The main source of nutrients for all animals is plants, either eaten directly or consumed indirectly by eating other animals that fed on plants. Every food chain on Earth traces back to photosynthetic organisms, primarily green plants on land and microscopic algae (phytoplankton) in the ocean, which convert sunlight into the chemical energy that animals cannot produce on their own.
Why Plants Are the Foundation
Plants capture solar energy and use it to transform carbon dioxide and water into carbohydrates, storing that energy in chemical bonds. This process, photosynthesis, is the primary energy input into the global food chain. Nearly all living organisms use the complex organic compounds derived from photosynthesis as their energy source. Without it, the sugars, starches, fats, and proteins that animals depend on simply would not exist.
In the ocean, the same principle applies through a different set of organisms. Phytoplankton, tiny floating algae, fuel aquatic food webs worldwide. Some of the planet’s largest animals rely on remarkably short feeding chains: phytoplankton feeds krill, and krill feeds great whales. Phytoplankton also produces fatty acids, amino acids, vitamins, and carotenoids that move up through the marine food web to fish, seabirds, and marine mammals.
The Three Macronutrients Animals Need
Regardless of species, animals require three broad categories of nutrients from their food: proteins, fats, and carbohydrates. Each serves a distinct role in the body.
Protein supplies amino acids, the building blocks for muscle, organs, enzymes, and immune cells. A primary function of dietary protein is providing essential amino acids that the body cannot manufacture on its own, plus nitrogen for building other biological molecules. When an animal doesn’t get enough energy from fats or carbohydrates, protein that would normally support growth and repair gets burned for fuel instead.
Fats are a concentrated energy source and carry essential fatty acids, long-chain molecules that the body cannot synthesize. These fatty acids are critical for cell membranes, hormone production, and absorbing certain vitamins. Carbohydrates, found abundantly in grains, grasses, and fruits, provide the most readily available form of energy. Sugars and starches are quickly broken down into glucose, the universal fuel for cells.
How Herbivores Extract Nutrients From Plants
Herbivores face a unique challenge: much of a plant’s energy is locked inside cellulose, the tough structural fiber in cell walls. No mammal produces the enzymes needed to break cellulose apart. Instead, herbivores rely on billions of specialized bacteria living in their digestive tracts. These cellulolytic bacteria break the complex chains of glucose units in cellulose into short-chain fatty acids, which the animal’s gut absorbs and uses for energy.
This symbiotic relationship is one of the major contributions gut microbes make to their host. In cattle, horses, rabbits, and many other herbivores, the microbial community in the rumen or large intestine converts otherwise indigestible plant fiber into usable fuel. The bacteria also generate compounds like acetate, succinate, and formate from plant cell walls, which support the production of additional fatty acids by other members of the microbial ecosystem. Without these internal partners, a cow grazing on grass would starve.
Carnivores and Omnivores Get Nutrients Secondhand
When a wolf eats a deer, it is consuming nutrients the deer originally extracted from plants. The energy has simply been repackaged into animal tissue. Meat, eggs, and milk offer certain nutritional advantages over raw plant material. Animal-based proteins score higher on digestibility and amino acid completeness than most plant-based proteins. Plant proteins from legumes tend to be low in sulfur-containing amino acids, while cereal proteins are low in lysine. When any essential amino acid is missing or limited, the body cannot fully use the rest for building proteins, and the surplus gets broken down and discarded.
Animal tissues also deliver nutrients like vitamin B12, which can only be produced by microorganisms and accumulates in animal flesh over time, and highly bioavailable forms of iron. This is why carnivores and omnivores can meet their nutritional needs with smaller volumes of food compared to herbivores.
However, the tradeoff is efficiency. Each step up the food chain loses a large portion of the original energy. Estimates of trophic transfer efficiency, the percentage of energy that passes from one level to the next, range widely but typically fall between 10 and 25 percent. That means a predator retains only a fraction of the energy its prey consumed, and that prey retained only a fraction of what it ate from plants. This cascading loss explains why ecosystems support far fewer predators than herbivores.
Minerals, Vitamins, and Internal Production
Beyond the big three macronutrients, animals need minerals and vitamins in smaller quantities. Most minerals enter the food chain through soil. Plants absorb phosphorus, calcium, magnesium, and trace elements from the ground, and animals acquire them by eating those plants or drinking mineral-rich water. Cereal grains are rich in phosphorus, though much of it is bound in a form called phytate that some animals struggle to absorb. In tropical regions where soil is phosphorus-deficient, grazing animals sometimes develop abnormal chewing behaviors and appetites for unusual materials, a condition called pica.
Sodium and chloride, the components of common salt, are often scarce in plant-based diets. Wild herbivores seek out salt licks, and livestock farmers routinely add salt to feed. Salt also serves as a vehicle for delivering trace minerals like iodine, copper, zinc, and manganese.
Some vitamins don’t need to come from food at all. The gut microbiome of ruminants like cattle and sheep can synthesize riboflavin, biotin, folate, vitamin B12, vitamin K2, thiamine, niacin, and several other B vitamins. The large intestine absorbs these microbe-produced vitamins through specialized transport systems, supplementing what the animal gets from its diet. Vitamin B12 is notable because it can only be made by microorganisms, never by plants or animals themselves. This is why it accumulates through the food chain and is most concentrated in animal tissues.
What Domesticated Animals Actually Eat
For the billions of livestock raised worldwide, the plant-based origin of animal nutrition is visible in every bag of feed. Corn is the primary energy source, and soybean meal is the main protein source for dairy cows in many countries. A typical dairy cow diet might contain around 27 percent corn, 25 percent corn silage, 17 percent alfalfa hay, 9 percent soybean meal, and smaller amounts of cottonseed meal, wheat bran, and mineral premixes. The premix supplies vitamins A, D, and E along with copper, iron, zinc, and manganese to fill nutritional gaps.
This formulation reflects the same basic principle at work in wild ecosystems. Whether a deer is browsing on forest undergrowth or a dairy cow is eating a carefully balanced ration, the nutrients flowing into the animal originated in plants that captured energy from the sun.
Detritivores: The Exception That Proves the Rule
Not all animals eat living plants or other live animals. Detritivores, creatures like millipedes, earthworms, and dung beetles, feed on dead and decaying organic matter. They consume fallen leaves, rotting wood, and animal waste, breaking large pieces into tiny particles and partially digesting them. This process reduces the carbon-to-nitrogen ratio of the material, making nutrients more accessible to soil microbes and, eventually, to plant roots.
Even these animals, though, are ultimately powered by photosynthesis. The dead leaf a millipede eats was built from sugars a living tree once made from sunlight. Detritivores simply access that stored energy at a later stage, closing the loop in the nutrient cycle by returning carbon and nitrogen to the soil where plants can use them again.