Intermittent fasting (IF) has been in the spotlight for the past couple of years. However, intermittent fasting is nothing new. Humans have fasted for most of their history, whether it’s during the typical overnight period, during more extended periods of food scarcity, or for religious reasons.
You’ve heard about some of the benefits of intermittent fasting: that it may help you lose weight, lower your risk of certain chronic diseases, and even give your brain a boost. But as with all diets, we need to take a look at the scientific evidence to see whether IF is actually beneficial for us.
What is Intermittent Fasting?
Intermittent fasting diets focus on when you eat, rather than what you eat.
During fasting times, little to no calories are allowed, and the main intake is usually from calorie-free drinks (such as; water, black tea and black coffee). But on non-fasting days there are often no rules or restrictions in place.
Here are two main types of intermittent fasting:
1- Time-restricted feeding: This involves shrinking the window of time during which you eat each day. Typically, that involves extending the duration of your regular overnight fast, to anywhere from 12 to 20 hours, by skipping either breakfast or dinner. The most popular example of this is the 16:8 diet – this includes an 8-hour eating window with a 16 hour fast.
2- Alternate day fasting: This involves fasting on a few days per week. For example, the 5:2 diet where you consume less than 600 calories for 2 days per week, and consume your usual intake on the remaining 5 days of the week. The 6:1 diet (also known as the ‘eat-stop-eat-diet’) is similar but involves a 24 hour fast one day per week, and your usual intake on the remaining 6 days.
In this way, IF is more lifestyle than a diet. Unless you follow the 5:2 approach, you technically don’t count calories or even have to restrict them.
Intermittent Fasting Burns Fat
When you fast, your body burns fat instead of sugar for energy, which leads to fat loss and gives your brain a boost (1-4).
During a fast, your cells switch from using glucose as their primary fuel source to using fat. Thus, your fat stores, chiefly triglycerides, get burned up for energy. This is why research has found that IF leads to weight loss, as well as an improved cardiovascular disease risk profile (1, 2).
When your body uses fat stores for energy, it releases fatty acids called ketones into the bloodstream. Ketones play a role in weight loss, but they have also been shown to preserve brain function, even offering some security against epileptic seizures, Alzheimer’s disease, and other neurodegenerative disorders (5, 6).
Intermittent Fasting Lowers Insulin and Improves Insulin Sensitivity
When you fast, your insulin levels drop, while your levels of human growth hormone and norepinephrine rise, helping you shed weight and resist chronic disease. In several studies, fasting has been shown to both prevent and reverse type 2 diabetes. Insulin sensitivity improves and levels of insulin drop dramatically (7, 8, 9).
Intermittent Fasting Increases Your Cells’ Resiliency and Health
During intermittent fasting, your cells get a little stressed out—in a good way—making you more resilient.
Fasting seems to cause cells to initiate a waste-removal process called autophagy. During autophagy, the body cleans house and starts regenerating itself—eliminating dysfunctional, damaged cells to make room for new, healthy ones. Autophagy may offer protection against diseases like cancer or dementia (10, 11, 14). IF also appears to protect the body against the negative effects of oxidative stress and inflammation (12, 13).
Intermittent Fasting May Improve Gut Health
Gut bacteria play an essential role in regulating your mood, protecting your immune system, and getting the most nutrition out of your food. Fasting may help protect your gut microbiome. And in turn, those bacteria could help protect your body while you're fasting.
Scientific research is revealing that fasting may restore microbe diversity in the gut, increase tolerance against ‘bad’ gut microbes, and restore the integrity of the gut lining (intestinal epithelium) (15,16, 17).
Intermittent Fasting Impact on Heart Health
IF has been linked with an improvement of multiple indicators of cardiovascular health, including blood pressure; levels of high-density and low-density lipoprotein (HDL and LDL) cholesterol, triglycerides, glucose, and insulin; and insulin resistance (18,19, 20).
It is important to note that current studies have looked at cholesterol and blood pressure levels, rather than heart disease itself. And most of these have examined alternate day fasting, rather than time-restricted feeding. Therefore more research is needed before we conclude that it can reduce the risk of heart disease.
When You May Not Want to Fast
Intermittent fasting has benefits for many people, but is it right for you?
The answer to this question depends on your health goals and your lifestyle.
Circumstances to be cautious of intermittent fasting include:
You have chronic fatigue or HPA axis dysregulation. For example; feeling tired when you wake up in the morning even if you get enough sleep, having afternoon energy crashes, not being able to fall asleep or stay asleep, your quality of sleep is poor, you have poor stress tolerance or poor exercise recovery.
Your hormones are not balanced
You’re a woman trying to maximise fertility
You suffer from any type of eating disorder
You’re under a lot of stress at work or at home
Evidence shows that intermittent fasting can have powerful effects on your body and brain.
However, intermittent fasting is not something that anyone needs to do. It’s simply one of many lifestyle strategies that can improve your health. Eating real food, exercising, and taking care of your sleep are still the most important factors to focus on.
At the end of the day, there is no one-size-fits-all solution when it comes to nutrition. The best ‘diet’ for you is the one you can stick to in the long run.
Intermittent fasting works great for some people, not others. The only way to find out which group you belong to is to try it out! If you feel good when fasting and find it to be a sustainable way of eating, it can be a very powerful tool to lose weight and improve your health.
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