TRUTH: Unless your are a Type 1 Diabetic, its not dangerous. You (or your Doctor) may be thinking of Ketoacidosis.
Ketosis is a state where your body is using Fat as it's primary source of Fuel.
Ketoacidosis is a state that Sometimes happens with Type 1 Diabetics who can not produce Insulin where the body has used all it's glucose and all cells are burning ketones. The body becomes acidic and can be a dangerous situation.
If you are not a Type 1 diabetic, you will always have some glucose in your system.
We Need Carbohydrates to Live
TRUTH: We do NOT need Carbohydrates to live. We need a few essential nutrients to sustain life.
We need water, energy (for fuel) which can be either carbs OR fat, minerals, major elements (like calcium, phosphorus, potassium and sulfur), trace elements (like iron and manganese), amino acids, fatty acids, vitamins and miscellaneous things (like choline, carnitine).
This is NOT a complete list. The point is, since carbohydrates fall in the "ENERGY" category, the body can choose FAT to use as it's energy instead of carbs. Fats are also a much better fuel source too! Use your own body fat for fuel!
If I Eat All That Fat I will Gain Weight!
TRUTH: Once you become 20 or 30 pounds overweight your body's chemistry gets locked. To reverse this, Stop triggering insulin. The secret to weight loss or gain is all about managing insulin levels.
When your body is in Ketosis, it is using your own FAT as it's primary energy source. To continue to fuel your body and release fat you have to eat fat.
TRUTH: They don't matter when insulin is ruling your body.
Insulin rules: If too much insulin is in the blood, it locks down the fat cells and they WILL NOT RELEASE FAT.
Once you become Keto Adapted, you can work on calorie balance. In the beginning the focus is on insulin levels.
Low Carb Causes High Cholesterol
TRUTH: This is partly true.
The cholesterol that goes up is the GOOD (HDL) cholesterol.
When you remove carbs your triglyceride levels drop.
Your heart gets healthier when you raise your good cholesterol and lower your bad cholesterol.
What About Exercise? I use Carbs for Fuel!
TRUTH: The Keto Lifestyle is excellent for endurance athletes.
For the rest of us, burning fat for fuel allows a steady production of ketones. Energy levels are very consistent which makes working out easier.
There are several adaptations to a Ketogenic Lifestyle that can accommodate a higher level of carbs before a workout for instance.
If I go Keto, I Will Start Eating Muscle
TRUTH: Not True. The human body was not meant to break down muscles for fuel.
Only in Extreme stages of starvation will we begin to use muscle as fuel.
Fat gets emptied first. When you are in a state of ketosis you are burning your fat and many of us have plenty of it.
The above information is paraphrased from the the book: