Many of us are in privileged positions: Whereas it was presumably common for some of our ancestors to experience famine, nowadays food is so abundant that people deliberately fast for extended periods for health benefits. This practice is rooted in science, for intelligent use of fasting can be therapeutic. The punctuated stress of fasting can trigger myriad adaptations that bolster resilience, protecting against a range of future stressors, and people can successfully use fasts of 5 to 21 days multiple times within a single year.
We should not take our good fortune for granted, however. The Ukraine War, climate change, and crises in the food supply chain are probably increasing the number of people who are desperately hungry.
With this in mind, the purpose of today’s post is to detail some of what we know about how starvation affects human biology. This blog won’t be strewn with practical tips, but it will shed light on what too many people are going through right now.
- Smart use of fasting can be therapeutic. Frank starvation, however, wreaks biological havoc and is ultimately fatal.
- Prolonged fasting triggers a metabolic switch to depending primarily on fatty acids and ketones for energy.
- A chronic energy deficit leads to a reduction in the mass of body tissues, including fat, skeletal muscle, and bone. This contributes to a slowing of metabolic rate.
- Insufficient energy intake reduces biological investment in non-essential process, such as reproduction.
- Immune functions are energetically costly, so starvation hampers the ability to ward off infections.
- Extended fasting can disrupt sleep, presumably to increase food-seeking behaviour.
- Going without food makes the gastrointestinal system less ready to digest and metabolise foods.
- Starvation seems to “programme” changes in biology that increase susceptibility of offspring to metabolic diseases.
Extended fasting and starvation are very different beasts
How going without food affects you depends hugely on your biology at baseline. To illustrate this, let’s look at a couple of poignant instances of food deprivation.
At one extreme, medically-supervised extended fasting can be used to treat obesity. Perhaps the most famous example of this is a case study of a very heavy Scottish man named Angus Barbieri man who did a medically-supervised fast for 382 days. Angus was provided essential nutrients (vitamins, minerals, essential fatty acids) during the fast, but that was all. After over a year without food, his weight crashed from 207 kg (over 32 stone) to 82 kg (under 13 stone), and in many ways his health improved dramatically.
At the other extreme, the effects of food deprivation on lean people are very different. Perhaps the most salient study for many people right now is Ancel Keys’ famous Minnesota Starvation experiment. Designed to better understand the physical and psychological strain of wartime starvation, 36 healthy young men who were conscientious objectors during World War 2 were given about 3,200 calories of food per day for the first 3 months. Then came the hard bit. For 6 months they were provided just 1,800 calories each day, with the intention of making them lose about 25% of their weight. To mimic what people had been eating in war-torn areas of Europe, their nutrition centered on lots of root vegetables, bread, and not much else.
As the starvation phase progressed, the men became irritable and impatient, and as their interest in women waned, their desire for food grew dramatically — one man had to be excluded from the analysis because he resorted to raiding rubbish bins for leftovers! Their physical function deteriorated remarkably. They became dizzy, weak, uncoordinated, and fatigued, and they experienced hair loss and collection of fluid in their legs (oedema). After this brutal starvation phase, the men didn’t exactly immediately bounce back to their best — they took 2 months to 2 years to report feeling normal again.
Despite the remarkable contrast between these two examples, there’s a lot of overlap in the biology of the changes experienced by the men mentioned above, so let’s look under the hood to understand more about fasting physiology.
Food deprivation triggers a metabolic switch in fuel preference from glucose to fatty acids and ketones
Your muscles and some of your other tissues store carbohydrate as glycogen, providing a fuel depot for certain activities (e.g., high-intensity exercise). Most of us store roughly 500 g glycogen in muscle and 80 g in the liver, and as you go without food, you begin to deplete these carbohydrate depots, in part to maintain your blood sugar levels (during fasting these tend to drop a little, before stabilising). As glycogen reserves become all-but empty, your body must maintain your blood sugar by using other sources, making glucose from non-glucose matter in a process named gluconeogenesis. This involves breaking down structures such as proteins in muscle for amino acids that can then be used to make glucose, and this degradation of proteins is driven by both reductions in circulating anabolic factors that help store nutrients (e.g., insulin, IGF-1) as well as increases in factors that help breakdown bodily tissues (e.g., stress hormones made by the adrenal glands).
These shifts in metabolic processes also increases the breakdown of fat tissue, raising fatty acids in the blood. The fatty acids are then burned for energy, elevating levels of a molecule (acetyl-CoA) that is converted by mitochondria in the liver into alternative fuel sources named ketones. Two of these (acetoacetate and β-hydroxybutyrate) are then used by mitochondria for energy in organs that consume lots of energy, such as brain and muscles. Ketones have many other roles too, helping to keep hunger at bay and serving signaling functions thought to help maintain resilience in the face of stress.
