Intermittent fasting has been around for centuries, but it’s getting more popular. Recently, it has become more of a fad diet. It’s promising research has been conducted regarding its potential benefits on disease prevention and obesity.
What exactly is intermittent fasting?
Intermittent fasting is time-restricted eating – meaning you do not eat for a certain period of time. It is followed by eating normally for a specific amount of time. There are various types of intermittent fasting. Some eat for an 8-10 hour period during the day and fasting for the remaining hours at night. Others may fast for 2 days out of the week while eating normally the other 5 days of the week. This is called 16:8 fasting or 5:2 fasting. What’s most popular is often skipping breakfast and choosing an 8-hour eating window like 11 a.m. to 7 p.m. This is the most attainable option for most people.
While fasting may seem extreme to some people, proponents argue that humans actually ate this way for most of history. Since our hunter-gatherer ancestors ate food that was available, they didn’t eat at set times throughout the day. Furthermore, fasting has been a part of religious traditions in past centuries, so it’s not necessarily a foreign concept.
Advocates for intermittent fasting claim that it is an effective and research-backed means of losing weight and improving health. Usually, as dietitians we run for the hills when we hear about new fads or trends popping up. But does this one have some merit to it?
Let’s take a look at the research:
Benefits for body and brain but evidence for humans is limited.
There is a large body of evidence that suggests fasting can benefit both the body and brain. However, most research has been conducted on animals, such as mice. not humans. So there is much, much more research needed to be done. Research has also been short-term studies, so we are unsure of the long-term effects at this time.
Fasting is beneficial for many reasons.
Fasting has been shown to improve biomarkers of disease, reduce oxidative stress and preserve learning and memory functioning, according to Mark Mattson, the Senior Investigator for the National Institute on Aging, which is part of the US National Institutes of Health. Mattson has investigated the health benefits of intermittent fasting on the cardiovascular system and brain in rodents and has called for “well-controlled human studies” in people “across a range of body mass indexes” (J Nutr Biochem 2005;16:129–37) according to “Intermittent fasting: the science of going without” in the Canadian Medical Association Journal.
One main hypothesis why fasting may improve health and promote weight loss is due to simple calorie reduction. That is, fasting is one approach to eating fewer calories overall if you’re restricted to eating at only a certain amount of time during the day. This can lead to less mindless eating and unnecessary snacking when you aren’t truly hungry (i.e. snacking at night in front of the TV). Studies on mice have shown that mice that ended up eating more calories overall on non-fasting days did not lose weight. They did not improve their health biomarkers. This shows one example of fasting (without calorie reduction) not having the same benefit. It may be that simply the reduction in calories is what’s beneficial. Simply put, if you overeat on non-fasting days, you aren’t doing your body much good.
Intermittent Fasting must be done long-term.
A randomized control trial on non-obese humans concluded that “alternate-day fasting was feasible in nonobese subjects, and fat oxidation increased. However, hunger on fasting days did not decrease, perhaps indicating the unlikelihood of continuing this diet for extended periods of time”. For intermittent fasting to be beneficial and feasible, it must be doable for the long-term. This study points that fasting for a full day (vs. during the night) isn’t necessarily doable long-term. The clients still felt hungry, increasing their risk of non-compliance. Not to mention, not eating for 2 whole days every week sounds plain miserable! We must consider how this will affect our life overall. You can say goodbye to lunchtime meals with pals and brunch in bed with your spouse. Fasting for a whole day is not feasible for most humans who live normal lives.
Another study on an alternate day fasting on obese adults showed that fasting every other day was no more superior to weight loss, body composition, or cardiac disease prevention than simply calorie reduction every day. Intermittent fasting is just one approach to cut back on calories. What matters most is picking sustainable habits that work for your lifestyle.
Benefits that could possibly improve metabolism and burn fat.
The premise of fasting on weight loss is also based on reprogramming your metabolism, which makes sense. It’s based on the idea that if you aren’t eating for a set amount of time, your body will turn to its other fuel source: stored fat. Anytime we consume carbohydrates, our bodies pump out insulin, which tells our body to store fat. Also, anytime excess calories (no matter where they come from) are consumed, you’re body will store the extra fat. Some think that insulin will not be secreted and our bodies will be in fat-burning mode when not consuming food.
Other studies have shown that Time-Restricted Feeding (⩽12 hr) “has been shown to both protect against and reverse the harmful metabolic consequences of diverse nutritional challenges, including high-fat and high-sugar obesogenic diets.” The mice on a time-restricted feeding regimen displayed reduced adiposity and liver steatosis. Likewise, they had improved glucose tolerance and reduced cholesterol levels when compared with mice fed ad libitum with the same high-fat diet. These changes may reflect improved homeostasis in multiple tissues. Importantly, these improvements occur “independently of energy intake”. Again, this study was done on mice – not humans.
