It should be mentioned briefly since it is occasionally a nutritional problem. Fat is used for energy continuously to some extent, but when there is a shortage of carbohydrate, fat is needed for this purpose in increased amounts. Fat is degraded by rapid fragmentation of long chain fatty acids to form 2-carbon and occasionally 4-carbon units. Two-carbon units may enter the tricarboxylic acid cycle, may be synthesized to fatty acids or cholesterol, or may condense to form aceto-acetic acid. Aceto-acetic acid may be used in the tissues but is not broken down in the liver. The formation of this compound is greatly increased in diabetes mellitus and in starvation. When the concentration of aceto-acetic acid in the blood reaches 20 mg. per 100 ml. ketonuria results. Some of the aceto-acetic acid is converted to B-hydroxy butyric acid and acetone. Accumulation of these substances results in acidosis. Diagnosis of ketosis is relatively easy as simple tests are available for measurement of ketone bodies in urine and in blood.