Definitions

The step in digestion where a nutrient actually crosses from the gut into the bloodstream, rather than passing through the digestive tract unused and leaving the body as waste. Absorption is the mechanism behind bioavailability — a nutrient that's poorly absorbed has low bioavailability, even if the food it came from was rich in that nutrient to begin with. How much gets absorbed depends on factors like the nutrient's chemical form, other compounds in the meal that help or hinder uptake, and conditions in the digestive tract at the time.

per NIH's Office of Dietary Supplements

How much of a nutrient in a food actually makes it into the bloodstream and becomes usable by the body, as opposed to the total amount that's simply present in that food on paper. Two foods can list the same gram amount of a nutrient and still deliver very different amounts to the body, because bioavailability depends on the chemical form the nutrient is in, what else is eaten alongside it, and how an individual's digestive system happens to process it. It's the reason nutrient totals based on raw food content alone can overstate what a person actually absorbs.

concept as described by NIH's Office of Dietary Supplements

Calcium works differently from iron and zinc here: its main absorption blocker, oxalate, is a property of the specific food rather than the overall diet, so no single blended multiplier exists the way NIH publishes one for iron and zinc. Instead, this tool uses a small table of per-food factors (currently spinach, Swiss chard, kale, broccoli, and tofu) drawn from real isotope-tracer studies that directly measured each food's calcium absorption against milk's, in the same trial, on the same subjects. That means these numbers reflect a controlled, single-food measurement rather than a real-time read of your own meal — but unlike the iron/zinc adjustment above, they also aren't a blend across typical eating patterns; they're a directly-sourced ratio for that one food. Foods without a directly-measured factor are left at face value rather than estimated, per this tool's no-fabricated-values rule.

per Heaney/Weaver calcium-absorption isotope studies (see Bioavailability, above)

An umbrella term covering the whole family of nutrient reference numbers that nutrition scientists publish — the RDA is just one member of that family. Depending on how much research exists for a given nutrient, scientists may publish a confident RDA, or fall back to an “Adequate Intake” (a reasonable working estimate used when the evidence isn't solid enough for a precise RDA), an upper safety limit, or a figure meant for planning across a population rather than judging one person's diet. When USDA or NIH materials use “RDA” and “DRI” in the same breath, DRI is the broader category and RDA is one specific type of number inside it.

per the National Academies of Sciences, Engineering, and Medicine

A vitamin that dissolves in fat rather than in water, which means it travels through the body attached to fat and can be stored in fatty tissue and the liver for later use. Because the body can build up a reserve, these vitamins don't have to be eaten every single day to maintain adequate levels — but that same storage capacity also means very large doses can build up to unsafe levels over time in a way water-soluble vitamins generally don't. Vitamins A, D, E, and K fall into this group.

per NIH's Office of Dietary Supplements

A food that has had a nutrient added to it during processing that wasn't originally there, or was only present in a much smaller amount — usually done deliberately to help close a known gap in what people typically eat. That's different from a food that's simply naturally rich in something to begin with; fortification is a manufacturing step. It matters for vegans in particular because a nutrient that's hard to obtain any other way on a plant-based diet, like vitamin B12, is commonly obtained through products manufactured with it added, such as fortified nutritional yeast or fortified plant milks.

per USDA/FDA food-labeling conventions

NIH's Office of Dietary Supplements finds that vegetarians and vegans need about 1.8x the standard iron RDA, since a plant-based diet's iron is entirely the less-efficiently-absorbed non-heme form. This tool applies that same 1.8x multiplier to your iron target rather than to any single food's iron content. Importantly, that 1.8x figure comes from studies of vegetarians eating their normal, varied diets over time, not a calculation of any one meal — it doesn't know whether you paired an iron-rich food with vitamin C or ate it alongside tea at the same sitting. It's a blended average across a typical vegan/vegetarian eating pattern, not a per-meal estimate, which is why pairing choices that boost or block absorption (see Non-heme iron) can still shift your real-world absorption above or below what the number shows.

