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Peer-reviewed scientific evidence:

Carbohydrate deficiency

Not getting enough carbs can cause problems. Without sufficient fuel, the body gets no energy. Additionally, without sufficient glucose, the central nervous system suffers, which may cause dizziness or mental and physical weakness, according to Iowa State University. A deficiency of glucose, or low blood sugar, is called hypoglycemia.

If the body has insufficient carbohydrate intake or stores, it will consume protein for fuel. This is problematic because the body needs protein to make muscles. Using protein for fuel instead of carbohydrates also puts stress on the kidneys, leading to the passage of painful byproducts in the urine, according to the University of Cincinnati.

People who don't consume enough carbohydrates may also suffer from insufficient fiber, which can cause digestive problems and constipation.

https://www.livescience.com/51976-carbohydrates.html

The energy value of digestible carbohydrates is generally accepted as 4 kcal/g for both sugars and starches. Fermentation of fiber in the gut will produce SCFAs that contribute calories, generally estimated to be ∼2 kcal/g. Few studies have linked carbohydrates to obesity. Indeed, observational data generally report that higher carbohydrate intake is linked to lower body weight.

Low-carbohydrate diets, especially those that avoid grains, are a popular notion for weight control. Yet most studies, including a recent systematic review and meta-analysis (7), found that low-carbohydrate vs. isoenergetic balanced diets had the same effect on weight loss, especially when looking at long-term effects on weight loss. There was little or no difference in weight loss and changes in cardiovascular risk factors up to 2 y of follow-up when overweight and obese adults, with or without type 2 diabetes, were randomly assigned to low-carbohydrate or isoenergetic balanced weight-loss diets.

Food patterns associated with health include consumption of dietary fiber, whole grains, vegetables, fruits, and dairy. The conclusions of recent research on carbohydrates and health from the 2010 Dietary Guidelines for Americans Advisory Committee report (8) are summarized below:

The primary role of carbohydrate is to provide energy to all cells in the body and dietary fiber. Adults should consume 45–65% of their total calories from carbohydrates, except for younger children who need a somewhat higher proportion of fat in their diets. A maximum intake of 25% of added sugars is suggested. Americans should choose fiber-rich foods such as whole grains, vegetables, fruits, and cooked dry beans and peas as staples in the diet. Dairy products are also a nutrient-dense source of carbohydrates. Carbohydrates are the primary energy source for active people. Sedentary people, including most Americans, should decrease consumption of caloric carbohydrates to balance energy needs and attain and maintain ideal weight.

https://academic.oup.com/advances/ar.../6/760/4616690

Recently, diets low in carbohydrate content have become a matter of international attention because of the WHO recommendations to reduce the overall consumption of sugars and rapidly digestible starches. One of the common metabolic changes assumed to take place when a person follows a low-carbohydrate diet is ketosis. Low-carbohydrate intakes result in a reduction of the circulating insulin level, which promotes high level of circulating fatty acids, used for oxidation and production of ketone bodies. It is assumed that when carbohydrate availability is reduced in short term to a significant amount, the body will be stimulated to maximize fat oxidation for energy needs. The currently available scientific literature shows that low-carbohydrate diets acutely induce a number of favourable effects, such as a rapid weight loss, decrease of fasting glucose and insulin levels, reduction of circulating triglyceride levels and improvement of blood pressure. On the other hand some less desirable immediate effects such as enhanced lean body mass loss, increased urinary calcium loss, increased plasma homocysteine levels, increased low-density lipoprotein-cholesterol have been reported.

https://www.ncbi..nih.gov/pubmed/16436102