Overall, there is some promising scientific evidence suggesting that circadian rhythm fasting (fasting overnight) when combined with a healthy diet and lifestyle, can be a particularly effective approach to weight loss. There is definitely more research that needs to be done long-term, and on humans. It’s exciting to see promising results in small studies.
Fasting is just one potential tool that people may benefit from. However, it’s important to note that if you try to implement Intermittent Fasting (IF) into your lifestyle, do so in a healthy way.. Pair it with a balanced diet and exercise.
How to do Intermittent Fasting the Healthy Way:
Try fasting naturally overnight for a 12 hour period.
This was once just normal eating habits, but as food, especially palatable food, has become more and more indispensable in our modern era, humans are now eating at all hours of the day. Try waiting to eat your first meal of the day at least 12 hours after your last meal or snack the night before. For example, if you finished up dinner at 7:30 pm, try to avoid eating anything else until 7:30 am the next day and see how you feel.
You can try stretching it to 14 hours maximum, but after 14 hours, your body starts to get stressed, and the beneficial effects may start to become negative. Keep it in that sweet spot of 12-14 hours of fasting at night, and always avoid fasting during the daytime! We need proper fuel during the day to have energy and function, but we don’t necessarily need those late night snacks when we’re not doing much.
Still consume adequate calories to function optimally!
Fasting is NOT starving. Just because you are limiting your eating to a smaller window during the day, it isn’t healthy to eat too little calories as our bodies need adequate carbs, protein, and fats to function optimally and prevent starvation mode. So if you skip breakfast, make your lunch a little larger, or add in at least a snack or two to reach your personalized calorie and nutrient needs. Fasting is not an excuse to starve yourself!!
Limit snacking at night to help your body go into fat burning mode while you sleep.
Still follow a healthy plant-centered diet that includes all food groups – lean protein, whole grains, fruits, veggies, & low-fat dairy.
Make it simple and sustainable.
Don’t go overboard. How can you incorporate the idea of temporary fasts into your lifestyle? Maybe you can just try to stop eating after dinner. Or maybe you can push dinner a little earlier and breakfast a little later. Or maybe you can try 14 hours of nighttime fasting but only 1-2 days per week to start. Make it a small change and you’re more likely to stick with it.
How to do Intermittent Fasting the Unhealthy Way:
Those with a history eating disorders.
If you have an eating disorder history, be careful. Never try intermittent fasting, as it could be a slippery slope into disordered eating habits.
Those with medical conditions need to be extra careful.
Those with diabetes or those taking diabetes medication, eating disorders, and those that are pregnant or breastfeeding. Advise your doctor before trying a fasting regimen if you have any of these conditions.
Those who won’t listen to their bodies.
If you are physically hungry and can’t focus, EAT! Everybody is different. Pay attention to what your body is telling you and feed it when it’s hungry. Don’t intermittent fast just to do it, do it only if it works with your lifestyle. For example, are you a morning exerciser and can’t fathom waiting 4 hours after your workout to eat? Listen to your body, it needs fuel for exercise. Maybe instead try moving around the hours you are eating to work better with your schedule.
Those who fast just to follow the trend.
This touches back on listening to your body. If you trying intermittent fasting and it just doesn’t work well for you, then stop doing it! Everyone’s body is different. Don’t continue to fast just because it’s a trend if it doesn’t make you feel good.
Those who don’t eat enough calories during your eating ‘window’.
Intermittent fasting should never be used as a means to just skip meals. You STILL need to focus on getting enough nutrition!
Those who think intermittent fasting is the magic bullet.
It’s not. There is no such thing! It’s still important to eat a healthy, varied diet, get adequate sleep, drink enough water, and exercise. And remember, no research is 100% conclusive and most research on IF has been done on mice, not humans. We don’t have all the answers yet!
Those who fasting during an entire 24 hour period.
We need fuel to function optimally. Fasting for an entire 24 hours one or two days per week is not sustainable. It could set you up for overeating later on. Make intermittent fasting a realistic and simpler part of your lifestyle by fueling up when you are awake and functioning. Do your fasting during the night while you’re sleeping.
Those who confine themselves to a specific way of eating for a short time.
This could potentially take you down a dangerous path. It leads to yo-yo dieting and the possibility of increasing your risk for disordered eating habits. Instead, aim to focus on a long-term sustainable healthy lifestyle for optimal health, rather than a short-term fix.
The Bottom Line
Is intermittent fasting going to be doable for you and fit into your lifestyle? Will it be sustainable long term? How will it affect your energy levels, exercise, or focus at work? These are the questions we need to be asking ourselves.
Overall, a healthy long-term eating pattern and lifestyle is what’s most important. If incorporating small time-restricted fasts into your lifestyle works for you, then that’s great! Be sure that you are still getting in enough nutrition during the day. You need all 5 food groups while focusing on whole plant foods and avoiding skipping meals. Just be mindful of the downsides as discussed, and always listen to your body first and foremost!