Every now and then, somebody says something so *well* that paraphrasing simply isn’t necessary.
“The point of a keto diet is to be in ketosis - what this means is that your body is breaking down fat at such a rate that there are ketones in your bloodstream. This happens after fasting, or with a low carb diet. It’s a normal metabolic state. If you’ve ever had steak and veggies for supper and then had eggs for breakfast (or skipped it all together), you’ve been in ketosis.
It’s also important to note that your body burns whatever fuel is available - glucose, FFA (free fatty acids), ketones, alcohol. Whatever there is more of is what it burns more (which is why alcohol “pauses” ketosis, your body will burn up the alcohol first).
Ketostix will measure excess ketones that are in your urine. However they are not the be all and end all of being in ketosis. If you are using all your ketones for energy, or if you’re getting rid of the excess through sweat or saliva, ketostix may be negative.
One of the goals of the keto diet is being “keto adapted”. This means that your body is primed for functioning with very little glucose. Different enzymes are involved for breaking down fat than breaking down glucose. When you first enter ketosis, you are using fat for energy, but at first it’s in limited amounts because you don’t have as many fat-converting enzymes. These get built up over time. This is what causes the tiredness at the beginning of the diet. Once the enzymes are in place, your cells change the way they get energy. It’s really amazing all the changes that have to happen internally for keto-adaptation to happen. It’s like you can’t just put diesel into a regular car engine and expect everything to work smoothly.
Once you are keto-adapted (which can take a few weeks to a month depending on the person), fat/ketones becomes the preferred fuel. Hormone levels are changed, glycogen (glucose stored in muscles and liver) is lowered, you carry less excess water. You’re able to function well - lots of energy, you can exercise, lift, build up endurance etc.
When you’re keto adapted and get an “overdose” of carbs (more than your body needs at the time) different things happen. First, glycogen gets replenished, which causes water retention. Secondly, insulin rises, which can affect other hormone levels as well. While your body processes the carbs, you are not burning ketones. Once the glucose is dealt with, you will go back into ketosis. When you’re keto adapted, this doesn’t take long because you already have the enzymes and are “primed” to use fat for energy.
When you’re starting keto, the more often you have sugar, the longer it takes to become keto adapted. When you are keto adapted, sugar will will still take precedence over fat for fuel (because excess blood sugar is fatal and so your body needs to handle the sugar first).
If your goal is weight loss, excess glucose is counter productive. There is no “need” for a carb up, even with endurance sports, but some people find the boost helpful depending on their goals (Peter Attia has said he doesn’t use any carbs unless he is cycling more than 3 hrs, so if you’re under that, going carb free is at the very least possible). Even building muscle while in ketosis is possible, just ask those over at /r/ketogains”