What actually changes between a child, a teenager, an adult and an older adult — and how one household with four requirements manages without four cupboards.
~13 min read
Nutrition is usually taught as though there were one body with one set of numbers. The requirement moves across a life, and in most homes across the kitchen table.
What the body needs and the daily foundation assumed average size, average absorption and no unusual demand. Five things move the numbers.
Growth
Building tissue costs far more than maintaining it, so a body laying down bone, muscle and blood at once needs more per kilogram than an adult.
Hormonal change
Puberty, the years in which the menstrual cycle ends, the slower shifts across a man’s decades — each alters how the body handles minerals and protein.
Absorption
Stomach acid production tends to fall with age, and with it the release of some nutrients from food.
Energy demand
It peaks in adolescence and falls with muscle mass, and eating less means less of everything.
Pregnancy and nursing
A different physiology: blood volume, thyroid output and several requirements change measurably, on a timetable.
Two rules govern this whole lesson, stated here so they are not mistaken for caution added at the end. Nothing here is a dosing instruction for a child — the label on the bottle in your hand is. And nothing in the pregnancy section is advice; it is physiology, and the decision belongs to a healthcare practitioner. A supplement does not replace food, and nothing here replaces professional medical care.
The phrase is closer to arithmetic than slogan. Per kilogram of body weight a child needs more of most things, not less: an infant runs near a hundred kilocalories per kilogram a day against an adult’s thirty to forty, and takes protein at one and a half grams per kilogram against the adult zero point eight. Halving an adult capsule makes the wrong amount of every ingredient, in a form the child cannot swallow.
Diagram to come
Two figures side by side, an adult and a small child, drawn to true relative height so the size difference is obvious. Beside each, a vertical bar showing requirement PER KILOGRAM of body weight — and the child’s bar must be visibly TALLER than the adult’s, which is the entire point of the drawing. Below, a second pair of bars showing TOTAL daily requirement, where the adult’s is taller. The reader must see both facts at once and understand that they do not contradict each other. No food icons; plain bars and plain figures.
Three nutrients carry most of the difference: iron, calcium and DHA.
Then the thing parents actually ask about, which is not a nutrient. Fussy eating is normal: wariness of unfamiliar food rises in the second year, peaks between two and six, and usually fades on its own. Treating it as an emergency makes the mealtime worse without improving the diet.
Where a children’s supplement earns its place, the reason is format before content: a child cannot swallow a capsule, so the useful products are chewables and liquids.
Iron tracks blood volume, and blood volume tracks growth — the reference figure for a one-to-three-year-old sits near seven milligrams a day, not far below an adult man’s eight, in a body a fifth of the weight. Calcium is the material the skeleton is built from, and the skeleton is under construction: the reference for a four-to-eight-year-old is around a thousand milligrams. And DHA is a structural fatty acid in neural and retinal membranes at every age — a statement about where a molecule sits, not about what a bottle does.
doTERRA’s a2z Chewable is a chewable multivitamin built around a fruit and vegetable blend with a botanical blend alongside it. IQ Mega is an omega-3 supplied as a liquid rather than a softgel, carrying the long-chain fatty acids described in what the body needs in an orange oil base that makes it possible to take by spoon.
Neither appears in the nutrient library this app treats as its authority on which product carries which nutrient, so both descriptions above are qualitative rather than a statement of contents — the panel on your own bottle is the answer, and it is also the only dosing instruction.
The label is the only dosing instruction
Serving sizes for children’s products are set by age band on the label, they differ between products and between markets, and no lesson can override them. Read that panel before the first serving, and again when the child moves into a new band. One warning printed on iron-containing supplements in several markets is worth taking at face value: accidental overdose of iron-containing products is a leading cause of fatal poisoning in young children. They live where a child cannot reach them, not on the breakfast table.
The range for children differs by market more than the adult range does — what is on the shelf in one of the sixty-five markets may have no equivalent in another. And one fact that sits awkwardly here: for most children, food and time beat a supplement.
Adolescence is the fastest growth since infancy: energy requirement in a fifteen-year-old can exceed an adult’s outright, not per kilogram but in total.
Iron rises in both sexes. Reference figures put a fourteen-to-eighteen-year-old boy near eleven milligrams a day against eight for an adult man; in girls monthly menstrual loss takes the figure to around fifteen — the largest single divergence between two people at one table, and the reason a family multivitamin is a compromise.
