Mumbai-based entrepreneur and CEO Abhishek Agarwal chose an unusual way to celebrate his birthday this year, by going without food for 90 hours. Agarwal, CEO of CreditVidya and Prefr, documented the experience on LinkedIn, describing it as a personal challenge to test "hunger vs willpower". He went without food for almost four days. Agarwal said that the first day felt easy, but the second became much harder. He described the third day as "silence", and the fourth day, which was his birthday, he said he had the clearest mind he had experienced in months.

After completing the fast, Agarwal said he felt stronger and more powerful and compared the experience to proving his own mental limits wrong. In his post, he wrote, "90 hours later, I realised we, humans, are probably capable of tolerating way more discomfort than our 10-minute-delivery brains would have us believe. I felt stronger, powerful & like a super hero in my own way. And trust me, once you prove your brain wrong once, you start questioning its other limits, nothing feels unachievable anymore!"

He also shared pictures of himself exercising during the fast. Agarwal's 90-hour fast experience is not proof that it is safe or beneficial for everyone. Going without food for almost four days causes major changes in the way the body produces and uses energy. Read on to know what actually happens inside the body during such a long fast.

What Happens When You Fast For 90 Hours?

1. Your Body First Uses Stored Glucose

When you stop eating, the body initially depends on glucose already available in the bloodstream and stored as glycogen in the liver and muscles. As the hours without food increase, these glycogen stores gradually get depleted. The body then has to find another source of energy. This is why a prolonged fast is very different from simply delaying a meal.

2. Fat Becomes A Major Source Of Energy

As glycogen stores fall, the body turns to stored fat for fuel. Fat is broken down and used for energy. At this point, the liver produces substances called ketones. This process, known as ketosis, can become more definite after a day or two without food. Fasting can therefore shift the body from relying on glucose to using more fat and ketones for energy.

3. Ketone Levels Rise

During a prolonged fast, the brain and other organs begin using more ketones as a fuel. This metabolic change can happen after several hours of fasting and becomes more significant as the fast continues. Some people report feeling mentally clear during this stage. However, individual responses can vary, and feeling mentally alert does not necessarily mean that prolonged fasting is beneficial for everyone.

4. Insulin Levels Drop

Without regular food intake, especially carbohydrates, the body does not need to release as much insulin. Insulin levels fall while hormones such as glucagon help the body move stored energy. These changes are part of the body's normal response to fasting. They help maintain blood glucose and make stored fat more available for energy.

5. Some Muscle Protein May Be Used

Although the body relies on fat during prolonged fasting, it does not use only fat. Some protein can also be broken down to provide amino acids that help the body make glucose. The body has mechanisms that reduce protein breakdown as ketosis increases, but prolonged food deprivation can affect the body's lean body tissue.

6. Hunger, Weakness And Headaches

Not everyone experiences fasting in the same way. Hunger may be strongest during the earlier stages, while some people may later report reduced hunger. Others can experience fatigue, headaches, irritability, nausea, constipation, difficulty concentrating or sleep problems. These symptoms can become more concerning when a fast is prolonged, especially if you don't consume any fluid or electrolytes.

7. Dehydration And Electrolyte Problems

A fast that involves only food restriction is different from a dry fast. If a person continues drinking water, they can still become dehydrated because the body loses fluids through urine, sweat and breathing. Electrolytes such as sodium and potassium are also important for nerves, muscles and the heart. Fluid or electrolyte disturbances can lead to serious complications.

Is A 90-Hour Fast Safe For Everyone?

No, a prolonged fast should not be treated as a health or weight loss practice. People with diabetes, certain medical conditions, a history of eating disorders, those who are pregnant or breastfeeding, and people taking medicines that affect blood sugar or blood pressure may have certain risks. It is recommended that you discuss fasting with a healthcare professional if you have certain medical conditions.

Disclaimer: This content including advice provides generic information only. It is in no way a substitute for a qualified medical opinion. Always consult a specialist or your own doctor for more information. NDTV does not claim responsibility for this information.