662 BOTANY OF THE LIVING PLANT 



FOOD VALUES. VITAMINS. 



The relative values of the various vegetable food-stuffs of which 

 analyses have been given above, may be assessed in the first instance 

 by considering their " fuel values ", i.e. the number of " Calories " 

 which each produces when completely oxidised (one [large] Calorie is 

 the amount of heat required to raise the temperature of one kilogram 

 of water by one degree C. — or that of one lb. of water by 4 degrees F.). 

 It has been calculated by physiologists that an average healthy man, 

 weighing 70 kg. (11 stone) and doing a moderate amount of physical 

 work, requires each day food with a total fuel value of 3300 Calories. 

 Assuming each gram of protein or carbohydrate to provide 4 Calories 

 and each gram of fat 9 Calories, the normal metabolic requirements of 

 the average man will be met by a diet comprising 100 grams of protein, 

 100 grams of fat and 500 grams of carbohydrate per day. 



The fuel value of any substance is however only one aspect of its 

 suitability for human food. As regards proteins, in particular, quality 

 is quite as important as quantity. Digestibility and palatability of the 

 food must likewise be taken into account. Among the drawbacks of a 

 purely vegetarian diet is the monotonous and unpalatable character of 

 many vegetable staple foods. Rice, for example, quite apart from its 

 very low fat and protein content, makes an insipid dish and hence an 

 unsatisfactory staple diet unless seasoned by the addition of highly 

 flavoured condiments such as curry or soy. The widespread instinctive 

 preference for a mixed diet is moreover biologically justified, not only 

 by the need for a proper balance between protein, fat and carbohydrate, 

 but also because adherence to such a diet is the natural way of ensuring 

 a sufficient and regular supply of the essential mineral salts and of the 

 no less indispensable vitamins. 



Vitamins are " accessory food factors ". Though they are not them- 

 selves food substances in the ordinary sense, a certain amount of each 

 vitamin must be supplied to the human body if full health is to be main- 

 tained. That amount is very small ; perhaps not more than one 

 millionth part of the daily food ration ; but, on the other hand, a 

 minimum supply of every vitamin is absolutely necessary, since lack of 

 any one results in a characteristic " deficiency disease ". The best 

 known of these ailments are scurvy and rickets. 



Scurvy has been known for centuries, especially among seafaring 

 peoples. Its connection with a diet lacking fresh vegetables and fruit 

 was first clearly demonstrated by Lind in 1757. Captain Cook was able 

 to keep his crews free of the disease by making them eat fresh food as 

 often as possible. It is now known that the active principle in fresh 

 foods which prevents the development of scurvy is the " anti-scorbutic 

 vitamin " or " Vitamin C ". 



Rickets is ordinarily caused by lack of sunlight aggravated by a diet 

 poor in natural fats. The specific " anti-rachitic " agent is " Vitamin 

 D ", one of the richest sources of which is cod-liver oil (and similar fish 

 oils). Most other natural foods contain comparatively little of this 



