6 Hours After Eating

After we eat, our bodies digest food and break down glycogen, which are the molecules that store energy. This process produces glucose — a molecule that is used as a major source of energy and absorbed into the bloodstream.


Glucose is used as our primary energy source if we’re on a normal eating schedule. This keeps us feeling well-fed and happy, because glucose goes to our liver and muscles, and fatty acids get stored for later use. Glucose fuels us for about six hours, and once it runs out, we begin to feel that “hangry” state when we’re hungry and grouchy.


Starvation Mode: 3 Days of No Food

Your glucose stores may last you for up to 24 or 48 hours, though they will mostly be depleted after six hours. Then, not only will you be hangry, but your body will be entering a state of ketosis, which involves elevated levels of ketone bodies in your system. Ketone bodies are produced from fatty acids when liver glycogen is entirely depleted, and are used for energy. Typically, if you’re fasting or starting a keto diet (a low-carb diet), your body enters ketosis. Sometimes this diet is used as a medical intervention to treat intractable epilepsy.

However, your brain can’t be fueled by fatty acids as its only source of energy, so it will continue to use the last remaining glucose stores in your body. Your brain actually uses 120 grams of glucose every day, which is a significant amount — so when glucose is depleted, your body has to figure out a back-up plan for the brain.

On day three, your brain will turn to the energy from ketone bodies — getting about 30 percent of its energy from them. By the time it’s day four, however, your brain will be getting nearly 70 percent of its energy from ketone bodies.


Eating Nothing for Over 72 Hours

The rough part happens after 72 hours of no eating — this is when advanced stages of autophagy begins. Once the fats are broken down, your body turns to breaking down protein in muscles, essentially wasting away your muscles. At this point, your brain’s requirement for glucose will have dropped from 120 grams per day to only 30 grams. But your brain will need to start getting energy from protein next. Breaking down protein and releasing amino acids into the bloodstream will produce more glucose; this transformation takes place in the liver, and your brain will be fueled by its much-needed glucose once again. Regardless, though your brain will be able to survive from protein, your muscles will slowly disappear.

Interestingly, the greatest amount of protein loss occurs during the first 72 hours. Afterward, the body adapts to conserve protein. Essentially, your metabolism slows down so much to the point that your body uses the smallest amount of energy as possible.

Your body may be able to survive for up to three weeks or even up to 70 days, depending on whether you’re also hydrated or have plenty of fat reserves to use up for energy. However, at a certain point, your immune system will be weakened due to lack of vitamins and minerals. Typically, two diseases can occur in end-stage starvation: marasmus and kwashiorkor. 

  • Marasmus is a form of severe malnutrition and energy deficiency, characterized by loss of muscle mass and edema, or stomach bloating. 
  • Kwashiorkor is the most common form of malnutrition in developing countries, caused by not getting enough protein and also characterized by fatigue, edema, and decreased muscle mass.

  • Read More: 9 Amazing Facts About Protein Coffee and How It Can Change Your Life
  • May 6, 2014 04:50 PM By Lecia Bushak 
    Consuming 500 calories a day is not a healthy diet. Normally, eating anything below 1200 calories per day will make your body assume there's a food shortage. The end result will cause your body to go into what is known as "Starvation Mode". During this mode, your metabolism will slow down and try to conserve energy. A slow metabolism will make it difficult for your body to lose weight. It's recommended to consume around 1200 - 1500 calories per day in an effort to lose weight.
Laura Motosko, MSEd, RD
Nutrition & Dietetics

For the general population, 500 calories per day is not a healthy diet. The calorie level is too low to obtain adequate nutrition including macronutrients carbohydrate, fat and protein as well as micronutrients, vitamins and minerals. The body begins to conserve energy and metabolism begins to slow when calories are below resting energy expenditure or what the body requires for essential functions. A very low calorie diet for weight loss of less than 1000 calories per day should be supervised by a physician.

I burn 500 calories by 9am each day.  How on earth would I run ME without any fuel for the next 12 hours?  

If you are eating anything less than 1200 calories per day, you'd better have this cleared through a doctor, registered dietician or nutritionist.  

Eat great foods to fuel your body.  Enjoy eating and be kind to yourselves!

500 calories a day is never healthy.  I know some of these so called diets out there even have you on this type of calorie deficit in the beginning but it is never a good idea.  What you have to remember is that your body needs a certain amount of calories per day just to do its basic functions.  So when you eat too few calories your body goes into starvation mode and slows down your metabolism.  Sure you might lose some weight in the beginning but long term you can actually gain more weight back then when you started.  For starters eat at least 1200 calories per to giver your body what it needs so you have a good balance and have good energy throughout the day.  If you really don't know if something is safe always consult your fitness professional first before trying anything extreme like this.  Sometimes more is better in this case.
This caloric level is way too low for any indvidual.  It will put the body into starvation mode. While you may lose weight initially, you will gain the weight back and probably even more once you get back on a regular diet.