With low carbohydrate diets glucose becomes unavailable as a source of energy for our body, leading to the production of ketones from fatty acids in the liver. The increase in plasma ketones is known as nutritional ketosis. The available evidence from basic and clinical studies indicates that both low carbohydrate and high fat low carbohydrate diets are effective for weight loss and are better than non-intervention. However, low carbohydrate diet and ketogenic diets induce unique metabolic changes and consistently improve some markers of cardiovascular risk, lowering elevated blood glucose, insulin, triglycerides, ApoB and saturated fat concentrations, reducing small dense LDL particle numbers, glycated hemoglobin levels, blood pressure and body weight while increasing HDL-cholesterol concentrations and reversing non-alcoholic fatty liver disease. Low carbohydrate diets are an efficient strategy for the management of obesity and metabolic syndrome. They may also benefit patients with polycystic ovary syndrome. They must be prescribed by trained professionals to balance the risks and benefits for each individual patient. Future research is required to improve the knowledge about individual responses to dietary interventions, their safety, tolerance, efficacy and long-term effects.