Starvation: When Your Body Eats Itself

We’ve all heard the phrase, “I’m starving!” or “I’m so hungry I could die”. These are common examples of hyperbole and exaggeration to sound more dramatic and humorous. But in reality, starvation is anything but a joke. 

About 1 in 11 individuals worldwide, or roughly 733 million people, faced undernourishment in 2023 (FAO et al., 2024).

What is Starvation? 

Starvation is the most extreme form of undernourishment: when the body does not receive sufficient calories to keep up with its energy demands. This could be from lack of food intake or malabsorption. Starvation is the process the body goes through after its sugar and fat stores have been used up, and begins breaking down muscle tissue. This leads to permanent and often irreversible damages, and eventually, death.

The human body can roughly last only 5 to 10 minutes without oxygen, and 3 to 8 days without water. In contrast, humans can last up to weeks without something to eat. This is because humans have stores of fat, glucose, and muscle tissue which the body will slowly break down for sustenance in an attempt to buy you time to find food. 

You’ve likely heard that 20% of your body’s energy goes to your brain, despite it only taking up about 2% of your weight. Your brain is your most important organ, crucial for survival and regular daily functioning. It is constantly at work, while you are awake or asleep. It is responsible for your thoughts and decision making, controlling muscles needed for voluntary action, as well as involuntary functions such as keeping your heart beating, maintaining body temperature, and regulating hormones. It is no surprise that with all these tasks, the brain has a very high metabolic rate. Starvation poses a serious risk to the brain, and the body adapts to prioritise the brain to keep it alive, even at the expense of other organs. A slowed metabolism, inability to regulate temperature, impaired kidney function, and weakened immune system are just some of the many effects of starvation

The effects of undernourishment can be categorised into 3 overlapping stages of bodily responses. Stages 1 and 2 can occur in relatively short periods of fasting, while stage 3 (starvation) only occurs in prolonged absence of food. Keep in mind that the actual duration of these stages varies, depending on factors such as age, sex, intake of calories, physical health, and state, and participation in physical exertion.

Stage 1: Glucose as Fuel

When you digest food, nutrients such as glucose are absorbed from the small intestine into the bloodstream. Cells use this glucose as fuel for cellular respiration to produce energy. When you don’t eat, your blood glucose level drops. To compensate for this, glycogen in the liver is converted to glucose to increase blood sugar levels and provide energy for your cells. This happens around 6 to 24 hours after an individual’s calorie intake is reduced.

Fig 1: a diagram depicting blood glucose regulation.

Individuals may experience mood changes, but no harmful neurological effects as of yet. Eventually, the brain enters an “alarm phase” and heightened stress levels. It generally takes around 1 to 3 days for glucose stores to fully run out, and the body will transition during this time to using fat as a fuel source. 

Stage 2: Fats as Fuel

When glucose starts to run out, the body turns to fats (lipids) and some non-crucial muscles, which are broken down into amino acids, as an alternative source of energy. Generally, by 72 hours, the body is relying almost exclusively on lipids, and can continue to do so for a few weeks. Keep in mind that the transition from using glucose to fat as fuel is gradual. Fats are broken down in the liver to produce ketones, in a process known as ketogenesis, and are converted into acetyl-CoA to be used in cellular respiration in substitution of glucose.

For one to three weeks with no food, the metabolism rate decreases; individuals experience a slow heart rate, severe weakness, and lethargy. Energy supply to “non-essential” and metabolically intense bodily functions are reduced, such as organs within the digestive system (which can slow digestion and cause diarrhea), reproductive organs, the voluntary muscles in your arms and legs, or your immune system. This also results in decreased “neural activity” in the brain, resulting in sensations of apathy or “emotional numbness”.

To truly understand the severity, consider that a healthy adult heart weighs roughly 300 grams. Even with the body directing its sparse energy away from non-essential parts and to crucial organs such as the heart, it can shrink to less than half its size, to a mere 140 grams, in the latter phases of starvation. 

Stage 3: Protein as Fuel

Stage three marks the most dangerous phase, where the body breaks down large amounts of protein to create amino acids (proteolysis), signifying true starvation. When the body starts breaking down muscle tissue, death becomes a very feasible possibility. As said by Dr. Nancy Zucker, director of the Duke Center for Eating Disorders at Duke University, "Once protein stores start getting used, death is not far.” Why is this? While the body initially breaks down non-essential muscles in your limbs, you only have so much spare muscle mass that is non-critical to life that can be used until you run out. Dr Zucker continues, "You're consuming your own muscle, including the heart muscle.” In the third stage, your body eventually starts to break down tissue within vital organs, such as your heart, lungs, kidneys, and liver, to fuel your brain.

