Is fasting one day a week good for your health? To answer this question, let's take a look at what happens in your body when you begin to eat and drink nothing but water.
After your cells use up the sugar that is in your bloodstream from your last meal or beverage, your body has to find another source of energy for your cells. And the first places that it turns to are your liver and your muscles. Both your liver and your 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 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 secnd 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 is built into human physiology to deal with times of famine.
Getting back to the big picture, it should be clear that from about the second or third 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 one day a week, 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 one day a week, 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 always mean more available resources for detoxification.
Rather than fast one day a week on water only, I think it makes more sense to do a juice fast one day a week, or even once a month. With a juice fast, you can supply your body with enough nutrients that you don't have to deplete the sugar stores in your liver and muscles, or break down a lot of your muscle tissue. At the same time, because the nutrients in freshly pressed juices are so easily digested, a one-day juice fast can ease digestive burden and enhance ongoing detoxification to some degree.
But let's be clear: the main benefit of a one-day juice fast is not significant detoxification; it's a concentrated period of rest for your digestive organs, and an opportunity for the organs that are responsible for ongoing detoxification (liver, kidneys, skin, and lungs) to do a little extra health-promoting work.
To sum things up, I would say that it's not good for long term health to fast one day a week on water only. If you want to give your body a period of rest and intense cleansing once in a while, it makes more sense to spend a day eating all raw fruits and vegetables, or drinking nothing but freshly pressed juices.