In summary, while reliance on carbohydrate for energy dwindles, your body breaks down more muscle to sustain blood glucose and fat tissue to provide fatty acids and ketones for fuel.
Extended fasting slows metabolic rate and thwarts reproductive function
Organs such as skeletal muscle are very metabolically active, burning lots of calories even at rest. The loss of some of this tissue therefore slows metabolic rate. This is compounded by loss of fat mass, for fat tissue makes a hormone named leptin that informs the brain of the body’s overall energy status: Dwindling fat mass produces less leptin, the drop in which triggers the brain to slow metabolic rate (e.g., by reducing the production of thyroid-stimulating hormone) and divert resources away from processes that are not essential to one's own survival, such as reproduction. The lowered thyroid and reproductive hormone activity in part explains why people who chronically undereat tend to feel cold, fatigued, and uninterested in sex and contributes to loss of menstruation in women of reproductive age. General slowing of many of your body’s systems under the duress of fasting requires less of your cardiovascular system, so heart rate and blood pressure decline as food deprivation continues.
A hungry immune system is less adept at fighting off pathogens
The immune system can also be very energetically demanding, an outward manifestation of which is the raised body temperature you experience as a fever. Furthermore, many nutrients (e.g., antioxidant vitamins) are needed to optimise immune function. As food deprivation reduces energy and micronutrient availability, it’s therefore not surprising that some immune activity declines.
Initially, this can be good for health, for chronic low-level activation of the immune system underlies many chronic diseases (e.g., diabetes, cardiovascular disease). As fasting continues, however, the immune system becomes so enfeebled that people become less able to fight infections, which can ultimately be fatal.
An absence of food promotes food seeking but ironically impairs ability to digest and metabolise food
While this subject hasn’t been well studied at all, many people feel that fasting initially sharpens their brain function, leading to feelings of mental clarity and alertness. This makes sense in the light of evolution, for animals that could successfully function in times of scarcity would be more likely to find food. Some people speculate that, if indeed present, improved cognition during fasting might relate to increased use of ketones by the brain.
If we return to the Minnesota starvation experiment, however, weeks of food deprivation appeared to worsen cognition, increase irritability, and impair interpersonal skills, and it is inevitable that malnutrition will eventually impair brain function.
Perhaps related to any effect of fasting on alertness, food deprivation seems to shorten sleep (people who have anorexia have quite short sleep, for instance). Presumably this is to increase time available to source food. In line with this, if you only allow certain animals access to food for a short period each day, their sleep-wake timing tends to shift so that they become more active in anticipation of food availability.
Returning to humans, once someone who has been without food for a long time happens upon something to eat, he or she is understandably likely to gorge. The irony, however, is that many aspects of gastrointestinal function are bound to have changed in the absence of food, hindering ability to digest and metabolise food. It is food that feeds the ecosystem of microorganisms in the gut, so starvation seems likely to reduce the diversity and abundance of these flora, perhaps contributing to difficulty breaking down food.
Some detrimental effects of starvation are handed down generations
Finally, there’s been a flurry of research in recent decades into developmental origins of disease, largely starting with the work of David Barker. Barker put forward the hypothesis that a stressful foetal environment potentiates the effects of a subsequent life of plentiful food availability on the development of chronic diseases such as obesity. Famously, near the end of The Second World War, people in a German-occupied part of The Netherlands had to survive on less than 800 calories or so per day for months on end, and some of these individuals were pregnant. The offspring of those who survived this enormously stressful “Dutch Hunger Winter” were later prone to an array of health problems, and there’s now a whole field of research into such “foetal programming”.
In short, if people survive starvation and successfully pass on their genes, the stress of food deprivation they experience could ripple through generations to come.
How to cope when food is scarce
Forgive me for stating the obvious, but the antidote to all the above is…
… wait for it…
… eating food.
Being serious, in survival contexts, any food is welcome. However, some foods are much better suited to these circumstances than others. I’ve expanded elsewhere on this site about the impressive merits of nut butters in contexts in which there are premiums on having energy-dense, nourishing foods (see The Science of Long Range Fuel section here), so I won’t retrace those steps.
Given what’s going on in Ukraine and the need for a better alternative to survival rations (which are generally just dry blocks of flour, sugar, and palm oil), we’ve formulated a nut butter product specifically for survival contexts. This nut butter is loaded with healthy fats, contains plenty of fibre, packs a whopping caloric punch, and is as tasty as anything we’ve ever made. It also comes in a handy resealable pouch. We’re currently working on using food science to maximise its shelf life, but in the meantime we're releasing this early version of it to support the people of Ukraine.