By Ari Snaevarsson, Features Editor
Now let us discuss how the body regulates fat loss. We are going to look at seven important hormones involved in this regulation: insulin, glucagon, epinephrine, growth hormone, cortisol, thyroid, and leptin. This will be a super brief crash course just to make sense of this holistically.
In brief, insulin rises postprandially (after a meal) and functions as a storage mode. The pancreas is alerted as to the presence of incoming nutrients and secretes insulin, which stores glucose and fats in skeletal muscle and adipose tissue (however, how it affects glucose uptake is vastly different than fat uptake). By inhibiting hormone-sensitive lipase (HSL), it also shuts off fat oxidation since it is no longer necessary to keep burning fat for energy when we have a fresh new supply of energy coming in.
Glucagon, also secreted by the pancreas, works as the complete counter to insulin. When we have gone some time without food, the pancreas secretes glucagon, which works to raise blood glucose levels. It does this by stimulating the breakdown of liver glycogen into glucose that can then be released into the blood; it also ramps up gluconeogenesis, the process of creating glucose using various gluconeogenic precursors, such as lactate.
Epinephrine, also known as adrenaline, is another hormone that works to raise blood glucose when fasting. This kicks in after glucagon has already started its work, so this requires longer periods without food to activate. Secreted from the adrenal glands, this systemically stimulates the “fight or flight” response, so our body now reacts as if a major stress is placed upon it (and our ancient brains are still unaware of the fact that eight hours without eating is harmless when you can always pick up a banana from Servo). Epinephrine, and its neurotransmitter counterpart norepinephrine, also increase glycogen breakdown and gluconeogenesis, but again, this is now happening systemically, so muscle glycogen is taking one for the team here, too.
Functioning similarly to epinephrine, this hormone works by increasing liver gluconeogenesis (meaning muscle proteins are used as substrate, which is not ideal) and conserving the amount of glucose used by the muscles and other organs. This should make intuitive sense, as we cannot afford, in times of food deprivation, to burn up what little glucose reserves we have left – these are now the sole property of the greedy brain and some other tissues.
Growth hormone (GH)
Effectively the last line of defense, growth hormone works not only by conserving glucose use by the muscles and increasing glucose output by the liver but also by… hold your applause… mobilizing and burning fats.
Leaving behind discussion of the fasting versus feasting cascade, we arrive at the all-important thyroid hormone. As these regulators are interconnected in one way or another, conversion of inactive T4 to active T3 is ramped up by the catecholamines (think epinephrine and norepinephrine here). Active T3 works both through the mitochondria and long-term gene expression to alter basal metabolic rate, the energy required to sustain vital, involuntary bodily processes. In this way, putting yourself on a long-term, very low-calorie deficit (VLCD) worries the brain; through T3, it will work to suppress your metabolism to match your intake. This would be akin to a car that is running low on gas making its engine more efficient to conserve gas.
Leptin rises when food intake is higher, so it feeds back to the brain and signals that we are satiated and our metabolism can live to see another day. Essentially, it suppresses hunger and raises metabolic rate. This happens both acutely (right after a big meal) and chronically (which is one reason obese individuals – without leptin deficiencies – have much higher leptin counts than lower weight individuals).
We have now explored both the basic physiology and the hormonal regulation of fat loss. Consider yourself somewhat of an expert now and feel free to tell your parents over reading days all about the processes an effective weight loss plan must work with.