Barley for Food and Health: Science, Technology, and Products

GLYCEMIC RESPONSE TO CARBOHYDRATE CONSUMPTION 185
contend that the GI is merely an approximate prediction of glycemic response.
Other physiological markers have recently been utilized as alternative methods
to indicate and measure glycemic responses to foods. The term incretin
refers to the hormones glucagon-like polypeptide (GLP-1) and glucose-dependent
insulinotropic polypeptide (GIP), which are now measured in glycemic studies.
Incretin hormones are involved in the interaction between glucose ingestion and
insulin response (Nauck et al. 2004; Gentilcore et al. 2006; Milton et al. 2007). It
is believed that dietary interventions with the potential of controlling postprandial
glycemic responses are more definitive if the level of incretin hormone secretion
is determined. Breath hydrogen (H 2 ) is frequently measured postprandially to
assess colonic fermentation of indigestible carbohydrates such as soluble dietary
fiber. The degree of fermentation is assessed by the quantity of H 2 expired over a
specified time period (Thorburn et al. 1993). An increasing amount of breath H 2
is indicative of increased fermentation in the large intestine, also resulting in the
formation of the short-chain fatty acids (SCFAs): acetic, propionic, and butyric.
Plasma free fatty acids (FFAs) are produced by lipolysis of triglyceride stores
and are involved in the dynamic energy balance in body cells. Insulin and other
hormones have a direct influence on serum FFA concentration, and high FFA levels
tend to decrease insulin effectiveness in moving glucose into body cells. Foods
with a high GI tend to result in elevation of FFAs, creating a cycle leading to
insulin resistance. Hence, a lowering of FFA levels following a dietary intervention
for glycemic effect would be seen as a positive benefit (Ferannini et al. 1983;
Wolever et al. 1995). FFA levels are also believed to be responsive to the presence
of SCFAs produced by colonic fermentation of indigestible carbohydrates.
Adiponectin, a hormone that is secreted from adipose tissue, modulates glucose
regulation and fatty acid catabolism. Because it has been associated with protection
from metabolic conditions related to diabetes, such as obesity, adiponectin
has recently been measured in glycemic studies (Kim et al. 2007). Insulin resistance
in susceptible individuals (impaired glucose tolerance, obesity, or hyperlipidemia)
can be detected or predicted by various indices, such as the homeostasis
model assessment (HOMA) and other methods (Behall et al. 2005). Insulin resistance
has been detected and measured using the euglycemic–insulinemic clamp
in experimental animals, in which decreased insulin uptake by peripheral tissues
is recorded (Yokayama and Shao 2006). Finally, interleukin 6 (IL-6), a
pro-inflammatory cytokine, is a metabolic marker of immune response to inflammation.
Elevated levels of IL-6 are associated with hepatic glucose production,
and thus have implications for glycemic evaluations. Increased levels of cytokines
inhibit translocation of glucose transporters to the cell surface in response to
insulin (Kishimoto 1989). Future research will undoubtedly clarify the roles of
these mechanisms in terms of carbohydrate metabolism for both healthy individuals
and those with metabolic syndrome or diabetes.
Effect of Barley on Carbohydrate Metabolism
Investigation of the effect of barley on carbohydrate metabolism has intensified
since Sato et al. (1990) reported that plasma glucose concentration in patients