Essentials of Nutrition for Sports

Function Structure

Protein is a basic structural material of all cells. The structural

proteins of muscles make up the largest proportion of total-body proteins. Biologic Activity

Biologically active proteins include enzymes, immunoglobulins,

hormones, neurotransmitters, nutrient transport and storage compounds, and cell membrane receptors.

Plasma proteins (e.g., albumin) contribute to oncotic pressure

that directs the flow of fluid and metabolic waste from the intracellular compartment into the

capillary venules. These proteins

(e.g., hemoglobin) also contribute to plasma buffering capacity and oxygen-carbon dioxide transport (e.g., hemoglobin, myoglobin).

Acute phase reactant proteins (e.g., ferritin, prealbumin) secreted

by the liver bind minerals such as iron and zinc rendering them unavailable to support microbial proliferation.

Many small nitrogen-containing molecules needed for normal

body functions are synthesized from amino acids. Some of the individual amino acids are precursors of the purines and pyrimidines needed for the synthesis of nucleic

acids, the hereditary units that

carry information from one generation to the next.

Other amino acids are precursors of small biologically important

molecules such as heme, small hormones such as thyroxine and epinephrine, creatine, neurotra

nsmitters, skin pigments, and

nitrogenous constituents

of phospholipids.

Some amino acids serve as stimuli for the release of hormones

from endocrine organs and the gastrointestinal tract.

Several of these amino acids, partic

ularly leucine, glutamine, and

arginine, are used as therapeutic agen

ts in the treatment of patients in

catabolic states or with hepatic encephalopathy.

Energy Production—Oxidation

Proteins can break down to provide energy: four calories per

gram.

Organs and tissues differ greatly in their ability to use amino

acids as energy sources. The liver has the capacity to oxidize most amino acids and, if they are in surplus, will oxidize them in preference to other energy-yielding molecules.

Most of the essential amino acids are not oxidized in other

tissues. The branched-chain amino acids—leucine, isoleucine, and valine—like many of the dispensable amino acids, can be oxidized by most tissues and organs.

Glutamine and glutamic acid are preferred energy sources for the

intestine and lymphocytes.

Deficiency

In the US and other developed countries, it is generally easy to

get the minimum daily requirement of protein. Deficiencies usually result from disease or eccentric diets.

Cereal with milk for breakfast, a peanut butter and jelly

sandwich for lunch, and a piece of fish with a side of beans for dinner adds up to about 70 grams of protein,

plenty for the average

adult

. Around the world, millions of people do not get enough protein.
Protein malnutrition leads to the condition kwashiorkor.

Kwashiorkor can cause growth failure, loss of muscle mass,

decreased immunity, weakening of the heart and respiratory system, and death.

The most common cause of protein deficiency is insufficient

energy intake, which is exacerbated when demand for both protein and energy is high.

Protein energy malnutrition signs and symptoms include: weight

loss, diarrhea, loss of lean body mass, muscle weakness,

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