Growth hormone deficiency is often associated with weight gain. What drives this problem? And what can it tell us about the fundamental mechanisms at work in fat regulation?
The Caloric Balance Hypothesis tells us that growth hormone deficiency can only make people fat by changing behavior. Somehow, a lack of hormones stimulates appetite and/or causes sedentary behavior. Thus, a positive caloric balance gets established, and the excess calories get dumped into the fat tissue by an as yet unidentified mechanism.
The Lipophilia Hypothesis says that the body strives to balance catabolic and anabolic hormones. Growth hormone deficiency could disrupt that balance, thus effecting a compensatory change in insulin levels. Changes in insulin provoke changes in the fat tissue and drive the establishment of a positive caloric balance.
So which idea has better support?
1. First let's look at a book called "Growth Hormone In Adults: physiological and clinical aspects."
In the introduction to a chapter on "the diagnosis of growth hormone deficiency in adults," the authors tell us that:
"It has...been shown that GH deficient adults have several abnormal clinical features including increased adiposity...reduced muscle strength and exercise capacity."
2. Here is another article: "Influence of growth hormone deficiency on growth and body composition in rats: site specific effects upon adipose tissue development."
The authors tell us that their data:
"clearly demonstrate for the first time that GH is required for differentiation of adipose sites in vivo. [In addition, the results] demonstrate distinct effects at different anatomical sites and suggest that GH may be one factor responsible for the differences described in numerous metabolic parameters and hormonal sensitivities of adipose tissue derived from different locations within the body."
So let's summarize:
A) Growth hormone deficiency indeed seems to be associated with weight gain.
B) GH deficiency affects the adipose tissue, insulin levels, and so forth. These researchers aren't talking about calories or appetites; they're discussing changes at the level of fat tissue.
Inferentially, the Lipophilia Hypothesis wins again.
3. Here is another curious study: "The effect of growth hormone on weight gain pulmonary function in patients with chronic obstructive lung disease."
The authors write:
"Defect of hormone (GH) in malnourished patients with COPD...subject received a balanced diet of 35 kcal/kg with a 1 g of protein per kg for one week (pre – GH)... the next three weeks they got exact diet plus an injection of growth hormone... The subjects had no significant weight gain during the first week of the diet alone... but they had substantial weight gain during the first week of GH treatment."
So there was no change in caloric intake – no change to "Calories In." The growth hormone therapy itself elicited changes in weight. Remarkable! Well, it is only remarkable if you believe the Caloric Balance Hypothesis. If you believe the Lipophilia Hypothesis, it makes complete sense.
4. "Clinical aspects of growth hormone deficiency in adults."
These authors tell us that:
"Several studies have shown that GHD in adults is associated with abnormalities in body composition (6 to 14), metabolic derangements... and suboptimal physical performance."