Fed, Fasting and Starved Metabolic States

What happens when you don’t eat enough? This post is going to talk about the biological consequences of fasting and starvation. Starvation and/or under-eating may occur for a variety of reasons: circumstance, prolonged extreme dieting, or fasting diets may put the body in a state of starvation. This post will detail what exactly happens in your body when you are starving.

Before we dive deep into the biological impacts of fasting, I thought this little epiphany I had while researching this post was worth sharing. 

Even though modern medicine has evolved tremendously, and we have adapted to incredible technology, at the end of the day we are still creatures, and much of our underlying biology is designed to help us survive as our hunter-gathering ancestors did. Our bodies have astounding protective mechanisms and tightly regulated systems that work to keep us alive.

Personally, I find this stuff fascinating. Ever since I had to read this paper many many years ago, I found the metabolic shifts the body undergoes while eating, post-meal, and during extended periods of time without food riveting.

I am not sure if anyone else will find this interesting (lol really selling myself here…but for real if this is to much science lingo and too in depth, let me know), but I think understanding this process is also important if you want to better understand fasting diets, consequences or physiological damages caused by eating disorders, or what “keto” really means. Hope you enjoy! 

*Disclaimer: As always, this is general information intended for healthy adults. Your needs may vary based on medical status, lifestyle, or life-stage. Please never replace generalized health information you’ve read online with individualized clinical care.

Forms of Energy Stores in the Body:

When your body has extra energy after a meal that isn’t immediately needed, it is stored. Your body tightly regulates the amount of glucose (energy/sugar) in the bloodstream and tucks the rest away into storage for a rainy day. This is your body’s way of defending against possible potential starvation scenarios.

Many of us are aware that fat (adipose triglyceride) tissue is one of the major forms of energy reserve in the human body. While adipose tissue serves other important roles, including insulation and padding for your organs, one of its main functions is to serve as an energy depot.

In addition to adipose tissue, glycogen (from carbohydrates) is stored in the liver and muscles. Protein is also stored in muscle. Although it can be broken down by the body and transformed into energy in dire circumstances, as it serves a variety of roles, the body makes great effort to spare it unless depleted of all other options. 

Why You Need Food:

To maintain proper function, the brain, nervous system, and cells needs energy. That energy comes in the form of glucose. Typically, your body tightly regulate levels of blood glucose to be able to properly fuel your heart, lungs, brain, and muscles to fuel your daily activities. 

The body does a great job tucking extra energy into storage for later, and harvesting it as needed. More details below.

The Well-Fed State:

In a well-fed state, glucose and amino acids are transported from the intestines into the bloodstream to be carried all over the body for energy. The liver has first dibs on using dietary glucose, where it can be converted to glycogen (for storage). Red blood cells (RBC) also use a lot of glucose in the fed state, because unlike other cells, they don’t have a mitochondria, which is the powerhouse of the cell used to create fuel. 

In the fed state, insulin is secreted by the pancreas, and stimulates the storage of fuel and protein kinase cascades. To put it simply, insulin (read more about hunger-related hormones here) promotes glucose to enter the muscle and adipose (fat) tissue for storage. Insulin also promotes the uptake of amino acids into muscles, favoring a build up of muscle proteins. 

Metabolism in the Post-Fed State:

Once you stop eating, your body can no longer harvests energy directly from ingested glucose but must start to rely on other sources of stored fuel. For the first few hours after a meal or overnight, glycogen in the liver is harvested as the major source of glucose. 

Metabolism in the Early Fasting State:

Carbohydrate stores are typically depleted after about a day. After 18-48 hours of no food intake, the body starts the evolve its energy-harvesting strategy. 

When glucose levels in the blood begin to drop, insulin decreases and a different hormone known as glucagon begins to increase. Glucagon signals a starved state to the body. As the brain and other tissues (including red blood cells) are dependent upon glucose for fuel, the body does everything it can to protect and fuel itself.

Glucagon stimulates the breakdown of glycogen (stored energy) from the liver, releasing the storage from the liver into the bloodstream as a form of fuel. Fatty acid stored in adipose tissue can also be oxidized for energy, and transported to the brain for energy.

Glucocorticosteroid hormones are also released. The combined presence of low insulin levels and glucocorticosteroid hormones send signals to the body to bring to hydrolyze proteins in muscle cells and to provide amino acids that can be used to create fuel. 

Large amounts of nitrogen are typically lost in urine during the early fasting rate, as a result of the high rate of muscle protein breakdown and synthesis of glucose from muscle glucogenesis. 

Metabolism in Prolonged Fasting:

If fasting or starvation is ongoing, eventually the body will deplete stores of glycogen. It goes through another metabolic shift.

The body’s goal at this point is to maintain vital physiological functions. It aims to save proteins as best it can.

Fat is broken down into compounds known as glycerol and fatty acids. Through a complex series of events and biological processes (I could go into detail, but I’m guessing no one reading this really wants me to since this is rather science-heavy already, but if you do, let me know) the fatty acids travel to the liver where they are broken into carbon units, which then form units called ketones.

Ketones may be used in place of glucose as a source of fuel for the brain and nervous system during periods of starvation. When ketones are feeding the brain, glycogenesis (the breakdown of muscle for fuel) slows down to try to spare muscle mass. 

You may have heard of the ‘keto’ diet. The keto diet strives to achieve ketosis. As you can tell, ketosis is your body’s line of defense against starvation. I am planning a whole post about that, but thought the metabolism of starvation may be useful background information to have before the keto diet is detailed.

During this time, the kidney is also harvested for glucose. The kidney also releases NH3 to try to neutralize the organic acids from the ketone bodies. 

After a couple weeks to a couple months (depending on the person), the body may run out of fat stores to harvest for energy, and will turn back to harvesting protein from muscle to make fuel. The metabolism slows down to protect the body from death.

Keep in mind that your heart is a muscle. Eventually, your body harvesting protein for fuel may cause organ failure, such as cardiac failure, resulting in death. Survival time depends on the amount of fat a person has before enduring starvation. 

Even for those with large fat stores, fasting state’s can cause physiological damage or death due to extreme ketosis.

It’s also important to note that you don’t have to be eating literally nothing to go into a starvation state. You burn a lot of energy just by thinking, breathing, and engaging in other life-sustaining activities, even if you are completely sedentary. Under-eating for a prolonged period (due to circumstance or due to an eating disorder) can put your body into metabolic states like those of starvation.

That wraps up this post for today! Stay tuned for posts on fasting, under-eating, and keto diets coming soon.

Thank you for reading and as always, feel free to leave questions or comments below, or say hello on Instagram, Twitter, or YouTube!

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