Without food and energy, the brain and other organs will eventually undergo cell death and extensive damage. A large concern for children facing starvation is that their brains are still developing, and any damage may be permanent. Symptoms of starvation can include dry and flaky skin, changes in hair colour, and large edemas in the abdomen and lower limbs of individuals. During the very late stage of starvation, victims can experience hallucinations, convulsions and irregular heart palpations.

Many people do not actually die purely from starvation, but from immune-related diseases, as their immune system is heavily weakened by nutrient deficiency and unable to properly defend itself.

Mental Effects

Long-term starvation affects an individual’s mental health both psychologically and biologically. When explained in a greatly simplified manner, the lack of nutrients reduces the proper synthesis of fundamental neurotransmitters and alters their signalling properties, distorting precise brain chemistry. Serotonin and dopamine are a few of the many affected chemical messengers, which can further induce feelings of hopelessness and depression.

An Example of the Effects of Starvation

Fig 2.  A photo of the "malnourished and emaciated" body of 18-year-old Saida Ahmad Baghili, at Al-Thawra in the Red Sea Port city of Hodeida. 

As depicted in Figure 2, Saida experienced the widespread poverty and food shortages in Yemen firsthand, weighing only 11 kilograms when she first arrived at the hospital. In the first few weeks of arrival, she struggled to speak and, at times, struggled to eat, drinking only liquid medical food. Her throat was damaged from malnutrition, and her stunted growth resulted in chronic bone fragility. Fortunately, Saida gained 5 more kilograms during her stay at the hospital (Deutsche Welle, 2016).

Individuals who have experienced severe starvation cannot be cured by simply providing an abundance of food. Their body’s internal chemistry (eg. electrolyte deficiencies) has changed to cope with the absence of food, and suddenly giving them regular food can cause complications and harm their system. This is known as re-feeding syndrome. If you are interested in learning more about how survivors of starvation recover, click HERE.

Extra Facts

• Due to obvious ethical reasons, no experiments have investigated how long a person can survive without food, and most estimates come from survivors who have been trapped or buried alive in rubble, hunger strikes, or supply lines cut off in history.

• Unlike most animals, the human brain has the special ability to use ketones (lipids) as an energy source, allowing muscles to be preserved for longer during extended periods of starvation.

End Notes

In summary, when your body is deprived of food, it turns inwards to find other sources of energy. It starts by consuming the glucose and glycogen stored in your blood and liver, then consuming the fat, until finally burning through the protein and crucial muscle responsible for bodily function needed for survival.

The next time your friend complains about missing lunch and “starving” from hunger, feel free to let them know that they likely have several more hours before their body begins to start breaking down fat, and several more days before they really start starving. 

Reference List

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Brink, S. (2016). What Happens To The Body And Mind When Starvation Sets In? [online] Goats And Soda. Available at: https://www.npr.org/sections/goatsandsoda/2016/01/20/463710330/what-happens-to-the-body-and-mind-when-starvation-sets-in.

FAO, IFAD, UNICEF, WFP and WHO (2024). The State of Food Security and Nutrition in the World 2024. [online] openknowledge.fao.org. Rome, Italy: FAO ; IFAD ; UNICEF ; WFP ; WHO ; Available at: https://openknowledge.fao.org/handle/20.500.14283/cd1254en [Accessed 4 Oct. 2025].

Hjalmarsson, D. (n.d.). The Starved Brain – Can What We Eat Determine How We Think? [online] National Eating Disorder Collaboration. Available at: https://nedc.com.au/eating-disorder-resources/find-resources/show/issue-59-i-the-starved-brain-can-what-we-eat-determine-how-we-think [Accessed 5 Oct. 2025].

NP Istanbul Hospital Editorial Board (2023). How Hungry Can We Go? [online] Npistanbul.com. Available at: https://npistanbul.com/en/how-hungry-can-we-go [Accessed 3 Oct. 2025].

PBS NewsHour (2025). Understanding the stages of starvation. [online] YouTube. Available at: https://www.youtube.com/watch?v=q_WO33IEsnU [Accessed 3 Oct. 2025].

Weiss, T.C. (2016). What Happens When We Starve? Phases of Starvation. [online] Disabled World. Available at: https://www.disabled-world.com/fitness/starving.php.

Welle, D. (2016). UN agency reports dire conditions for Yemeni children. [online] dw.com. Available at: https://www.dw.com/en/un-agency-reports-22-million-yemeni-children-hungry-and-in-need-of-care/a-36743234 [Accessed 4 Oct. 2025].