A human body is, at its most stripped-down level, a collection of about two dozen chemical elements organized into water, proteins, fats, minerals, and carbohydrates. Roughly 99 percent of your mass comes from just six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. But the interesting story is not the ingredient list itself. It is how those ingredients are arranged into trillions of cells, how much of you is simply water, what role trace metals play, and how relentlessly the body replaces its own parts while a few structures persist for an entire lifetime.
The Elemental Ingredients
If you could break a person down into a jar of pure elements, the proportions would look surprisingly simple. Oxygen alone accounts for about 65 percent of body mass, mostly because it is a major component of water and is locked into virtually every organic molecule. Carbon comes next at around 18 percent, forming the backbone of proteins, fats, sugars, and DNA. Hydrogen adds about 10 percent, and nitrogen roughly 3 percent. Calcium and phosphorus together contribute another few percent, concentrated heavily in bones and teeth. Everything else, from iron to zinc to cobalt, makes up well under one percent combined.
That “everything else” is far from trivial. Twenty elements are considered essential for normal biological functioning in humans. Half are metals and half are non-metals. The essential metals include four main-group elements (sodium, potassium, magnesium, and calcium) and six transition metals (manganese, iron, cobalt, copper, zinc, and molybdenum).1PubMed. Essential metals in health and disease Iron, for instance, is central to hemoglobin’s ability to carry oxygen through the blood. Zinc is involved in hundreds of enzyme reactions. Cobalt sits at the core of vitamin B12. These elements are present in milligram or even microgram quantities, but without any one of them, critical biochemical pathways shut down.
Mostly Water
The single largest molecular component of your body is water. How much water depends on your age, sex, and body composition. In children between ages 3 and 10, total body water sits around 62 percent of body mass, with little difference between boys and girls. In adult men, the percentage holds relatively steady before declining to about 57 percent after age 60. In women, body water drops more noticeably during adolescence, settling around 50–55 percent for most of adulthood and falling to roughly 50 percent after age 60.2PubMed Central. Body water percentage from childhood to old age People who carry more body fat tend to have a lower water percentage across all age groups, because adipose tissue contains much less water than muscle does.
This water is not just sitting around filling space. It is the solvent for nearly every chemical reaction in the body, the medium that carries nutrients to cells and waste products away, and a key player in temperature regulation through sweating and evaporation. Even bone, which feels about as dry as a biological tissue can get, contains roughly 10 percent water.3PubMed Central. Chemical and Biochemical Basis of Cell-Bone Matrix Interaction in Health and Disease
Proteins, Fats, and Carbohydrates
After water, the next largest fraction of your body is protein, making up roughly 15 to 20 percent of total mass. Proteins do an astonishing range of jobs: they form muscle fibers, carry oxygen, catalyze chemical reactions as enzymes, transmit signals as hormones, and defend against infection as antibodies. The most abundant single protein in the body is generally considered to be collagen, the structural scaffold found in skin, tendons, ligaments, and bone. Research in mice found that collagen makes up about 12 percent of total protein in females and 17 percent in males, with albumin, hemoglobin, histones, and actin also ranking among the most plentiful proteins.4Scientific Reports. Collagen constitutes about 12% in females and 17% in males of the total protein in mice Though these numbers come from mouse studies, the general hierarchy is expected to be broadly similar in humans.
Fat, or lipid, makes up a wide range of the body depending on a person’s build. In a study of male cadavers, total adipose tissue ranged from about 18 to 44 percent of body mass.5PubMed Central. Adipose tissue density, estimated adipose lipid fraction and whole body adiposity in male cadavers Not all of that adipose tissue is pure fat; its lipid fraction ranged from about 54 to 85 percent in the same study, with the remainder being water, proteins, and connective tissue. Beyond stored energy, lipids are critical components of every cell membrane and serve as insulation for nerves.
Carbohydrates are the smallest of the major molecular categories. Your body stores carbohydrates primarily as glycogen, with about 500 grams packed into skeletal muscle and roughly 100 grams in the liver.6PubMed Central. The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise That is less than a kilogram total, a tiny fraction of body weight compared to fat or protein. Glycogen serves as a rapidly accessible fuel source, especially during fasting and exercise, and the liver’s supply is particularly important for maintaining stable blood sugar between meals.7PubMed. Carbohydrate storage in cells: a laboratory activity for the assessment of glycogen stores in biological tissues
DNA by the Numbers
Every nucleated cell in your body carries a complete copy of your genome, and those copies add up to a surprising amount of material. A single diploid nucleus in a male cell contains about 6.27 billion base pairs of DNA, stretched out to roughly 205 centimeters in length and weighing about 6.41 picograms. Female cells, with two X chromosomes instead of an X and a Y, are slightly larger: around 6.37 billion base pairs, about 208 centimeters long, and 6.51 picograms.8PubMed Central. On the length, weight and GC content of the human genome Two meters of DNA crammed into a cell nucleus only a few millionths of a meter across. Multiply that by the trillions of nucleated cells in the body and the total length of DNA in a single person stretches into the hundreds of billions of kilometers.