per NIH's Office of Dietary Supplements, Iron fact sheet

The U.S. government's biomedical research agency, whose Office of Dietary Supplements publishes the nutrient targets — RDAs and the broader family of DRIs — that this tool compares your intake against. Where USDA supplies the “how much of a nutrient is in this food” side of the math, NIH supplies the “how much of it do you actually need” side, so the two sources together are what let this tool turn a list of foods you eat into an estimate of nutrient coverage.

per NIH's Office of Dietary Supplements

Iron comes in two chemical forms depending on where it's from. Heme iron comes from the hemoglobin and myoglobin in animal flesh (meat, poultry, fish) and is absorbed fairly efficiently and consistently. Non-heme iron is the form found in plant foods like legumes, grains, and leafy greens, and it's absorbed considerably less efficiently — and that efficiency swings a lot depending on what else is in the meal, since some compounds (like vitamin C) boost non-heme absorption while others (like the tannins in tea or the phytates in whole grains) suppress it. Because a vegan diet's iron is entirely non-heme, this is central to why raw iron content in plant foods can't be compared directly against absorption assumptions built around heme iron.

per NIH's Office of Dietary Supplements

The average daily amount of a nutrient that's enough to meet the needs of nearly everyone in a given age and sex group. It's deliberately set higher than a bare minimum so it still covers people whose needs run above average, which is why it's the number most people mean when they casually refer to a nutrient's “daily requirement.” Not every nutrient has a confidently established RDA — some only have a looser estimate instead (see DRI, below).

per NIH's Office of Dietary Supplements and the National Academies

A specific, standardized quantity of a food used as the reference point for reporting its nutrient content, so different foods can be compared on a consistent basis instead of an arbitrary one. A serving size isn't a recommendation about how much to eat in one sitting — it's a measurement convention. This tool's per-food nutrient figures are calculated per serving as sourced from USDA FoodData Central, then scaled by how often you say you eat that food: Daily assumes one serving once a day, Often assumes one serving a few times a week, and so on for every frequency — always one typical serving per occasion, never more. If you usually eat more than one serving at a time (two cups of oats instead of one, for example), your actual nutrient intake from that food runs higher than what this tool shows, since the math has no way to know that unless the frequency you picked already accounts for it.

per USDA FoodData Central conventions

The federal agency responsible for, among other things, tracking what's actually in the food Americans eat. Its FoodData Central database is the source for every raw nutrient figure used in this tool — how much iron, calcium, vitamin B12, and so on is present in a given food per a standardized amount. This tool doesn't estimate or guess those figures; it looks them up from USDA's published data and then applies its own adjustments (like bioavailability) on top.

per USDA FoodData Central

A vitamin that dissolves in water instead of fat, so the body doesn't stockpile it in tissue the way it does with fat-soluble vitamins — any amount beyond what's currently needed is mostly filtered out through urine rather than saved for later. That makes regular, frequent intake more important, since reserves run low faster, though it also generally means a lower risk of toxic buildup from overconsuming it. Vitamin C and the B-vitamin family (B1, B2, B3, B5, B6, B7, B9, and B12) are the water-soluble vitamins.

per NIH's Office of Dietary Supplements

NIH's Office of Dietary Supplements finds that vegetarians and vegans may need up to 50% more zinc, since the phytates common in grains and legumes — major staples on a plant-based diet — bind zinc and reduce how much of it gets absorbed. This tool applies that same 1.5x multiplier to your zinc target as a whole, the same way it handles iron above. Like the iron adjustment, this is a blended average drawn from typical vegetarian eating patterns rather than a calculation of what else was on your plate at any single meal, so it can't account for a specific meal's own mix of phytate-rich and phytate-free foods.

per NIH's Office of Dietary Supplements, Zinc fact sheet