exceeded 460 kg (1000 lbs) with the heaviest person weighing 635 kg (1,400 lb),
and having a BMI of 186. Fat is a very efficient way to store energy as it contains
about twice the amount of energy per gram dry weight as glycogen (i.e., carbo-
hydrate) or protein and is stored with very little water (Power and Schulkin 2009 ).
Glycogen is primarily stored in muscle and the liver, which are relatively limited in
size compared to expandable visceral and subcutaneous adipose depots
(Schmidt-Nielson 1994 ). Estimates are that the average person stores enough
energy in fat to satisfy minimal energy requirements for more than a month (Power
and Schulkin 2009 : 246). Endogenous factors also play a role in fat accumulation.
Multidisciplinary research on the gut microbiome shows that not only do bacteria
regulate energy extraction, but they may be involved in the regulation of fat storage
(Backhed et al. 2004 ; Turnbaugh et al. 2006 ).
Fat has insulative, endocrine, immune, and metabolic functions. White fat pre-
dominates in humans with the percentage of brown fat decreasing after infancy
(Kuzawa 1998 ). Brown fat is especially important in thermoregulation in infants,
and both brown and beige fat can be activated on cold exposure (Nedergaard et al.
2007 ). Brown and beige fat contain a higher number of mitochondria than white fat
and are capable of releasing heat energy for thermal regulation via uncoupling
protein reactions to produce non-shivering thermogenesis (Rao et al. 2014 ).
Establishing useful parameters for non-shivering thermogenesis for use as a bio-
marker of brown fat activity is confounded by metabolic facultative and adaptive
responses to a wide range of ambient temperatures in human environments (Cannon
and Nedergaard 2011 ). Diet-induced thermogenesis (DIT), the amount of energy
expended due to the cost of processing food for use and storage, as measured by
indirect calorimetry in a respiratory chamber, has not been a reliable biomarker of
reported satiety or food intake (de Graaf et al. 2004 ).
Adipose tissue is comprised primarily of adipocytes but also has many other cell
types with a range of functions including: preadipocytes, fibroblasts, vascular
endothelial cells, mast cells, and macrophages (Fain 2006 ). Adipocytes function to
store energy in the form of lipids (triglycerides) and their secretions regulate ap-
petite, eating, digestion, absorption, and metabolism by targeting peripheral organs,
the gastrointestinal tract, and parts of the CNS. Non-adipocyte cells in adipose
tissue release interleukin and other inflammatory adipocytokines associated with
obesity’s co-morbidities (Fain 2006 ). Table10.3 lists some of the important
bioactive molecules secreted by adipose tissue and involved in the regulation of
appetite and eating (Power and Schulkin 2009 ).
Adipose tissue stores, synthesizes, secretes, and has receptors for a number of
peptide and steroid hormones and enzymes. The researchfield of adipobiology
developed quickly after the discovery of leptin in 1994 with a growing list of
adipose-secreted signaling proteins collectively designated adipokines (Fiore et al.
2014 ). The most well-characterized adipose hormone is leptin, in part because of
intense, but failed, efforts to develop pharmaceuticals that could mimic some of its
effects on satiety, food consumption, metabolism, and obesity (Power and Schulkin
2009 ). In the intestine, leptin functions to regulate lipid and carbohydrate meta-
bolism. Leptin also modulates sweet taste sensitivity showing diurnal variation with
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