By mass, though, DNA is a negligible fraction of you. All that genetic material adds up to roughly 50 grams in total, about the weight of a small egg. The body’s information storage is dense and compact but light.
Trillions of Cells, Give or Take
Counting the cells in a human body is harder than it sounds, because different cell types vary enormously in size. Red blood cells are tiny and numerous; muscle cells are huge and comparatively rare. A landmark 2016 estimate placed the number at about 30 trillion human cells in a 70-kilogram adult male.9PubMed Central. Revised Estimates for the Number of Human and Bacteria Cells in the Body A more recent and detailed analysis in 2023 arrived at roughly 36 trillion cells in the average adult male, 28 trillion in the adult female, and about 17 trillion in a child.10PubMed Central. The human cell count and size distribution The sex difference comes largely from differences in body size and the proportion of certain tissue types.
Red blood cells dominate the count. They are so small and so numerous that they make up the vast majority of cells by sheer number, even though they collectively weigh only a few kilograms. Muscle cells, by contrast, are far fewer in number but contribute an enormous share of total body mass. This means “what are you made of” gets a very different answer depending on whether you count by number or by weight.
Bacteria and the Microbiome
For decades, a popular factoid held that bacteria in the human body outnumber human cells by ten to one. That ratio has been revised sharply downward. A careful recalculation found the actual ratio is much closer to one to one, meaning you carry roughly as many bacterial cells as human cells.11PubMed. Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans Most of these bacteria live in the large intestine, and their total mass is estimated at only a couple hundred grams. By cell count you are about half microbe, but by mass the microbiome is a tiny sliver of you.
Bacteria are not the only microbial residents. Fungi, archaea, and viruses also colonize the gut, skin, mouth, and other surfaces. Some of these organisms contribute to digestion, immune training, and vitamin production. Others are simply along for the ride. The composition of your microbiome shifts with diet, medication use, age, and even geography, making it one of the most variable components of what you are “made of.”
The Mineral Skeleton
Bones feel solid and permanent, but they are active tissue with a complex internal chemistry. About 60 percent of bone mass is inorganic mineral, predominantly a form of calcium phosphate called hydroxyapatite. Roughly 30 percent is organic material, mostly collagen proteins that provide flexibility and tensile strength. The remaining 10 percent is water.3PubMed Central. Chemical and Biochemical Basis of Cell-Bone Matrix Interaction in Health and Disease This combination of rigid mineral and flexible protein is what gives bone its remarkable ability to resist both compression and bending.
The mineral component itself is more chemically complex than simple hydroxyapatite crystals. Research into the fine structure of bone mineral has shown that a significant fraction of the phosphate ions are in a different chemical form (monohydrogen-phosphate) and are concentrated on the surfaces of mineral particles rather than embedded in the crystalline core.12Scientific Reports. Bone mineral: new insights into its chemical composition This surface chemistry matters because it influences how bone interacts with the surrounding fluid, how minerals are deposited or resorbed, and how bone responds to mechanical stress over time. The skeleton is not just a scaffold; it is a mineral reservoir that the body draws on and replenishes continuously to regulate calcium and phosphate levels in the blood.
What Gets Replaced and What Does Not
One of the more striking facts about the human body is that most of it is not the same material it was a few years ago. Cells die and are replaced at different rates depending on the tissue. The lining of the gut turns over in a matter of days. Red blood cells last about four months. Skin cells shed and regenerate on a cycle of a few weeks. Bone remodeling takes years, but it happens continuously. The net effect is that most of your atoms are swapped out over the course of roughly seven to ten years, though this is a rough average rather than a precise clock.
Not everything turns over, however. A few structures persist with essentially no molecular replacement for an entire human lifetime. The crystalline proteins of the eye lens are one example. Radiocarbon dating of eye lens crystallins has confirmed that these proteins are formed during development and never replaced or remodeled afterward.13PubMed Central. Radiocarbon dating of the human eye lens crystallines reveal proteins without carbon turnover throughout life Dental enamel is the other well-known case of lifelong permanence. What makes the lens proteins especially remarkable is that, unlike enamel, the eye lens is a soft structure under almost continuous deformation as it changes shape to focus light, yet its core proteins are never swapped out. This permanence is part of why cataracts accumulate with age: damaged lens proteins cannot be cleared and replaced the way damaged proteins in most tissues can.
Certain neurons in the brain also appear to be lifelong, though individual molecules within those neurons do get replaced. The distinction matters: a neuron that lives for 80 years is not made of the same atoms it started with, but it is the same cell, maintaining its connections and its place in a circuit. The body’s approach to replacement is a patchwork, not a blanket policy.
