With the release of Disclosure Day, Steven Spielberg’s new film about aliens, a question as old as science fiction itself resurfaces: if aliens were to arrive on Earth, would they come to conquer us, to study us… or perhaps to eat?
We must begin here with a word of caution. There is no scientific evidence that extraterrestrial beings have visited Earth, let alone any information about what they eat. NASA states that there is no data to support the claim that unidentified anomalous phenomena (UAPs) are alien technology, and the US Department of Defence has also found no verifiable evidence of extraterrestrial technology or activity.
But the literal meaning of “extraterrestrial” is simply “from outside Earth”. In that broad sense, the only extraterrestrials whose diet we know of are, oddly enough, humans – specifically the astronauts who spend weeks or months in space. While their experiences do not tell us what an alien would eat, they do show us that leaving Earth changes the way we eat.
In microgravity, appetite, taste perception, muscle mass, bone health, hydration and energy expenditure all change. Even for our own species, eating off-planet means adapting diets, nutrients, textures, food preservation and metabolic control.
Read more:
Astronauts don’t eat enough because food tastes bland in space. We’re trying to work out why
This article is therefore a purely speculative exercise, though not an absurd one. Biology allows us to ask reasonable questions, even about imaginary organisms. How much do they weigh? Do they move around a lot? Do they maintain a constant body temperature? Do they breathe oxygen? Do they have a large brain? Do they live in gravity similar to that on Earth?
Even with these pieces of information, we cannot guess at what “the alien diet” consists of. But we can figure out the minimum energy needs of a hypothetical living being.
UFOs and calories
The word “alien” is not a biological category. In popular culture, they range from “little green men” to reptilian beings, tall humanoids, luminous creatures, and mechanical, non-organic intelligence. But these classifications belong in UFO folklore and fiction, not zoology.
Science does, however, have tools for estimating a creature’s metabolism. In land animals, one guideline is that basal energy expenditure (the minimum energy an organism needs at rest to maintain vital functions) increases with body mass, though not proportionally.
This means a mouse consumes a lot of energy per gram of body weight, while an elephant, despite expending a great deal more energy in total, uses much less per gram of its mass. In other words, the larger an organism is, the more energy it needs, but each kilogram of its body is usually “cheaper” in terms of energy use.
Applied to a hypothetical living being, this allows us to make rough estimates. If we imagine a warm-blooded, active organism whose basic functioning is similar to that of a mammal or a bird, a creature weighing around 30 kilograms might need about 900 kilocalories (kcal) a day just to maintain itself at rest. A creature weighing 70 kilograms would require around 1,700 kcal per day, similar to an adult human at basal metabolic rate. And an alien weighing 150 kilograms could need over 3,000 kcal per day, even without moving around a lot.
These figures only indicate the minimum energy required to sustain basic functions: breathing, maintaining body temperature, repairing tissues, circulating fluids and keeping the nervous system active. They do not account for movement, stress, reproduction, thermoregulation, digestion, brain activity and any other behaviour an interplanetary visitor might typically engage in.
An alien that walks, runs, digs, flies or travels halfway across the planet to abduct cows would therefore need considerably more energy than its basal metabolic rate. In that case, the question would no longer be simply how much it weighs, but what it does, how it moves and how much energy it needs to survive in our environment.
Imaginary bioenergy
From this starting point, we can look at the metabolisms of three classic, imaginary forms of extraterrestrial life:
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The little green or grey man: With its slender body, large head and apparently little muscle mass, this alien would weigh between 25 and 40kg. If it were a warm-blooded, active organism with a large brain, its basal metabolic rate would be between 800 and 1,100 kcal per day.
However, a large brain is costly to run. In humans, the brain consumes around a fifth of energy at rest. If these grey or green humanoids had oversized brains, their diet would have to be high in energy and in continuous supply, unless they had evolved to have highly efficient biological mechanisms or technological support.
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The “reptilian”: This is more difficult to calculate. If it was truly reptilian in the physiological sense, it would be ectothermic (or cold-blooded), meaning it would not expend much energy maintaining a constant internal temperature. In that case, a 100kg creature might require less daily food than a mammal of the same size, provided it lived somewhere warm enough.
However, if it were an intelligent, bipedal, muscular and active predator, its energy expenditure could rise to levels comparable to or higher than those of humans. A 150kg endothermic reptilian might need 3,000 kcal a day at rest, and considerably more if it engages in physical activity.
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The tall humanoid, weighing between 80 and 100kg, would be the easiest scenario to imagine. If its physiology were similar to that of humans, it would require between 1,900 and 2,300 kcal per day at rest, and between 2,500 and 4,000 kcal during activity. On a space mission, we would also need to take into account spacesuits, spacecraft, different gravity, microbiota, hydration and adaptation to stress.
A fourth possibility is the post-biological entity: artificial intelligence, a hybrid organism, or a synthetic body. In this case, “food” would no longer consist of proteins, fats or carbohydrates, but electricity, heat, chemical fuel or nuclear energy. An alien robot wouldn’t eat rice or pasta, it would simply need to recharge its batteries.
Alien food on Earth
If an extraterrestrial visitor were based on carbon, water and chemistry similar to that found on Earth, our planet offer them a somewhat risky buffet. There is liquid water, as well as salts, organic carbon, sugars, fats, amino acids and minerals – but also potential toxins, pathogens, allergens and other incompatible molecules.
Earth’s food would not necessarily be edible for them. Terrestrial protein might be of no use if their digestive systems used different amino acids. Our sugars might prove useless if their metabolism could not handle them. Our bacteria may devastate alien life – or perhaps not infect it at all.
The last of these possibilities was explored by H.G. Wells in his 1898 novel The War of the Worlds, in which invading Martians are not defeated by human weapons, but by terrestrial microorganisms against which they have no evolutionary defences.
In astrobiology, it is generally thought that life requires three things: a source of energy, a liquid medium, and suitable chemical elements. But that does not mean that all living beings in the universe share the same diet. On Earth, for instance, a koala is almost entirely dependent on eucalyptus, while a cow needs its particular microbiota to digest the cellulose in grass. Diet is not just about energy: it is about biochemistry, the microbiome and evolution.
So if aliens were to arrive, they might not be looking for human “food” but for raw materials: water, nitrogen, phosphorus, iron, salts, lipids, microbial biomass or simple organic molecules. The classic image of aliens abducting cattle could be speculatively reinterpreted not as cosmic malice, but as nutritional sampling. Not that it makes any difference to the cows, unfortunately.
Extraterrestrial nutrition
This exercise reminds us that nutrition is not simply a list of foods, but a science of energy exchange between an organism and its environment. Eating is about solving a series of physical problems: how to obtain energy, build tissue, eliminate waste, and avoid poisoning.
Among humans, dietitians and nutritionists play a vital role because they translate that science into practical health advice: they tailor energy intake, protein, micronutrients, hydration and lifestyle habits to individual needs. We do not consume “calories” in the abstract – each of us eats within the context of our culture, gut microbiota, medical conditions, age, budget and a life story.
If we ever made contact with non-terrestrial biological beings, we would not just need diplomats, linguists or engineers. We would also need experts who could figure out what molecules these life forms tolerate, what energy they require, what poisons them, what microorganisms they carry, and what resources they could use without destroying the planet’s ecosystems. In other words, we would need alien nutritionists.
If aliens ever do reach our planet, they might not come to steal things, conquer us or reveal cosmic secrets. Perhaps they would simply come to eat better, but if they really wanted to understand our diet, they would have to learn one final Earthly lesson. Here, eating isn’t just about consuming energy – it’s about spending time together.
