You come to my shop, over in Health, Lex.
I've unraveled what I believe to be the physiological issues behind the hardgainer (ectomoprh) somatype - at the molecular level. We can talk common traits of lifestyle and behavioral issues that are part of the problem, and then talk about solutions.
I have a couple hundred very keen hardgainers on another forum who I worked with to develop an conceptural model of this functional phenotype. Most of them are using diet and exercise recommendations that fit their current CNS condition; the program helps to modify the CNS response chemistry to slowly push the expressed phenotype back into a mesomorph somatype.
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Cholesterol biosynthesis biochemistry and gene controls are pretty well elucidated at this time. "Cholesterol balance is maintained by a series of regulatory pathways that control the acquisition of cholesterol from endogenous and exogenous sources and the recycle of endogenous cholesterol, facilitated by its conversion to bile acids and recovery from gut. Over the past decade, investigators have discovered that a family of membrane-bound transcription factors, sterol regulatory element-binding proteins (SREBPs), mediate the end-product repression of key enzymes of cholesterol biosynthesis. Recently orphan members of another family of transcription factors, the nuclear hormone receptors, have been found to regulate key pathways in bile acid metabolism, thereby controlling cholesterol balance."
Note, for instance, that insulin is known to regulated lipid metabolism, and that various natural steroids and physterols also play a role in activation of AP-1, a nuclear receptor transcriptional regulator that also controls sterol regulatoryt binding elements.
Source: THE ROLE OF ORPHAN NUCLEAR RECEPTORS IN THE REGULATION OF CHOLESTEROL HOMEOSTASIS. Joyce J. Repa and *David J. Mangelsdorf. Ann. Review Cell Develop. Biol. 16: 459-481 (2000).