Isotopes and the Subtle Fingerprint of Diet
The elements in your body are not all identical at the atomic level. Most elements exist as a mix of stable isotopes, atoms with the same number of protons but different numbers of neutrons. Carbon, for instance, comes as carbon-12 and the slightly heavier carbon-13 (distinct from the radioactive carbon-14 used in dating). Nitrogen appears as nitrogen-14 and nitrogen-15. Oxygen has three stable forms. The relative abundance of these isotopes in your tissues is not random. It reflects what you eat, where your food was grown, and how your metabolism processes it.14PubMed Central. Stable Isotope Abundance and Fractionation in Human Diseases
Different enzymes and transport proteins handle heavy and light isotopes at slightly different rates, a phenomenon called isotope fractionation. The result is that different tissues accumulate slightly different isotopic signatures. Your bone collagen, your hair keratin, and your blood proteins each carry a subtly different ratio of heavy to light carbon or nitrogen. Forensic scientists use these isotopic signatures to determine where a person lived and what they ate. Researchers are also exploring whether shifts in isotope ratios can serve as early markers for metabolic diseases, since changes in enzyme activity can alter how isotopes are distributed across the body.
What Happens to Your Chemistry After You Die
After death, the organized chemistry of the body unravels, and the elements that make up a person reenter the surrounding environment. This process is not a uniform dissolution. Different elements follow different timelines and pathways. A study tracking the elemental changes in soil during human decomposition identified three distinct groups of elements based on when and how they appeared.15PubMed Central. Soil elemental changes during human decomposition
Sodium, potassium, phosphorus, and sulfur were clearly cadaver-derived, appearing in the soil relatively early. Their persistence varied: phosphorus lingered in organic forms, sodium and potassium moved through the soil’s chemical exchange system more readily, and sulfur was released gradually as microbes broke down soft tissue. A second group, including calcium, magnesium, and manganese, showed up in concentrations higher than the body alone could account for, suggesting that decomposition chemistry was also mobilizing these elements from the surrounding soil itself. Calcium and magnesium appeared to come partly from the soil’s own reserves, while manganese was solubilized by the acidic conditions decomposition creates. A third group, including iron, copper, zinc, cobalt, and aluminum, increased only late in the process, apparently freed from soil minerals by prolonged acidification.
The net effect is that a decomposing body creates an ephemeral hotspot of nutrient cycling. The pulse of organic matter feeds microbial communities, alters soil chemistry, and redistributes elements in ways that can persist for months or years. Ecologists sometimes refer to these sites as “cadaver decomposition islands” because of how sharply the local chemistry differs from the surrounding landscape. The elements that once made a person do not vanish; they scatter into a widening circle of biological and geological recycling, eventually incorporated into plants, microbes, and the mineral matrix of the earth itself.
Why the Proportions Vary So Much Between People
The standard breakdown of human composition is usually given for a “reference man,” a 70-kilogram adult male used as a benchmark in radiation protection and physiology research. Real people deviate from this template considerably. A trained endurance athlete might carry less than 10 percent body fat, while a person with obesity might carry 40 percent or more. That single variable alone reshuffles the relative proportions of water, protein, and mineral. Higher body fat means a lower water percentage, less protein per kilogram, and a different distribution of energy reserves.
Age changes the balance too. Infants have proportionally more water and less mineral than adults, partly because their skeletons are still mineralizing. Older adults tend to lose muscle mass (and with it, both water and protein) while bone mineral density declines, shifting the ratio toward fat. Pregnancy adds its own temporary reconfiguration: increased blood volume, expanded extracellular fluid, fetal and placental tissue, and extra fat stores. Even hydration status on a given day can shift body water by a liter or two, which is enough to change percentage calculations meaningfully.
Sex differences are consistent but moderate. Women generally carry more essential body fat and less skeletal muscle than men, which accounts for most of the difference in water percentage noted earlier. Hormonal differences drive these patterns, particularly during and after puberty. The 2023 cell-count estimates reflect this as well: the roughly 8-trillion-cell gap between adult males and females is driven primarily by differences in body size and tissue proportions, not by any fundamental difference in cell biology.10PubMed Central. The human cell count and size distribution
Trace Elements and the Thin Line Between Essential and Toxic
Several of the elements your body depends on are toxic at only modestly higher concentrations. Iron is the classic example: too little causes anemia; too much damages the liver and heart. Copper follows a similar pattern: essential for enzymes involved in energy production and connective tissue formation, but dangerous in excess. Manganese is needed for bone formation and antioxidant defense, yet occupational overexposure can cause neurological damage resembling Parkinson’s disease.
The body manages this balancing act through tightly regulated absorption, transport, and excretion pathways. Iron absorption in the gut is controlled by a hormonal feedback loop that ramps up when stores are low and dials down when they are adequate. Copper is bound to transport proteins in the blood and excreted through bile. Zinc competes with copper for absorption, which is why high-dose zinc supplements can inadvertently cause copper deficiency. These systems generally work well, but they can be overwhelmed by genetic disorders, extreme dietary imbalances, or environmental exposures. The margin between “enough” and “too much” for several trace metals is surprisingly narrow, and the consequences of getting it wrong in either direction can be serious.