In the healthy individual the many metabolic activities of the body are maintained in an optimum state by a multitude of regulating mechanisms. A disease is the morbid state resulting from a disturbance of healthy metabolism; in this sense any disease can be regarded as a metabolic disease. However, the term metabolic disease is conventionally restricted to those disorders that are ascribable to primary disturbances in metabolic processes. The number of these is considerable. They can be classified as nutritional deficiency diseases, inborn errors of metabolism, endocrine disturbances, liver and renal disease and miscellaneous metabolic diseases.
Because many metabolic processes are intimately inter-related, ill-health may be expressed as a complex of symptoms. An example is "metabolic syndrome" which is likely to comprise metabolic abnormalities involving triglycerides, high density lipoproteins, insulin resistance (type II diabetes), hypertension and obesity. Mounting evidence shows conditions such as heart disease, Alzheimer’s and cancer are metabolic diseases.