lower sweet taste thresholds in the morning associated with lower plasma con-
centrations of leptin. Changes in leptin concentrations do not affect thresholds for
salty, bitter, sour, or umami stimuli (Nakamura et al. 2008 ).
Leptin acts on regions in the hypothalamus, hindbrain, and vagus nerve (Power
and Schulkin 2009 ). Low serum leptin levels, a biomarker for negative energy
balance and low energy stores, have an association with reported hunger and ini-
tiation of feeding but high serum leptin levels, as noted above, have a low corre-
lation with reported fullness, satiation, satiety, meal size, and duration of eating in
humans (de Graaf et al. 2004 ). This may be due, in part, to the already high leptin
levels and leptin resistance in overweight and obese individuals (Guyenet and
Schwartz 2012 ).
Leptin produced in the gastric mucosa may have a direct effect on the vagus
nerve that innervates the stomach (Peters et al. 2005 ; Power and Schulkin 2009 ),
and leptin produced by the placenta as measured in cord and maternal serum
concentrations is positively correlated with maternal fat mass, placental weight, and
fetal weight and size (Hogg et al. 2012 ; Jakimiuk et al. 2003 ). Hyperleptinemia in
pregnancy is not associated with decreased food intake (Jakimiuk et al. 2003 ).
A few other adipokine biomarkers are worth noting in this brief review:
(1) Tumor necrosis factor-α(TNFα) is secreted primarily by subcutaneous fat and is
involved in the regulation of adiponectin and interleukin-6. TNFαdecreases glu-
cose uptake and metabolism and fatty acid oxidation in the liver. The pathways are
complex, but the end result is a reduction in insulin signaling. (2) Adiponectin is the
most abundant of the adipokines but its level decreases with obesity, thereby
reducing hepatic insulin sensitivity and increasing insulin resistance and risk for
type 2 diabetes (Lihn et al. 2005 ). Circulating levels are higher in females than
males and decline during pregnancy (Power and Schulkin 2009 ). (3) Neuropeptide
Y (NPY) secreted by adipose tissue is also found in the pancreas. It acts in the CNS
to increase appetite and food intake and may be important in the activity of visceral
Table 10.3 Some hormones, adipokines, and other peptides produced and secreted by adipose
tissue
Hormone/adipokine/peptide Functions
Leptin Affects appetite and food intake, fetal and pubertal
development, bone development, immune function
Tumor necrosis factorα
(TNF-α)Represses genes involved in the uptake of and storage of
non-esterified fatty acids and glucose
Adiponectin Enhances insulin action
Interleukin 6 Regulates insulin signaling, has central effects on energy
metabolism
Neuropeptide Y (NPY) Involved in angiogenesis of adipose tissue, regulated leptin
secretion
Resistin Effects insulin action, increasing insulin resistance
Aromatase Converts androgens to estrogensPower and Schulkin ( 2009 ), after Table 11.1. Bioactive molecules produced by adipose tissue,
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216 L.S. Lieberman