Do you need carbohydrates to stay healthy?
Your body does not need a minimum amount of carbohydrates, but which carbohydrates you eat does matter, especially if you have diabetes or want to prevent it.
Carbohydrates are the only macronutrient for which your body has no absolute minimum requirement to survive. Your liver can produce glucose from other nutrients (proteins and fats) for the organs that need it. No studies show that cutting out carbohydrates makes you ill, as long as the rest of your diet is varied and nutritious.
That does not mean it makes no difference which carbohydrates you eat. Carbohydrates that raise blood sugar gradually, such as wholegrains and legumes, improve blood sugar control in people with diabetes. They also lower bad cholesterol and fats in the blood, and slightly reduce blood pressure. This effect has been shown repeatedly. How you choose your carbohydrates is therefore more relevant than whether you eat them at all.
Eating few or no carbohydrates can be useful for some people. Among people with type 2 diabetes, 57% achieved remission after six months on a low-carbohydrate diet, compared with 31% in the control group. Weight and blood fats also improved noticeably. However, after twelve months most of this benefit had faded, and there were signs of a rise in bad cholesterol. These effects are not yet well understood. If you have diabetes and want to try this approach, discuss it with your doctor.
In children with epilepsy who do not respond to medication, a ketogenic diet (almost no carbohydrates, a large amount of fat) works remarkably well: up to 85% saw their seizures reduced by more than half. Side effects such as vomiting and constipation occur regularly. In studies, children always followed this diet under medical supervision, so if you are considering it, discuss it with your doctor as well.
For healthy people without diabetes, excess weight, or epilepsy, the available studies contain no clear evidence that cutting out or sharply restricting carbohydrates provides an additional health benefit. Existing dietary guidelines assume that 45 to 65% of your energy comes from carbohydrates, but that figure is a practical guideline, not a hard physiological minimum.
This answer draws on several large summaries of existing research, including several on low-carbohydrate diets in diabetes and epilepsy. The researchers themselves rate the quality of the evidence as moderate to low for diabetes remission, and as low to very low for the epilepsy studies. The four available epilepsy studies together cover 385 children, which is a small number; the researchers describe the evidence for almost all outcomes as low or very low. The evidence is strongest for the effect of low-glycaemic-index carbohydrates on blood sugar control. For healthy people without any condition, direct research into whether carbohydrates are strictly necessary is lacking.