Bone is the other clock, and it does not come round again. The skeleton accrues roughly nine-tenths of its adult mass by the end of the teenage years, giving this stage the highest published calcium figure of any except nursing — about thirteen hundred milligrams a day from nine to eighteen. Vitamin D governs how much of it the gut takes up: calcium is the material and vitamin D the gatekeeper, and that is the whole of what either does here.
The practical problem is not knowledge. Feeding a teenager well is mostly out of your hands, and their intake swings week to week by nature rather than by failure. The intervention with the best return is the fridge rather than the cupboard: protein and fruit needing no preparation, at the hours they actually eat.
This section names no product, and that is deliberate. Pregnancy is the one stage where the nutritional decisions are genuinely medical and a lesson is the wrong instrument. What follows is physiology. The decision that comes after it belongs to your healthcare practitioner.
Folate shows why timing matters more than intention. The neural tube — the structure the brain and spinal cord form from — closes within about four weeks of conception, before most people know they are pregnant, which is why public-health guidance on folate addresses the months before conception rather than after. Whether it applies to you, in what form and amount, is a question for your practitioner and not one a lesson can answer.
Iron demand changes because the blood changes. Plasma volume expands by forty to fifty per cent while red cell mass rises by less, so haemoglobin falls by dilution even in a normal pregnancy. Reference figures for iron rise from around eighteen milligrams a day to around twenty-seven. Iron is also the nutrient where taking more than you need is least harmless — one more reason this is a blood test and a conversation with your practitioner rather than a decision made at a shelf.
Thyroid output rises, and iodine is the element those hormones are built from. Published figures move from about a hundred and fifty micrograms a day to around two hundred and twenty in pregnancy and two hundred and ninety while nursing, the higher because iodine leaves in the milk. Both figure and source are practitioner territory: iodine has a narrow useful range, and seaweed varies too widely between harvests to be a measured intake.
DHA is structural: it sits in neural membranes and in the retina, and the fetus accumulates it fastest in the final three months, drawn across from the mother — where a fatty acid travels, not a recommendation about a bottle. Whether anything is needed at all depends on what is already in the diet, and that assessment is your practitioner’s.
One further caution belongs here, and it is not nutritional. The standing rule across this platform applies without exception: if you are pregnant or nursing, speak to your healthcare practitioner before using essential oils — before using them at all, not before using a particular one. The safety guide sets out why this group is treated separately.
The rule this section runs on
Nothing above is a recommendation to take anything. Every figure in it is a population reference, and a population reference cannot know your blood work, your diet, your medication or your stage. Supplementation in pregnancy and while nursing is a decision for a healthcare practitioner, made with those things in front of them. Upper limits matter more here than at any other stage — preformed vitamin A is the standard example — so more is not a safe default.
Nothing dramatic happens in the forties and fifties, which is what makes them easy to get wrong: the changes are invisible year to year and large over a decade. Absorption is the first — acid production tends to decline from around this point, and the nutrient most affected is vitamin B12, for reasons the next section takes apart.
Muscle is the second. From the fourth decade it declines slowly in the absence of a reason not to, and two inputs give it a reason: loading it, and protein arriving at each meal rather than as a daily total — the response appears to need twenty-five to thirty grams in one sitting, so three servings beat one large dinner. Magnesium belongs alongside it.
Then the hormonal transitions, which need careful words rather than cautious ones. In women, the years in which the menstrual cycle ends bring a substantial change in oestrogen, and with it changes in bone turnover, in where body fat is carried, and in sleep and temperature regulation. In men the equivalent is slower: testosterone declines gradually across decades. Both are normal physiological change rather than something going wrong, and nothing in this course is claimed to alter either.
One practical consequence is frequently missed. Once menstruation has ended the iron requirement falls sharply — from around eighteen milligrams a day to about eight — and many multivitamins are formulated at the higher figure. Reading a supplement label is the check.
doTERRA’s women’s health range pairs Bone Nutrient Complex, covered below, with Phytoestrogen Essential Complex — a blend of plant-derived compounds, soy and flax among them, which the nutrient library does not model, so the description is qualitative. This platform makes no claim about what those compounds do to human hormonal physiology, and anything in that category is a conversation with a healthcare practitioner before it is a purchase.
