fiber foods to each meal to increase your daily caloric intake.
If you are new to intermittent fasting, as mentioned earlier, I suggest keeping a food and mood journal the first couple of weeks to monitor for any side effects (i.e. headaches, fatigue, hormonal imbalances, etc) you may experience at the start of intermittent fasting.
Let’s Chat! Have you tried intermittent fasting? If so, what’s your experience been with it? Do you have any questions about intermittent fasting I didn’t address in this post? If you enjoyed this post, please consider sharing. Thanks so much!
Hi There! I’m EA, a registered dietitian nutritionist (RDN) in sunny San Diego. I create easy, DELICIOUS gluten free recipes & low FODMAP recipes for a wide variety of diets. Learn more about my nutrition philosophy and my nutrition coaching for digestive health, autoimmune disease, wellness nutrition, & vibrant aging. Sign up for my newsletter, or get in touch~I’d love to connect with you!