Of the eighty-two chemical elements, thirteen enter uniformly into the composition of the body and ten more are occasionally found. Of all these, several exist in very small proportion and their uses are unknown, several are found more abundantly but are not indispensable to life, and certain elements - namely, carbon, hydrogen, oxygen, and nitrogen - are necessary ingredients of the tissues of the body. These elements form compounds which, as they occur in the structure of the various tissues, have the following characteristics:

First, although the elements are but few in number, their molecular arrangement is very complex.

Second, their compounds are comparatively unstable and are readily converted in the body or by chemical analysis into other forms.

All food is composed of combinations of these simpler chemical elements which, for the most part, must be subjected to alteration in the body itself to prepare it for assimilation by the tissues. The nutrition of the body, therefore, involves four distinct processes, viz.:

1. The secretion of digestive fluids and their action upon food in the alimentary canal.

2. The absorption of the ingredients of the food when digested into the blood vessels and lymphatic vessels.

3. The assimilation of the absorbed nutritious products by the tissues.

4. The elimination of the waste material.

The following analysis exhibits the relative proportion of the elements of which the human body is composed:

Approximate Chemical Analysis of a Man (Moss)

(Height, 5 feet 8 inches; weight, 148 pounds).

Oxygen

92.4 pounds.

Hydrogen

14.6 "

Carbon

31.6 "

Nitrogen

4.6 "

Phosphorus

1.4 "

Calcium

2.8 "

Sulphur

0.24 "

Chlorine

0.12 "

Sodium

0.12 "

Iron

0.02 "

Potassium

0.34 "

Magnesium

0.04 "

Silica

?

Fluorine

0.02 "

Total

48.00 pounds.

All these elements are necessarily derived from food and water plus the oxygen of the air which is breathed.

The three predominating elements - oxygen, hydrogen, and carbon - are the great force producers of the body, although they are tissue formers as well, and to them must be added nitrogen, as serving in this double capacity, although its relation to tissue formation and renewal is greater than its capacity for supplying energy.

The common elements which enter into tissue formation chiefly and which bear no direct relation to the main sources of the force production in the body are chlorine, sulphur, phosphorus, iron, sodium, potassium, calcium, and magnesium in different combinations. Bone tissue, for example, contains about 50 per cent of lime phosphate. If this substance is deficient in the food of the young growing infant, the bones are poorly developed and so soft that they yield to the strain of the weight of the body and become bent and out of shape. This constitutes one of the principal symptoms of rickets.

Lack of iron salts in the food impoverishes the colouring matter of the red blood-corpuscles on which they depend for their power of carrying oxygen to the tissues, and anaemia and other disorders of deficient oxidation result.

The lack of sufficient potash salts, especially potassium carbonate and chloride, is a factor in producing scurvy, and the condition is intensified by the use of common salt. A diet of salt meat and farinaceous foods with absence of potatoes and fresh fruit and vegetables may cause it.

The lack of sodium chloride interferes with many of the functions of the body immediately concerned with nutrition, such as absorption (osmosis), secretion, etc., and alters the density and reactions of the different body fluids.

These few illustrations suggest the diversity of roles exhibited by the elements and the need for a correctly balanced diet.

In order to determine what such a diet should consist of it is necessary to study the value of the principal classes of foods in force production and in nutrient power or tissue building, but before proceeding further with this discussion it will be advisable to adopt a simple comprehensive classification of the foods in general use by man.

The following table of analyses made by Dujardin-Beaumetz is quoted by Yeo to show the proportion of nitrogen present in different foods, and also the combustible carbon and hydrogen.

"The hydrogen existing in the compound in excess of what is required to form water with the oxygen present is calculated as carbon. It is only necessary to multiply the nitrogen by 6.5 to obtain the amount of dry proteids in 100 grammes of the fresh food substance: "

Nitrogen.

C + H.

Combustibles calculated as carbon.

Beef (uncooked)

3.00

11.00

Roast beef

3.53

17.76

Calf's liver

3.09

15.68

Foie-gras

2.12

65.58

Sheep's kidneys

2.66

12.13

Skate

3.83

12.25

Cod, salted

5.02

16.00

Herring, salted

3.11

23.00

Herring, fresh

1.83

21.00

Shiting

2.41

9.00

Mackerel

3.74

19.26

Sole

1.91

12.25

Salmon

2.09

16.00

Carp

3.49

12.10

Oysters

2.13

7.18

Lobster (uncooked)

2.93

10.96

eggs

I.90

13.50

Milk (cow's)

0.66

8.00

Cheese (Brie)

2.93

35.00

Cheese (Gruyere)

500

38.00

Cheese (Roquefort)

4.21

44.44

Chocolate

1.52

58.00

Wheat (hard sourthern, variable average)

3.00

41.00

Wheat (soft southern, variable average)

1.81

39.00

Flour, white (Paris)

1.64

38.50

Rye flour

1.75

41.00

Winter barley

1.90

40.00

Maize

1.70

4400

Buckwheat

2.20

42.50

Rice

1.80

41.00

Oatmeal

1.95

44.00

Bread, white (Paris, thirty per cent water)

1.08

29.50

Breat, brown (soldiers' rations formely)

1.07

28.00

Bread, brown (solders' rations at present)

1.20

30.00

Bread from flour of hard wheat

2.20

31.00

Potatoes

0.33

11.00

Beans

4.50

42.00

Nitrogen.

C + H. Combustibles calculated as carbon.

Haricots (dry)

3.92

43.00

Lentils (dry)

3-87

43.OO

Peas (dry)

3.66

44.OO

Carrots

0.31

5.50

Mushrooms

0.60

4.52

Figs (fresh)

0.41

15.50

Figs (dry)

0.92

34.00

Plums

0.75

28.00

Coffee (infusion of 100 grammes)

1.10

9.00

Tea (infusion of 100 grammes)

1.00

10.50

Bacon

1.29

71.14

Butter (fresh)

0.64

83.00

Olive oil

Trace

98.00

Beer, strong

0.05

4.50

Wine

0.15

4.00

To estimate the equivalent chemical elements in the different classes the following standards are adopted:

1. To obtain the amount of nitrogen in proteid foods, divide the quantity of food by 6.50.

2. To obtain the carbon in fat, multiply the quantity of fat by 0.765.

3. To obtain the carbon in carbohydrate food, multiply by 0.444.

4. To obtain the carbon in proteid food, multiply by 0.535.

Estimates vary somewhat as to the average quantity of the elements carbon and nitrogen consumed per diem. In a general way it may be said that the consumption of carbon is 320 grammes, and that of nitrogen about 20 grammes. (See Quantity of Food, page 287).