Page 430 - Small Animal Clinical Nutrition 5th Edition

444 Small Animal Clinical Nutrition

insulin responsive and thus declines in the face of decreased
VetBooks.ir insulin (Gavin and Moeller, 1983). T levels begin to decrease
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within 24 hours of fasting and may be 40 to 50% lower as com-
pared to values in fed animals by Day 3 of food deprivation
(Vagenakis et al, 1975). Thus the net effect is a hypometabolic
state, which becomes a beneficial adaptation allowing conserva-
tion of body functions until appropriate caloric intake resumes.
By Day 5 of food deprivation in all mammals, endocrine
changes have mandated a metabolic shift from exogenous fuel
usage in the fed state to endogenous fuel usage in the forms of
fatty acids and ketone bodies. Decreased insulin levels trigger
lipolysis in an effort to conserve protein stores and maintain
blood glucose levels for glucose-dependent tissues. By undergo-
ing fatty acid oxidation, amino acids are partially spared, which
helps maintain muscle protein stores throughout starvation until
the end stages. Body proteins are only partially spared because
muscles will catabolize branched chain amino acids. Nitrogen is
exported with pyruvate as alanine and to a lesser extent gluta-
mine, which may be deaminated and transaminated for hepatic
protein synthesis. Additionally, their ketoanalogs are used for
hepatic and renal glucose synthesis (Felig et al, 1969).
The respiratory quotient (RQ) can provide an indication of
which substrate(s) are undergoing catabolism. An RQ of 0.7
indicates fat catabolism, 0.8 indicates protein catabolism and
1.0 indicates that carbohydrates are being used as the primary
fuel. Studies involving well-fed dogs measured an RQ of 0.94
at rest, indicating high carbohydrate use. The RQ of the same
dogs after five to 15 days of starvation was 0.8 indicating a shift
Figure 25-3. Graph of glucose production and source by the liver
and kidneys. (Adapted from Owen OE, Felig P, Morgan AP, et al. from primarily carbohydrate use to that of lipid and/or protein
Liver and kidney metabolism during starvation. Journal of Clinical catabolism (Himwich and Rose, 1927). This is a testament to
Investigation 1969; 48: 574-583.)
the importance of fat as a primary fuel source after three to five
days of food deprivation.
mammals, serum ketones can reach 2 to 3 mM within a few Refeeding management of the food-deprived patient is
days of starvation, with levels increasing to between 7 and 8 aimed at matching the number of “no food” days with the pat-
mM after one week. This is greater than the normal glucose tern of fuel use (i.e., adaptation from using a mixture of fuels to
concentration of 5 mM (Engelking and Anwer, 1992). primarily using fat). This is often in the patient’s dietary histo-
Increased serum ketones contribute to the adaptive response ry. As described, the different proportions of stored carbohy-
of the patient towards conservation of endogenous protein and drate, fat and protein that is/are used to maintain blood glucose
glucose sparing for the non-adaptive, glucose-dependent tis- and provide an energy source mainly depends on the duration
sues (erythrocytes and renal medullary cells). Enzymatic of food deprivation. To minimize the metabolic complications
changes in peripheral tissues and brain caused by increased of refeeding, the refeeding formula should contain a complete
serum ketone concentrations promote ketone use and decrease balance of nutrients and should have a carbohydrate, fat and
the demand for glucose. Ketosis in food deprivation is an protein source similar to that which the liver has become adapt-
appropriate physiologic response and may not lead to severe ed or is estimated to be using from body stores. Oftentimes this
ketoacidosis except in diabetic dogs and cats.Thus,ketone bod- requires a feeding formula that is lower in carbohydrate and
ies serve as a readily diffusible lipid-source fuel for muscles, kid- higher in fat and protein sources as compared to a mainte-
ney cortex, peripheral nerves and the brain during periods of nance-type food. As described, patients deprived of food are in
starvation. Ketone body production is usually maintained until a state of catabolism, which can often be reversed by refeeding
adipose tissue is depleted. if body protein losses have not exceeded 25 to 30%.
In addition to adaptation from mixed fuels to fatty acids as a
primary fuel source, as food deprivation continues metabolism Disease State
begins to decrease. By the third day of food deprivation in all In contrast to patients undergoing simple starvation, patients in
mammals, the basal metabolic rate decreases to promote con- disease states are often inappetent or anorectic due to their dis-
servation of resources for long-term survival. Decreases in ease process. Although similar in their decreased dependence
blood glucose levels result in a decrease in serum insulin release. on glucose for fuel and increased propensity for lipolysis,
The conversion of thyroxine (T ) to triiodothyronine (T ) is patients in disease states have increased energy requirements
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