What is starvation mode? – The Metabolic Phases of Starvation

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Obligatory disclaimer: I am not a medical doctor, and the content presented in this website is intended for information purposes only. Such content should not be construed as medical advice, consultation, diagnosis or treatment.

Last week we went through a very basic reading of the Wikipedia page on Starvation Mode / Starvation Response, and we focused on the definitions it contained:

Starvation response in animals is a set of adaptive biochemical and physiological changes that reduce metabolism in response to a lack of food.

Starvation mode is a state in which the body responds to prolonged periods of low energy intake. During short periods of energy abstinence, the human body burns primarily free fatty acids from body fat stores, along with small amounts of muscle tissue to provide required glucose for the brain. After prolonged periods of starvation the body has depleted its body fat and begins to burn primarily lean tissue and muscle as a fuel source.

This week we’ll begin going through the key points from one of the papers that Wikipedia uses as references, we’ll check the accuracy of Wiki’s information and we’ll fill in some of the gaps, should we find any, in that information.

First paper: Wang, Tobias; Hung, Carrie; Randall, David (2006). “The Comparative Physiology of Food Deprivation: From Feast to Famine”. Annual Review of Physiology68 (1): 223–251.

Bear in mind, this is a rather long paper, so we’ll just stick to the basics. After a quick glance, it’s clear the paper basically answers what starvation mode is under the heading The Phases of Fasting and Starvation in Mammals and Birds. Given the nature of this blog, we’ll just stick to mammalian matters.

According to the paper, when faced with absolute food deprivation, mammals go through three distinct metabolic phases. These are characterized on the primary fuel available and the associated changes in overall body mass. Fasting occurs during the first two phases, while starvation happens during the third one. Let’s understand what the definitions of fasting and starvation. Quoting directly from the paper:

In humans, fasting often refers to abstinence from food, whereas starvation is used for a state of extreme hunger resulting from a prolonged lack of essential nutrients. In other words, starving is a state in which an animal, having depleted energy stores, normally would feed to continue normal physiological processes.

In other words, a fasting mammal will voluntarily forgo food, but a starving one will feed or attempt to do so in order to keep normal physiological processes going on. As an example of fasting, consider how migratory birds can travel vast distances, sometimes non-stop, and feed after reaching their destination.

So, what are the three metabolic phases and what happens during each one of them?

  • Phase I: This phase follows immediately after the last meal has been absorbed from the gastrointestinal tract. During this phase glycogenolysis (the breakdown of glycogen) maintains blood sugar levels constant and keeps metabolism going. Glycogen is mostly stored in the liver, with a lower amount stored in the muscles. This phase can last for hours.
  • Phase II: This phase begins when the liver’s glycogen stores are depleted. As some organs, like the brain, require glucose to function, gluconeogenesis becomes necessary to keep things running. Although there is a contribution of amino acids from proteolysis of muscle protein (in english: breaking down of muscle protein to get amino acids), adipose tissue provides the bulk of the material for the synthesis of glucose by providing glycerol. This phase can be maintained during weeks in humans.
  • Phase III: Should starvation proceed as fat stores are depleted, gluconeogenesis is carried on at the expense of muscle. This process eventually kills the animal.

So to summarize, after our last meal is absorbed from the gastrointestinal tract our body keeps functioning on the glycogen that’s stored in our liver. After some hours, that glycogen storage runs dry and our body needs to generate glucose. That glucose comes mainly from our fat and a little from our muscles. After some weeks of this, our fat deposits dry up and only muscles remain to be consumed. Death follows.

This raises a couple of questions that I’ll keep in the back of my mind and try to answer in future posts:

  • It’s clear that if fat loss in your goal, phase II is where you want to be at. An appropriate diet and/or exercise regime will keep your glycogen supplies low enough so that your fat deposits are kept under consumption. However, if you are like me and enjoy practising some sort of sport then you also want to minimize your muscle loss. How can this be done? Eating additional proteins comes to mind as a possible solution.
  • Following the previous question’s line of thought, can diet or exercise be tweaked so that muscle can also be put on whilst on phase II? If you’ve ever lifted weights then chances are you have experienced noob gains, an increase in the weight that you can lift despite being a newbie lifter. The usual explanation is that noob gains are a result of becoming better at performing the lift through improved technique, greater motivation, the benefit of even marginal experience and other factors, rather than actually putting on muscle and becoming stronger. However, if you are a long term weightlifter, how far would you be able to lose fat while also gaining muscle?

Anyhow, now that we know a little more of how the body reacts to being deprived of food, we can be a little more critical towards Wikipedia’s definitions of starvation mode. As a reminder, those definitions are:

Starvation response in animals is a set of adaptive biochemical and physiological changes that reduce metabolism in response to a lack of food.

Starvation mode is a state in which the body responds to prolonged periods of low energy intake. During short periods of energy abstinence, the human body burns primarily free fatty acids from body fat stores, along with small amounts of muscle tissue to provide required glucose for the brain. After prolonged periods of starvation the body has depleted its body fat and begins to burn primarily lean tissue and muscle as a fuel source.

I find the first definition is more appropriate, and more elegant as well. The second definition seems to be the result of someone trying to mix the lay man’s terminology with proper science, but still falls short due to the ambiguity of “short periods” and “prolonged periods”, but now I’m just being picky.

Next week we’ll look over the other papers wikipedia provides and, if necessary, add their content to our Rebuttals to Fatlogic section. Should those additions be unnecessary, we’ll take a look at starvation mode’s close friend, metabolic damage.

Hope you enjoyed the post and see you next week!

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