One of the few things all religions agree on is the benefits of fasting. Whether you do it for fitness or spiritual reasons, the act of abstaining from food can be a powerful, and free, tool.
As a means toward losing weight, fasting helps you to both reduce the amount you eat as well as well as gives your body time to burn fat. When you eat food, you consume carbs which become glucose that becomes insulin. This form of energy is what your body must burn through first before it starts to burn fat. This is why eating a low-carb diet (i.e. keto) helps with weight loss because you eat less carbs, which means you have less glucose to burn through, so you start burning fat sooner. And obviously the longer you go without eating, the longer your body is burning fat.
On the topic of insulin, there is a theory that obesity is actually caused by insulin resistance. When people form bad habits and eat a lot throughout the entire day, they are pumping their body full of lots of insulin all the time. Over time your body becomes worn out and insulin resistant. But in the same way that someone who is losing their hearing turns the volume up louder, your body produces more insulin in response to the insulin resistance, which then causes more insulin resistance and burn out, and it is a vicious cycle. Modern medicine treats insulin resistance by giving people more insulin, which is why they say diabetes isn’t reversible and you have to be on insulin the rest of your life. But people are finding that through extended fasts, you can deplete the reserve of insulin and begin to heal the insulin resistance and reverse type 2 diabetes. Insulin resistance is also why people who are overweight have a harder time losing weight/fat. Their body produces vast reserves of glucose/insulin so they are practically always burning that energy and never get to the point that they are burning fat. And the longer someone has been obese, the worse the insulin resistance, and the harder it will be to lose weight.
As for the spiritual benefits of fasting, in Mark 2, Jesus assumes we will be fasting when he says, “But the time will come when the bridegroom [Jesus] will be taken from them, and on that day they will fast.” In Mark 9 we find out one reason why fasting is needed when the disciples were unable to heal the demon possessed boy. They asked Jesus why they weren’t able to and he responds saying, “This kind cannot be driven out by anything but prayer and fasting.”
So aside from giving us greater spiritual strength, what else does fasting do for yourself from a spiritual point of view? Isaiah 58 says, “Is not this the kind of fasting I have chosen… to share your food with the hungry and to provide the poor wanderer.” This idea of using your savings from a fast to serve the needy is actually deeply embedded in the Latter Day Saint tradition where members of that faith are called to a 24 hours fast one Sunday every month for that very purpose.
Fasting also serves to strengthen our resolve and give us an opportunity to earn grace for others. By voluntarily denying ourselves things we build strength to turn away from other temptations when they come. We also may “offer up” the suffering we experience during a fast for a special intention or as a reparation for our sins. The most common and popular season of fasting in the US is Lent where even non-Christians choose to “give something up for Lent.” But it is also common to hear people throughout their 40 day fast to lament that they “can’t have sweets” or that they are “dying for a drink.” To this point, in Matthew 6, Jesus says “When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show others they are fasting. Truly I tell you, they have received their reward in full. But when you fast, put oil on your head and wash your face, so that it will not be obvious to others that you are fasting, but only to your Father, who is unseen; and your Father, who sees what is done in secret, will reward you.”
The best online resource I’ve found to give you easy to understand information and instructions on fasting is by Dr. Jason Fung.