With intermittent fasting becoming a popular approach to losing unnecessary fat, many have asked if fasting for short periods of time is good for their health. To answer this, let's take a look at what happens in your body when you begin to ingest nothing but water for a day.

After your cells use up the sugar that's in your bloodstream from your last meal or beverage, your body has to find another source of energy for your cells. The first places that it turns to are your liver and skeletal muscles. Both your liver and muscles store sugar in the form of glycogen, and when needed, glycogen can be broken down to glucose, which all of your cells can use to produce energy for their ongoing activities.

During a water-only fast, your glycogen stores are depleted within about 24 hours, give or take a few hours. After your glycogen stores are used up, most of your cells begin burning fatty acids for energy - these fatty acids come from your fat reserves, including fatty tissue that surrounds your organs.

Two groups of cells - your red blood cells and your brain cells - cannot use fatty acids to fuel their energy needs. Your red blood cells and brain require glucose, and once glycogen/glucose from your muscles and liver are used up, your brain and your red blood cells get their glucose from the following two sources:

From glycerol, which is a component of your fat tissues. From your muscles - some of your muscle tissues get broken down, and the amino acids from your muscle tissues are used to produce glucose for your brain and red blood cells.

Clearly, it's not in your best interest to rapidly eat up your muscles to meet the energy requirements of your brain and red blood cells during a water-only fast. Your body knows this, and somewhere between the second and third day of water-only fasting, your liver begins churning out ketones, which, during a water-only fast, come primarily from the breakdown of fatty acids from your fat reserves.

Once your liver generates large numbers of ketones, your brain is able to use ketones to fuel itself. At this point, only your red blood cells require glucose that must still be derived from breakdown of your muscles, but with your brain no longer dependent on breakdown of your muscles for energy, the rate at which your muscles are catabolized will be such that your muscles are spared as much as possible - this state is called "protein sparing" - it's a survival mechanism that's built into human physiology to deal with times of famine.

Getting back to the big picture, it should be clear that from about the 2nd or 3rd day of a water-only fast, your body meets it energy requirements by burning through your fat reserves.

Since the bulk of the toxins in your body are stored in your fat reserves, the longer you fast on water only, the more fat you'll burn and the more toxins you'll eliminate from your system.

This is why we see elimination of lipomas, atheromas (accumulated waste in your blood vessels), and other conditions related to toxin accumulation during a prolonged water fast.

Put another way, your body does not experience significant detoxification during the first 12-24 hours of a water-only fast.

Your body begins to eliminate large quantities of toxins only after it begins to burn your fat reserves at a rapid rate. And this doesn't happen until you've used up the glycogen stores in your liver and muscles.

So when you fast for about 24 hours, you deplete the stores of sugar in your liver and muscles, and you begin to break down your muscles - these are the main things you accomplish during the first day of water fasting. Significant detoxification only begins to occur if you continue past day one of fasting.

This is not to say that there are no benefits to fasting for 16 to 24 hours at a time, or that you don't eliminate any toxins during a one-day fast.

You are eliminating toxins with every breath that you take. And your body will always increase its rate of ongoing detoxification whenever you get more rest and/or eat less food, because less digestive burden and more physical rest lead to more resources being available for detoxification.

Rather than fast for 16 to 24 hours at a time, I think it makes more sense to focus on eating minimally processed foods, and to eat only when you are truly hungry - with this approach, you will supply your body with fuel it needs for your everyday activities without depleting the sugar stores in your liver and muscles or causing breakdown of some of your skeletal muscle tissue.

If your goal is to give your body a period of rest and intense cleansing once in a while, you might spend a day having nothing but freshly pressed vegetable-based juices. Or you might eat only raw vegetables and perhaps a small quantity of antioxidant-rich fruits like berries. If this type of intermittent cleansing is of interest to you, please feel free to have a look at the following articles:

Full Body Cleanse Series

How to Enhance Your Health With Juices and Juice Fasting

Intermittent fasting can be an effective way of improving insulin sensitivity and losing unnecessary fat, but the most important long term determinants of body weight and health will always be eating minimally processed foods, not overeating, being physically active, getting sufficient rest, and striving to to be emotionally balanced.