So let me explain to you what “Keto Flu” really is. Everybody that ends up on a Keto diet inevitably ends up cutting out a ton of processed food. With all the processed food, you end up cutting out most of your sodium intake. We have been told sodium is bad for us, just like saturated fat right? Not exactly, YOU ACTUALLY NEED MORE SALT THAN EVER UNDER A VERY LOW CARB DIET. When you cut back your carbs, you don’t end up with blood sugar spikes every time you eat and digest food, because well, your not eating any sugar. As a result, your body doesn’t need to elevate your circulating insulin levels to keep your blood sugar at normal levels. Normally, your kidneys tend to store and reuse a lot of sodium, under normal conditions. When your insulin levels are very low and stable, your kidneys through various horomonal mechanisms go into a diuretic type mode, excreting lots of sodium, potassium and water. The net effect is, if YOU DO NOT REPLACE YOUR SALT YOU GET LOW BLOOD PRESSURE.  Symptoms of low blood pressure include, dizziness, weakness and fatigue.
Another aspect of Keto-flu can include the nausea and diarreah many new keto dieters sometimes experience. If you go from a high carb diet, straight to Keto in one giant leap. You are probably going to have a bad time. Let me go over some biology.
When you eat foods, all your macro nutrients are broken down into their constituent parts such as simple sugars, amino acids, fiber and lipids. When you consume fat, various pancreatic enzymes and bile salts from your liver and gallbladder all work to break the fats/lipids down into cholesterol, triglycerides and other components. ***IT takes time for the liver, gall-bladder and pancreas to up-regulate the production of bile/enzymes in order to accommodate a large amount of fat. If you ingest a large amount of fat and your organs don’t produce enough of these things to break it down, you can get very nauseated from undigested fat.
Another thing that can happen with new keto-dieters is diarrhea. You may or may not know, there is a diverse world of bacteria, yeast and other microorganisms in your gut. Sometimes, they are beneficial and assist in digesting food, and sometimes they are simply taking advantage of a free meal. Every organism in your gut is constantly vying for space and nutrients in your gut, its like a world war 2 that never ends with some sides taking more territory than others. If your like most people and have taken rounds of antibiotics or consumed a standard american diet with tons of sugar/carbs and alcohol. It is like you just wiped England and France off the map and threw money and food out of helicopters to the German army. Basically, you probably messed up the balance of power, letting e. coli, H. pylori and Candida A. to start taking over where they shouldn’t. This is called Gut Dysbiosis and is part of the reason Keto and Paleo can be so beneficial for auto-immune disorders. We now know 70% of our immune system is located in our gut ecosystem, and these gut bacteria have connections to our nervous system and can initiate cravings as if they were telling us to feed them with certain food.
What this means for Keto: Most these “bad” microorganisms overgrow because we ate too much sugar, drank too much booze or took a ton of antibiotics. When you stop eating carbs, you cause mass genocide in your gut as many sugar dependent microorganisms die by the billions from starvation. The rotting corpses of these microorganisms can release chemicals and initiate inflammation inside your gut. leaving you with a few days of diarrhea and nausea at the beginning of your keto experience.
Let me lay down two rules of happy ketosis dieting. If you don’t follow these rules, you probably won’t be happy and it might sabotage your keto experience.
RULE ONE! . Supplement with 1 TEASPOON of salt per day, whether you want to or not! (magnesium supplements can help also)
Practical considerations: Easiest to dump a teaspoon of salt into heavily buttered vegetables.
RULE TWO!. If your new to keto, reduce your carbs GRADUALLY to let your stomach and body adapt instead of being shocked into keto.