72 Introduction to Human Nutrition
Thus, superimposed infection, altered gastro-
intestinal function, and metabolic changes that often
accompany chronic disease states would all be
expected to reduce the effi ciency of dietary nitrogen
and amino acid utilization. The metabolic response
to acute infection in healthy young men has been
characterized in experiments involving different types
of intracellular infection, and involves an increased
loss of body nitrogen, together with increased losses
of several other nutrients including potassium, mag-
nesium, phosphorus, and vitamin C. This increased
loss clearly implies increased needs for nitrogen,
amino acids, and other nutrients.
In addition to the catabolic response of body nitro-
gen metabolism to infection and trauma, there is a
corresponding anabolic component that is of major
importance during recovery from these stressful con-
ditions. Anabolic responses occur not only during
recovery but also in the early phase of illness, when
anabolism is associated with increased production of
immunocompetent cells such as phagocytes and other
leukocytes, and the induction of several tissue enzymes
and immunoglobulins.
During recovery from infection two characteristics
of the anabolic period that follows are that the
increased nitrogen retention seen during this period
is greater than that measured during the preincuba-
tion phase, and its duration is much longer than
the catabolic period. This may be due, in part, to
the effect of protein depletion antedating an acute
episode, which may be the case in poor communities.
However, in spite of the potential for disease states to
increase protein and amino acid needs there are too
few studies that help to assess precisely their quantita-
tive infl uence on nutrient utilization and dietary
requirements.4.9 Perspectives on the future
The purpose of this chapter was to provide a general
overview of human protein and amino acid metabo-
lism and a basis for an improved appreciation of the
metabolic determinants of the requirements for
protein (nitrogen) and for specifi c amino acids. With
the recent beginning of the postgenome era, func-
tional genomics, proteomics, and metabolomics will
take on an increasingly important basic and applied
research focus in biology. Thus, it will be even more
critical for students to understand the physiology of
human protein metabolism at its various levels of
biological complexity (cell, organ, and whole body)
and its nutritional corollaries.
There are certain areas of research in protein nutri-
tion where more knowledge will equip nutritionists
to make the best use of available food supplies. An
example is the infl uence of the ratio of total essential
or indispensable amino acids to total nitrogen and
amino acid requirements for different physiological
states. Another is the need for a functional defi nition
of protein requirements (e.g., indices) for maximum
resistance to disease and enhanced physical perfor-
mance. These are some of the challenges facing nutri-
tionists in the future. It is hoped that this chapter will
serve as an appropriate catalyst for further learning in
this area of human nutrition.Acknowledgment
This chapter has been revised and updated by Naomi
K Fukagawa and Yong-Ming Yu based on the original
chapter by Vernon R Young and Peter J Reeds. It is
dedicated to their memory. For more information on
this topic visit http://www.nutritiontexts.comTable 4.17 Agent, host, and environment factors that affect protein
and amino acid requirements and the nutritional status of the
individual
Agent (dietary) factors
Chemical form of nutrition (protein and amino acid source)
Energy intake
Food processing and preparation (may increase or decrease
dietary needs)
Effect of other dietary constituents
Host factors
Age
Sex
Genetic makeup
Pathologic states
Drugs
Infection
Physical trauma
Chronic disease, cancer
Environmental factors
Physical (unsuitable housing, inadequate heating)
Biologic (poor sanitary conditions)
Socioeconomic (poverty, dietary habits and food choices, physical
activity)