Intermittent fasting is when you don't eat for a determined amount of time, and eat all of your calories during an 'eating window'.
Eg: fast for 16 hours, eat for 8. Repeat every day. Most people do this by skipping breakfast and lunch, and eating around dinner time. Or you can do a 24 hour fast every 3 days or so. There are a ton of variations that you can look up.
The whole idea is to deprive your body of external energy sources so that it can dig into its internal energy sources aka your fat tissue. The concept is there but most of the information we have about it comes from people's subjective experiences whereas the scientific literature haven't found any significant differences in terms of fat loss between people who do IF, and people who simply calorie restrict. Yes, it is considered to be safe when done right. It's just another way to control your eating habits and cut some fat. As long as you hit your daily caloric and macro goals, and eat good foods, you're safe. A lot of people report reduced craving for junk food on the days they fast, but for others IF triggers episodes of binge eating. So it really comes down to what works for your body and lifestyle.
as I understand it:
You can only burn fat when producing glucogon.
This is the opposite of producing insulin when you have high levels of sugar.
so, by fasting for 14-16 hours, you naturally have low sugar levels (not consuming sugars or carbs) and thus produce more glucogon which then allows you to burn fat.
Lots of good information here but I would like to add that not only can it help with weight loss but it can also improve health. A condition called Nonalcoholic Steatohepatitis (NASH) or just fatty liver disease can occur in certain individuals. Nonalcoholic fatty liver disease occurs in every age group but especially in people in their 40s and 50s who are at high risk of heart disease because of such risk factors as obesity and type 2 diabetes. When people develop fatty liver disease they have a form of hepatitis that leads to inflammation of the liver causing scarring and damage. This damage can eventually lead to the need for liver transplants. Anecdotally most of the cases that I see (I'm a nurse) are morbidly obese patients with a long history of adult onset diabetes. While this might not always be the case, obesity and diabetes are certainly precipitating factors.
Fasting causes us to catabolize our own stored energies. Fat is by far the most abundant stored energy. We use this energy when we can't find food to eat. These energy stores sustain us until we can find food. Humans have only in the last few thousand years (since the advent of farming) or last few decades (since the abundance of high calorie food) have consumed an overabundance of food. Obviously there are constant famines globally but the globalization and mobilization enables us to get food to these areas when it would have been impossible decades ago. The vast majority of Americans over eat, under exercise and are overweight.
Before the advent of farming we were hunter gatherers. We ate what we found, when we found it. We would binge on apples when we stumbled on an apple tree. We binged on bison when we killed one. We didn't always eat 3 square meals according to the Food Pyramid with snacks in between. Fasting was not a choice but a way of life. This way of life more hundreds of thousands of years is what we adapted to.
These is a lot of evidence that fasting, just like our ancient ancestors did, is quite healthy. We are forced to catabolize our stored fat. There is a plethora of information pointing to fasting diets and their ability to decrease fatty liver disease and diabetes in patients. [Here](https://www.sciencedaily.com/releases/2016/05/160509085347.htm
) is an article discussing the benefits of fasting for fatty liver disease and [here](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5394735/
) is an article showing it's benefits for diabetes. A 5 minute Google search will give plenty of other examples.
I am no expert and I do not recommend fasting without consulting your doctor. I do however find this stuff fascinating and believe for certain people, under supervision of a doctor, that it can not only lead to weight loss but help with other chronic diseases.
I have discussed this with my doctor and I currently fast for a 24 hour period and a 36 hour period weekly. This along with a bit more exercise has helped me to lose about 20 pounds so far. I have not had labs drawn yet but I hope to on my next physical.
been doing a 24 hour fast twice per week for the last two months. it is pretty easy, it feels good, and it makes me have less cravings during the week on regular eating days.
I do this on accident. I just am not hungry for a day, then will eat for the next 3 or 4 a little more than "normal" (about 3k calories, I'm 6'2" so I "need" more than 2k/day)
Also 16 hrs may seem tough at first glance, but if you stop eating at 7pm and wake up at 6am that's 11 hours right there. Make it to lunch at noon and that's another 6 hours. So eat normal, just don't snack at night and skip breakfast.
Everyone else is correct, but if you’re considering trying it just remember to keep your head strong. If you have an addictive personality it can be a slippery slope to an ED and other problems. I tried to do it but couldn’t stick to the “intermittent” idea. I was addicted to the idea of challenging myself to go further, 8 hour of fast eventually turned into 4 days with restricted calories on the fifth.
I tried IF and 2 months later I started getting stomach pains that persisted for about a year after. Unsure if related.