183
Chapter eight: Fasting
ghrelin and dropping levels of leptin may function to suppress the drive to forage while
optimizing rates of lipid oxidation (Crocker et al. 2012). The high levels of ghrelin also
run counter to the observed reductions in GH, as increased ghrelin is usually associated
with increased GH production in fasting animals. This suggests a loss of the ability of
ghrelin to stimulate GH secretion in fasted adult seals. Further, the observed decreases
in leptin were not associated with any significant changes in thyroid hormones. An
exception was that changes in total T3 were directly related to changes in daily energy
expenditure. Cortisol levels also did not change during the fast.
8.1.4.2 Cetaceans
Only a single study has examined multiple hormone changes in a fasting cetacean. This
study was conducted on two fasting adult bottlenose dolphins, a species that likely rarely
experiences prolonged fasts (Ortiz et al. 2010). The results confirmed the expected switch
to lipid metabolism, but the swiftness of the response was more typical of mammalian
species not adapted to regular fasting episodes. Plasma fatty acids doubled by 24 h and
increased 2.5-fold by 38 h of fasting. Conversely, BUN decreased 17% by 24 h of fasting and
22% by 38 h. Plasma glucose decreased 25% between 14 and 24 h and levels returning to
baseline by 38 h of fasting.
Neither plasma total T3 nor free T4 were changed. Mean total T4 increased 19% by 38 h
of fasting, while mean rT3 showed an initial 30% decrease by 24 h of fasting, but returned
to baseline levels by 38 h. The increase in total T4 might be due to decreased clearance
rates (versus increased production) while the eventual recovery of rT3 by 38 h might reflect
preferential deiodination of T4 to decrease cellular metabolism. While measured plasma
cortisol levels were undetectable, these results may not be typical given values reported in
other studies (Thomson and Geraci 1986; Ortiz and Worthy 2000).
8.2 Toolbox
8.2.1 Methods of measuring mass loss/body condition
Most pinniped studies use serial measures of body composition and body mass to calculate changes in the mass of specific tissues over time (usually differentiated into lipid and
fat-free or lean mass). While a variety of methods can be used to determine body composition (whole-body dissections, direct imaging, ultrasonic measurements of lipid depth, and
bioelectrical impedance), the most common method currently employed uses dilution of
a chemical marker to indirectly estimate body composition through measures of whole
body water content.
This method is based on the knowledge that different tissue types have different water content. The differential water content of tissues is the basis for how your
home scale takes body fat measurements. While the technique has been detailed elsewhere (Reilly and Fedak 1990; Iverson et al. 2010), briefly, a small dose of isotopically
distinct water (either deuterium oxide or tritiated water) is injected into the animal,
and allowed time (usually several hours) to equilibrate with the rest of the animal’s
body water. A blood sample is analyzed for the resulting concentration of the chemical marker in the serum which, combined with the known amount injected, yields
an estimate of the total body water. This value is then converted to estimates of lean
and lipid mass through published mathematical equations previously generated from
empirical studies (often involving carcass analysis). The accuracy of the technique is
dependent upon the applicability of the mathematical models converting body water
Chapter eight: Fasting
ghrelin and dropping levels of leptin may function to suppress the drive to forage while
optimizing rates of lipid oxidation (Crocker et al. 2012). The high levels of ghrelin also
run counter to the observed reductions in GH, as increased ghrelin is usually associated
with increased GH production in fasting animals. This suggests a loss of the ability of
ghrelin to stimulate GH secretion in fasted adult seals. Further, the observed decreases
in leptin were not associated with any significant changes in thyroid hormones. An
exception was that changes in total T3 were directly related to changes in daily energy
expenditure. Cortisol levels also did not change during the fast.
8.1.4.2 Cetaceans
Only a single study has examined multiple hormone changes in a fasting cetacean. This
study was conducted on two fasting adult bottlenose dolphins, a species that likely rarely
experiences prolonged fasts (Ortiz et al. 2010). The results confirmed the expected switch
to lipid metabolism, but the swiftness of the response was more typical of mammalian
species not adapted to regular fasting episodes. Plasma fatty acids doubled by 24 h and
increased 2.5-fold by 38 h of fasting. Conversely, BUN decreased 17% by 24 h of fasting and
22% by 38 h. Plasma glucose decreased 25% between 14 and 24 h and levels returning to
baseline by 38 h of fasting.
Neither plasma total T3 nor free T4 were changed. Mean total T4 increased 19% by 38 h
of fasting, while mean rT3 showed an initial 30% decrease by 24 h of fasting, but returned
to baseline levels by 38 h. The increase in total T4 might be due to decreased clearance
rates (versus increased production) while the eventual recovery of rT3 by 38 h might reflect
preferential deiodination of T4 to decrease cellular metabolism. While measured plasma
cortisol levels were undetectable, these results may not be typical given values reported in
other studies (Thomson and Geraci 1986; Ortiz and Worthy 2000).
8.2 Toolbox
8.2.1 Methods of measuring mass loss/body condition
Most pinniped studies use serial measures of body composition and body mass to calculate changes in the mass of specific tissues over time (usually differentiated into lipid and
fat-free or lean mass). While a variety of methods can be used to determine body composition (whole-body dissections, direct imaging, ultrasonic measurements of lipid depth, and
bioelectrical impedance), the most common method currently employed uses dilution of
a chemical marker to indirectly estimate body composition through measures of whole
body water content.
This method is based on the knowledge that different tissue types have different water content. The differential water content of tissues is the basis for how your
home scale takes body fat measurements. While the technique has been detailed elsewhere (Reilly and Fedak 1990; Iverson et al. 2010), briefly, a small dose of isotopically
distinct water (either deuterium oxide or tritiated water) is injected into the animal,
and allowed time (usually several hours) to equilibrate with the rest of the animal’s
body water. A blood sample is analyzed for the resulting concentration of the chemical marker in the serum which, combined with the known amount injected, yields
an estimate of the total body water. This value is then converted to estimates of lean
and lipid mass through published mathematical equations previously generated from
empirical studies (often involving carcass analysis). The accuracy of the technique is
dependent upon the applicability of the mathematical models converting body water