185
Chapter eight: Fasting
Determining changes in resting metabolism over the course of a fast is complicated
by the fact that the animal is simultaneously losing body mass, which will inherently
decrease total energy expenditures. In addition, body mass loss during different phases
of the fast may be due to loss of tissues that are metabolically active (protein) or relatively metabolically inert (lipid stores). For metabolic depression to occur, the decrease in
metabolic rate during the fast must not be a mere consequence of body mass loss. Some
authors have suggested that true metabolic depression can be said to occur if there is a
clear decrease in this rate when it is expressed per unit body mass (mass-specific metabolism) (Cherel et al. 1988). While some might argue that this incorrectly assumes a specific
relationship between body mass and metabolic rate, it is certainly a conservative way of
calculating changes in metabolism.
8.2.3 Plasma metabolites as biomarkers of fasting phase
We can analyze the metabolites present in a blood sample for clues about the fasting
state of an animal. Metabolites are the end-products and the intermediaries of all chemical reactions in the body. They can, therefore, provide evidence about the rate and
amount of specific substrate types that are broken down across time during fasting.
Examples of these metabolites are mentioned earlier in this chapter and include glucose, FFA, and BUN.
Metabolites can be analyzed in two ways to study fasting. Specific known biomarkers associated with metabolic processes can be analyzed in a plasma or serum sample. Glucose and BUN, for example, are routinely measured in human and veterinary
clinical applications and are available on standard blood chemistry panels. FFA vary
by chain length and number of double bonds, meaning that researchers must decide
whether to simply measure all circulating lipid, or to measure the specific composition
of the lipid pool. The latter requires more sophisticated analytical methods, including
gas chromatography. Metabolites can also be analyzed by large-scale screens of the
metabolome (the entire metabolite population), taking advantage of rapidly advancing
technology and analytical methods to find patterns and signals in large data sets. The
benefit of evaluating the entire metabolome is that more data are retrieved from the
single sample, which allow further investigation into the details of substrate turnover
and the interaction between fasting and other physiological processes (e.g., lactation,
stress, molt).
Metabolites are small molecular weight chemicals that are identified by metrics such
as their mass, charge, pH, and hydrophobicity. Metabolomics platforms at research facilities and in industry combine liquid or gas chromatography to separate metabolites by
their chemical characteristics from a biosample, and then the mass of separated metabolites is determined by mass spectrometry. Knowing the specifics of mass along with
charge, etc., identifies metabolites from catalogs of hundreds of known (and synthetic)
chemicals; these platforms are also able to quantify each metabolite within a biosample.
Using metabolites as the target of large-scale screening is especially useful for studying
marine mammals because they can be consistently identified by their composition and
does not rely on comprehensive genomic information (needed to identify proteins and
transcripts based on their sequences), which is not yet available for many marine mammal species.
A limitation to using metabolite signatures to study fasting is that they require tissue
or blood sample collection. Samples must also be quickly processed (e.g., to separate the
Chapter eight: Fasting
Determining changes in resting metabolism over the course of a fast is complicated
by the fact that the animal is simultaneously losing body mass, which will inherently
decrease total energy expenditures. In addition, body mass loss during different phases
of the fast may be due to loss of tissues that are metabolically active (protein) or relatively metabolically inert (lipid stores). For metabolic depression to occur, the decrease in
metabolic rate during the fast must not be a mere consequence of body mass loss. Some
authors have suggested that true metabolic depression can be said to occur if there is a
clear decrease in this rate when it is expressed per unit body mass (mass-specific metabolism) (Cherel et al. 1988). While some might argue that this incorrectly assumes a specific
relationship between body mass and metabolic rate, it is certainly a conservative way of
calculating changes in metabolism.
8.2.3 Plasma metabolites as biomarkers of fasting phase
We can analyze the metabolites present in a blood sample for clues about the fasting
state of an animal. Metabolites are the end-products and the intermediaries of all chemical reactions in the body. They can, therefore, provide evidence about the rate and
amount of specific substrate types that are broken down across time during fasting.
Examples of these metabolites are mentioned earlier in this chapter and include glucose, FFA, and BUN.
Metabolites can be analyzed in two ways to study fasting. Specific known biomarkers associated with metabolic processes can be analyzed in a plasma or serum sample. Glucose and BUN, for example, are routinely measured in human and veterinary
clinical applications and are available on standard blood chemistry panels. FFA vary
by chain length and number of double bonds, meaning that researchers must decide
whether to simply measure all circulating lipid, or to measure the specific composition
of the lipid pool. The latter requires more sophisticated analytical methods, including
gas chromatography. Metabolites can also be analyzed by large-scale screens of the
metabolome (the entire metabolite population), taking advantage of rapidly advancing
technology and analytical methods to find patterns and signals in large data sets. The
benefit of evaluating the entire metabolome is that more data are retrieved from the
single sample, which allow further investigation into the details of substrate turnover
and the interaction between fasting and other physiological processes (e.g., lactation,
stress, molt).
Metabolites are small molecular weight chemicals that are identified by metrics such
as their mass, charge, pH, and hydrophobicity. Metabolomics platforms at research facilities and in industry combine liquid or gas chromatography to separate metabolites by
their chemical characteristics from a biosample, and then the mass of separated metabolites is determined by mass spectrometry. Knowing the specifics of mass along with
charge, etc., identifies metabolites from catalogs of hundreds of known (and synthetic)
chemicals; these platforms are also able to quantify each metabolite within a biosample.
Using metabolites as the target of large-scale screening is especially useful for studying
marine mammals because they can be consistently identified by their composition and
does not rely on comprehensive genomic information (needed to identify proteins and
transcripts based on their sequences), which is not yet available for many marine mammal species.
A limitation to using metabolite signatures to study fasting is that they require tissue
or blood sample collection. Samples must also be quickly processed (e.g., to separate the