HUMAN NUTRITION - World Food: Production and Use - page 48

In spite of this latter reaction, energy content of food is universally measured in calories,
which is a measure of heat.
The main source of carbohydrates (for production of energy or calories) come from
starchy foods. These are primarily from grains such as maize, rice, wheat, and root crops,
such as potatoes and cassava. See Figure 2.6 for examples of starch-rich crops, although
other small grains and root crops, barley, oats, millet, sweet potato, and others are also
important in certain localities. Of these sources grains are generally higher in other
dietary essentials, such as protein, than are root crops. Usually carbohydrate-rich
foods are consumed in larger amounts, and thus considered staple foods, than other com-
ponents of the diet. Generally speaking, carbohydrate-rich foods are less expensive than
protein-rich foods and this also contributes to their being important foods.
Figure 2.7 shows the kilocalorie content of four major food carbohydrate sources.
Note that although potato and cassava are lower in carbohydrate on a 100-g wet-weight
basis, that is, they contain considerably more water than the grain; thus on a dry-weight
basis all three have similar amounts of carbohydrates. Note also that grains will usually
not be eaten dry and so comparing these sources of carbohydrates on a raw (wet) weight
basis can be deceiving.
When comparing the energy content of foods, it is also important to also keep in
mind the other components contained in that food source, that is, protein, fat, fiber,
vitamins, and minerals. Some carbohydrate sources provide substantially more of these
essentials than do others. Fat in the diet is another source of energy and thus can add
Figure 2.7. Kilocalorie content of rice, corn, potato, and cassava on both a wet (raw) weight
and dry bases per 100 g of each. Water content is in terms of grams and uses the same scale
as kilocalorie. (Data from Agricultural Research Service Nutrient Data Laboratory USDA
National Nutrient Database for Standard Reference, Release 17, http: // www.nal.usda.gov /
fnic / foodcomp / Data / SR17 / reports / sr17page.htm.)