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The Bizarre Changes in Physiology: What Happens When the Human Body Goes into Starvation Mode?
Starvation mode is a series of adaptive biochemical and physiological changes in animals (including humans) that occur when food is scarce or when there is extreme weight loss. These changes are designed to prolong survival and preserve body fat and muscle mass by reducing metabolic rate and decreasing allostatic energy expenditure. In the human physiological response, when faced with hunger, the body seeks to use stored fat as a source of energy to preserve muscle and other important tissues.
When the human body is in a state of hunger, it will consume food in the intestines and glycogen in the liver and muscles within the first day, and then begin to use fat and muscle protein as energy sources.
Capacity and composition of the starvation response
Estimates of starvation responses (i.e. metabolic adaptations) showed that the eight individuals who lived in Biosphere 2 for two years lost an average of about 15% (range: 9–24%) of their body weight in response to the harsh conditions. They compared the eight individuals to a group of 152 people after they came out of isolation and found that their total daily energy expenditure decreased by 750 kilojoules (180 kcal). Of this, a total of 250 kilojoules (60 kcal) of energy was saved due to the reduction in fat-free body mass and fat mass.
Human energy needs
A person's energy needs are generally composed of basal metabolic rate (BMR) and exercise-related activity heat production (ERAT). This energy can be obtained from sources such as protein, fat and carbohydrates. When food intake is insufficient, the body uses glycogen in the liver for energy supply. As glycogen is consumed, the body begins to break down fat into esters and free fatty acids and transport them to the brain for energy generation.
Research has found that when people enter a state of hunger, they will undergo a series of physiological adaptation processes, which allows humans to maintain necessary physiological functions when facing long-term hunger.
Blood sugar and ketone body production
During the initial stages of starvation, the body relies primarily on fatty acids as an energy source. However, while the fat body is being used, the brain still needs glucose. When fat is broken down into ketone bodies, these are used by the brain as an alternative fuel, reducing reliance on glucose. For example, on the third day of fasting, the brain can divert about 30% of its energy sources to the absorption of ketone bodies, which can effectively protect the glucose in the body to maintain basic physiological functions.
Time series of hunger
During the second and third days of starvation, fatty acids become the main source of energy, but glycogen is still needed to maintain brain function. Once the body enters a long-term starvation period, muscles begin to degrade to release amino acids for the liver to synthesize new glucose. This process leads to a loss of muscle mass, which in turn affects the body's functions. Eventually, if the fat reserves are exhausted, the body can only rely on protein to survive. In the long run, this will inevitably lead to organ dysfunction and ultimately death.
When nutrients in the body are completely exhausted, the human body will face the challenge of how to survive from the existing resources. This is not only a physiological struggle, but also a survival instinct.
Bacterial responses to starvation
It's not just humans; bacteria also show strong adaptability in the absence of nutrients. When faced with antibiotics, starved bacteria can develop resistance, just as in chronic infections, where biofilm formation has been implicated. In a biofilm, nutrients consumed by surrounding cells allow cells within the biofilm to survive, which further promotes bacterial survival and reproduction.
Conclusion
From a physiological point of view, the human body's response to hunger is full of subtle regulatory mechanisms, and the various functions in the body influence each other through multiple pathways to adapt to the harsh environment. In the face of hunger, the body's survival instinct demonstrates great resilience and adaptability. When we are faced with food shortages again, perhaps we can think about whether this body’s adaptability can also inspire and help us in other challenges in life.
