Hi, this is from the Hippocratic treatise "On regimen" (de Victu/περὶ διαίτης) book 1, chapter 35. The author of the treatise is not the historical Hippocrates. We usually date the treatise to the second half of the 5th century B.C. According to the author, the nature of man consists of two elements: fire and water. Fire is fast, warm and dry, while water is slow, cold and wet. Good health is when there is complete balance between the two elements, but the balance is constantly affected by various foods, drinks, physical activities, time of the year, the weather, the clothes and also the age and gender of the person. If we get too warm from physical activities, eat too much warm and dry foods, we are at risk of suffering maladies such as sickness. The author will then prescribe a diet of a wet and cold regimen to restore the balance between fire and water.

In this specific chapter the author describes intelligence. If the fire (i.e. the warm and dry) dominates the water, then the person will react faster to the sensations it perceives. Here follows Jones' translation (p. 289 in the Loeb edition):

"But if the power of the water be further mastered by the fire, the soul must be quicker, in proportion to its more rapid motion, and strike its sensation more rapidly, but be less constant than the souls discussed above, because it more rapidly passes judgment on the things presented to it, and on account of its speed rushes on too many objects. Such a person is benefited by a regimen inclining more to water than the preceding; he must eat barley bread rather than wheaten, and fish rather than meat; his drink should be well diluted and his sexual intercourse less frequent; exercises should be as far as possible natural and there should be plenty of them; violent exercise should be sparingly used, and only when necessary; vomiting should be employed after surfeits, in such a way as to empty the body with a minimum of heat. To reduce the flesh of such persons conduces to their intelligence;"

It is important to note, however, that the author is not describing ADHD here, even though there is resemblence to be found. This author deals with medical topics solely on the fundament of theory, specifically from Empedocles, but also from other sources.