The close
chemical relation between the different carbohydrates makes such a
conversion easily possible.
*Fats.*--The fats used as foods belong to one or the other of two classes,
known as solid fats and oils. The solid fats are derived chiefly from
animals, and the oils are obtained mostly from plants. Butter, the fat of
meats, olive oil, and the oil of nuts are the fats of greatest importance
as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen,
and oxygen. They are rather complex chemical compounds, though not so
complex as proteids. Since neither fats nor carbohydrates contain
nitrogen, they are frequently classed together as _non-nitrogenous_ foods.
*Purpose Served by Carbohydrates, Fats, and Albuminoids.*--These classes of
nutrients all serve the common purpose of supplying energy. By uniting
with oxygen at the cells, they supply heat and the other forms of bodily
force. This is perhaps their only purpose.(50) Proteids also serve this
purpose, but they are not so well adapted to supplying energy as are the
carbohydrates and the fats. In the first place they do not completely
oxidize and therefore do not supply so much energy; and, in the second
place, they form waste products that are removed with difficulty from the
body.
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