Occasionally I like to read websites/blog posts/etc about why our bodies need carbohydrates to see if they have any good points.
They never do.
Here are some of the arguments and counterarguments:
Part of many proteins and fats (lipids) that our body needs for many bodily processes
The liver is capable of synthesizing up to 150g of glucose per day from fat/protein -- more than enough for our daily requirements.
Providing nutrients for the good bacteria in our intestines that helps us digest our food
Carbohydrates actually provide nutrients for the bad gut bacteria, the bad dental bacteria, as well as fuel for tumors and cancer cells.
Protecting our muscles because carbohydrates are the first source of energy for our body, without it, protein from our muscles will be used meaning that our body will effectively eat its own muscles!
The amount of protein you eat has to do with how much your muscles get broken down. In the absence of carbohydrates, your body will use dietary fat/protein/alcohol as energy and failing that, will break down body fat. Low-carb adequate-protein diets are actually muscle-sparing, while high-carb restricted calorie diets tend to be muscle-destroying.
Carbohydrates should be the body's main source of energy in a healthy balanced diet, providing about 4kcal (17kJ) per gram. They are broken down into glucose (sugar) before being absorbed into the bloodstream. From there, the glucose enters the body's cells with the help of insulin. Glucose is used by your body for energy, fuelling all of your activities, whether going for a run or simply breathing.
The body is perfectly capable of using ketones (from dietary fat), fatty acids (from body or dietary fat), or synthesizing its own glucose from dietary protein or triglycerides (body fat again) for energy.
Carbohydrate contains fewer calories gram for gram than fat, and starchy foods can be a good source of fibre, which means they can be a useful part of a weight loss plan. By replacing fatty, sugary foods and drinks with high-fibre starchy foods, it is more likely you will reduce the number of calories in your diet.
Carbohydrates (especially fructose) increase appetite; protein and fat lower appetite. Therefore you're more likely to reduce your calories by favoring fats/protein over carbohydrates.
Also, high fibre foods add bulk to your meal helping you feel full.
Being "full" isn't the same thing as being "sated" -- You only stay full as long as your stomach has expanded, whereas you can stay sated for hours and hours. Pursuing satiation over fullness also puts less strain on your digestive system -- meaning you don't feel bloated after meals.
Healthy sources of carbohydrates such as starchy foods, vegetables, fruits, legumes and lower fat dairy products are also an important source of nutrients such as calcium, iron and B vitamins.
Non-starchy Vegetables have considerable amounts of nutrients without many carbs; additionally meat has higher absorbable iron than vegetable sources and fish has much much higher b vitamin content than vegetable sources. High-fat dairy products have the same exact nutritional profile as their low-fat counterparts.
Vitamin B12 isn't even in the list above, while copious in animal sources.
Some starchy foods (such as wheat, rice, other grains) actually have antinutrients that inhibit nutrient absorption.
Ketosis as a result of a low carbohydrate diet can be linked, at least in the short term, to headaches, weakness, nausea, dehydration, dizziness and irritability.
This is actually caused by mass electrolyte dumping as water weight is flushed. Over time these symptoms disappear when one enters ketosis, although they can also be fixed in the short-term by electrolyte intake.
In the absence of carbohydrates in the diet your body will convert protein (or other non-carbohydrate substances) into glucose, so it's not just carbohydrates that can raise your blood sugar and insulin levels.
Which is why you don't eat excessive protein, you eat adequate protein. Dietary fat doesn't get converted into glucose.
Grains, especially wholegrains, are an important part of a healthy balanced diet.
If this was true, we would have died out before the agricultural revolution.
If your body doesn't get enough glucose, you can experience low blood sugar, or hypoglycemia. This may make you feel physically and mentally fatigued, shaky, dizzy or lightheaded, and it can decrease your performance during exercise or everyday tasks.
These are literally just withdrawal symptoms of glucose addiction.