Epigenetic changes happen as DNA “reads the environment” and makes adjustments by up-regulating or down-regulating what is expressed (read) from it based on exposure to the environmental stimuli. Your DNA dictates who you are, yes, but each attribute governed by your DNA has a range, not finite value. The most familiar epigenetic phenomena I can think of that we all know is the sun tan.
You are aware that the more time you spend in the sun, the easier it is to tan. Exposure dictates genetic behavior.
Generally, sun tans happen when cells called melanocytes oxidize the amino acid Tyrosine and create melanin, which protects you by dissipating over 99.9% of absorbed UV radiation, the proliferation of which makes your skin darker. Increased exposure to UV radiation from the sun induces production and induction of cytokines, growth factors, and neuropeptides, such as alpha-melanocyte-stimulating hormone which lead to increased melanin proliferation in the exposed skin. More sun exposure = faster, easier tan.
If you hide in your basement all winter then take a vacation to Florida in May, you burn to a crisp because all of these systems have down-regulated. Though your DNA has the code to make protections, exposure (or lack thereof) dictates that that part of the DNA is not being read to produce them. This is simply efficiency. Why run those reactions when there is no sun? Your body’s ultra-efficiency is often what gets you into trouble in unnatural environments (Suddenly sunny in January or swimming in a sea of Frappuccino).
Gradual increased exposure to the sun will cause these systems to “up-regulate” and ramp up the systems that produce melanin to protect you without burning you to a crisp. Environmental exposure dictates up or down-regulation of what is “read” from the genome. How efficiently an organism deals with epigenetic changes dictates how well it evolves genetically.
If a tree with a tap root that reaches a deep underground stream is faced with drought and loss of the stream what does it do? It grows fine feeder roots right under the soil surface to collect as much rain water as it can right from the soil surface as soon as it falls from the sky. Environmental exposure dictates genetic behavior.
Heat-shock protein expression is absent in the antarctic fish (big surprise).
Of course this is true for all of your metabolic enzymes. We have systems / pathways in place to utilize many different fuel sources, but the modern Western diet (environment) is glucose-based, so fat-burning pathways go away. Hitting the gym does not bring them back. An insulin-free diet does.
We eat glucose non-stop, often in enough volume to trigger the release of insulin: Toast or yogurt for breakfast, bagel for mid-morning snack, sandwich for lunch, pasta for dinner, something sweet before bed…day after day for decades. When glucose / insulin is omnipresent, your fat-burning pathways down-regulate. This is simply efficiency. Why run those reactions when you are in perpetual glycolysis?
When you finally decide to eat right and go to the gym, you don’t lose any fat because you ‘burn’ glucose preferentially. If you happen to burn up your stored sugar, your liver will tear down your skeletal muscle (proteolysis) and turn it into sugar (trans-amination) and protect your fat, which is meant to save your life.
Sadly, when you do burn up your stored sugar, it is actually stored as a compound with lots of water, which is heavy. So loss of that water + sugar gives the illusion of having lost a lot of weight, which comes right back as soon as you refill those glycogen stores. This leads to heartbreak that causes many to just give up the idea of diet & exercise.
If exposure dictates genetic behavior, then you will metabolize fats and cholesterol more efficiently when you eat more fats and cholesterol and up-regulate those metabolic pathways-this must be done in the absence of insulin. But be careful. If you have been in the Glucose basement for a long winter, do not rush into the Florida bacon. Gradually move from lean meats to fatty. Years ago when I first read Dr. Atkins book, he explained that everyone is afraid of meat and cholesterol but your cholesterol actually goes down on his diet-but he wouldn’t explain why! Epigenetics was not a hot topic in the early 70s.
Once those adipose-burning enzymes are up and running, you are called “fat-adapted”; You burn fat preferentially instead of burning glucose preferentially. Ascertaining what fuel you are burning can be done via Respiratory Quotient, just remember that your muscle shows up as glucose once trans-aminated.
If you then frequently burn fat as ketones via extremely low glucose load, you can become “keto-adapted”. This is why it takes some people 3 days to become ketotic and some people 3 weeks: enzyme up-regulation. And then the fat melts away sometimes too fast. You don’t have to live this way, but if you need to swing the pendulum, you can do this for months or years.
And this is why genetic twins can have vastly different metabolic behavior-same genes, different reading of those genes.
This is done by diet, not exercise, but exercise will help all of this along tremendously.
Inversely, the Inuit, who only ate fish and blubber and got all of their carbs & phyto-nutrients from lichens they scraped from rocks, most-likely had severely down-regulated glucose metabolism enzymes. Some of the people in the “Paleo” crowd do, too. They spend so much time metabolizing fats that they need to schedule “Glucose overfeeds” so they don’t down-regulate those enzymes. Take as long of a pause as you need to come up with whatever that says about modern society. It’s ridiculous but at least they understand the system.
So when someone like Mark Sisson writes a book with the strapline: “Reprogram Your Genes for Effortless Weight Loss, Vibrant Health, and Boundless Energy” this is what he is talking about, though it probably looks like pseudoscience to the cynical diet-book recidivist. I refer to this concept again & again while talking metabolism.