Nutrients come from a surprisingly wide range of sources: the food you eat, the water you drink, the bacteria living in your gut, and even industrial processes that add vitamins to everyday staples like flour and salt. Your body needs roughly 13 vitamins and 15 minerals, plus essential amino acids and fatty acids, to function properly. Most of these must come from outside your body because human cells can’t manufacture them on their own.
Plants: Where Most Nutrients Begin
Nearly every nutrient in the human food chain traces back to plants and the process of photosynthesis. Plants absorb sunlight through chlorophyll and use that energy to split water molecules into oxygen and hydrogen. The hydrogen equivalents then combine with carbon dioxide from the air to build carbohydrates, fats, proteins, and other organic molecules. This is the fundamental step that converts inorganic raw materials (water, CO2, sunlight) into the energy-rich compounds that feed virtually every living thing on Earth.
But plants don’t just produce calories. They also pull essential minerals from the soil through their roots. Potassium, for example, enters root cells through specialized ion channels in a passive flow when soil levels are adequate. When potassium runs low, plants activate high-affinity transport systems that actively pump the mineral inward. Nitrogen and phosphorus follow similar pathways, often with help from symbiotic fungi. These fungal networks extend thread-like structures called hyphae deep into the surrounding soil, digesting organic material and funneling nutrients like phosphorus and nitrogen back to the plant’s roots. The minerals a plant absorbs become the minerals you get when you eat it.
Animal Foods and Bioavailability
Animals concentrate and transform the nutrients they consume from plants (or from other animals), often into forms the human body absorbs more efficiently. Iron is a clear example. Plant-based iron, called non-heme iron, has an absorption rate of just 1 to 10%. Iron from animal tissue, known as heme iron, is absorbed at 25 to 30%. The chemical structure of heme iron allows it to pass through the intestinal wall more readily, which is why a small serving of red meat can deliver more usable iron than a larger portion of spinach.
This pattern holds for other nutrients too. Vitamin A from animal sources like liver arrives as a ready-to-use compound, while the beta-carotene in carrots and sweet potatoes must be converted by your body first, and the conversion isn’t always efficient. Protein from eggs, fish, and meat contains all the essential amino acids in proportions closely matching human needs, while most individual plant proteins are missing one or more. None of this means plant foods are inferior overall. They carry fiber, thousands of protective plant compounds, and certain vitamins that animal foods lack. But the source of a nutrient shapes how much of it your body actually uses.
Your Gut Bacteria Make Nutrients Too
Your intestines host trillions of bacteria, and many of them are quietly manufacturing vitamins you need. Gut microbes produce vitamin K2, which plays a role in blood clotting and bone health, along with most of the water-soluble B vitamins: B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6, B7 (biotin), B9 (folate), and B12. Researchers have identified 103 essential enzymes involved in these biosynthetic pathways across the gut microbiome.
How much of this microbial vitamin production your body actually absorbs is still being studied, and it varies based on factors like age and geographic origin. But the contribution is real enough that disruptions to gut bacteria, from prolonged antibiotic use, for instance, can measurably affect vitamin K and B vitamin status.
Drinking Water as a Mineral Source
Water isn’t just hydration. Tap water and mineral water contain measurable amounts of calcium, magnesium, sodium, fluoride, copper, and other minerals. The contribution varies widely depending on where you live and how hard your water is. In areas with hard water, drinking water can supply up to 36% of the adult recommended daily allowance for calcium. Magnesium from water typically covers 3 to 7% of daily needs, and copper from tap water accounts for 6 to 10% of the estimated safe intake.
Fluoride stands out as the mineral most significantly delivered through water. In communities with fluoridated water, drinking water alone provides 26 to 54% of total daily fluoride intake. Other minerals like iron and zinc contribute smaller fractions, generally under 5% of daily requirements.
Fortified and Enriched Foods
A large share of the nutrients in modern diets comes not from the food itself but from what’s added during manufacturing. Large-scale food fortification is the practice of adding micronutrients to commonly consumed staples like salt, flour, cooking oil, and sugar during processing. Salt iodization is the most widespread example. Between 1990 and 2008, the share of households globally consuming iodized salt rose from 20% to 70%, dramatically reducing iodine deficiency disorders.
Mandatory wheat flour fortification began in 1942, and 85 countries now require it. Across North and South America, folic acid is added to wheat flour to reduce the risk of neural tube birth defects. Some countries go further. Rwanda, for instance, fortifies maize and wheat flour with vitamin A, iron, zinc, folic acid, niacin, B1, and B12, while also adding vitamin A to cooking oils and sugar, and iodine to salt. If you eat bread, cereal, or table salt in most developed countries, you’re consuming industrially added nutrients whether you realize it or not.
Supplements and Synthetic Production
Many vitamins in supplements and fortified foods aren’t extracted from fruits or vegetables. They’re produced through microbial fermentation or chemical synthesis. Vitamins C, B2, B12, and D2 are all manufactured using bacteria, yeasts, or fungi grown in industrial fermentation tanks. Other nutrients, like niacin (B3) and pantothenic acid (B5), are made through a combination of chemical steps and microbial processes. For vitamin C specifically, a large-scale mixed co-culture fermentation process now produces the bulk of the global supply.
The synthetic versions of most vitamins are chemically identical or very similar to their natural counterparts, which is why they work effectively in supplements and fortified foods. A few exceptions exist where the natural form is better absorbed, but for the majority of essential vitamins, your body handles the synthetic version just as well.
Nutrient Levels in Food Are Declining
Even when you eat whole, unprocessed food, you may be getting fewer nutrients than previous generations did from the same crops. Research tracking the nutritional content of fruits and vegetables over the past 50 to 70 years has documented significant declines. High-yielding commercial varieties of apples, oranges, tomatoes, potatoes, and other staples have lost 25 to 50% or more of their nutritional density.
The specific mineral losses are striking. Across numerous studies in multiple countries, calcium content has dropped 16 to 46%, iron 24 to 27%, zinc 27 to 59%, and copper 20 to 76%. A review of 43 different fruits and vegetables found consistent declines in protein (6%), calcium (16%), phosphorus (9%), iron (15%), vitamin A (18%), riboflavin (38%), and vitamin C (15%) over a half century. The causes include breeding crops for higher yields and larger size rather than nutrient content, depleted soils from intensive farming, and rising atmospheric carbon dioxide levels that dilute mineral concentrations in plant tissue. This means that eating a varied diet rich in whole foods matters more than ever, and it partly explains the growing role of fortification and supplementation in meeting daily nutrient needs.