This is the stage where the gap between what is eaten and what is absorbed opens widest, and appetite often falls while requirement rises — the reason this group needs a practitioner more than a shelf.
Three requirements move clearly. Vitamin B12, for the reasons below, is absorbed less reliably from food — which is why several countries advise older adults to obtain it from fortified food or a supplement. Vitamin D falls on two counts: skin makes less from the same sunlight, and time outdoors falls too. And protein rises, to nearer one to one point two grams per kilogram a day.
For bone, the nutrient library records Bone Nutrient Complex as carrying calcium, magnesium, manganese, phosphorus, boron and vitamin K — the mineral fraction bone is built from, plus the vitamin directing where calcium is laid down. Vitamin D is not among them; it sits in Microplex VMz, worth knowing if you are holding one bottle and assuming it covers the subject. Bone is a living tissue rebuilt from supplied materials, which is what these nutrients are and all that is claimed for them.
Diagram to come
A simple body outline with four call-outs, each an inset showing a mechanism rather than an organ portrait, and each a two-state pair with the younger state above and the older below. One at the stomach: acid droplets, sparser in the older panel, with a protein-bound vitamin failing to be released. Two at the stomach wall: a carrier protein leaving the lining to meet the freed vitamin, with fewer carriers below. Three at the small intestine wall: the folded villi surface, shallower and flatter below. Four at the skin: a UV ray striking the skin and a molecule converting beneath it, with a visibly smaller output arrow below. The comparison is the drawing, not the caption.
Four mechanisms run independently, all reducing what reaches the blood from the same plate. The first is stomach acid. B12 in food is bound to protein, and freeing it requires hydrochloric acid and pepsin in an acidic stomach. Acid output tends to decline with age, and when it does the vitamin stays attached and passes through. A tablet contains B12 already free, so it skips the step that is failing. The same acidity governs the release of non-haem iron and of calcium from some of its salts, so the effect is not confined to one nutrient.
The second is intrinsic factor. Even freed, B12 must bind a protein secreted by the stomach lining, travel the small intestine attached to it, and be taken up by a specific receptor in the terminal ileum — the most elaborate absorption route of any vitamin, with more points at which it can fail. Production of that carrier falls alongside acid output.
The third is surface. The small intestine absorbs across an enormous folded area — villi carrying microvilli, turning a few metres of tube into tens of square metres. That surface flattens somewhat with age and its cells turn over more slowly. Less surface and older cells mean less uptake per meal.
The fourth is not the gut at all. Vitamin D begins as a cholesterol-like molecule in the skin, 7-dehydrocholesterol, which ultraviolet B light converts directly. That starting material falls over a lifetime: published comparisons put what a seventy-year-old makes at around a quarter of what the same exposure makes at twenty. Add shorter time outdoors and more covering, and the same sunlight yields much less. None of the four is a disorder; all four are ordinary ageing, and all four argue that the form a nutrient arrives in matters more as a person gets older.
Two practicalities decide more outcomes than any nutrient above. Swallowing becomes harder, so liquids, chewables and split servings are the difference between a supplement taken and one abandoned in a drawer. And several prescribed medicines are usually in the picture, with interactions that are real and specific. Nothing here replaces professional medical care; it is why at this stage a new supplement is raised with the practitioner who holds the whole list.
A well-planned plant-based diet is good nutrition, and the gaps it leaves are known rather than mysterious — a short list, all of it coverable deliberately. Six matter: the three below, and zinc, iodine and calcium, which are folded after them.
Vitamin B12 — non-negotiable
There is no reliable unfortified plant source. Not one. Algae and fermented soy have both been proposed and neither stands up — some carry inactive analogues that occupy the measurement without doing the job. Fortified food or a supplement.
Iron — the form, not the amount
Plant foods contain plenty of iron, in the non-haem form — absorbed far less readily than the haem form in meat. Several countries publish a plant-based reference roughly one point eight times the general one.
EPA and DHA
Plant foods supply ALA, and the body converts it onward badly — the daily foundation gives the rates, low enough that flax does not replace the long-chain fats. The direct plant source is algal oil, where the fish got them.
Zinc
Present in legumes, nuts and whole grains, and bound in those same foods by phytate, which holds the mineral where the gut cannot take it up. Soaking, sprouting, fermenting and leavening all break phytate down — why sourdough and soaked beans are not only a matter of taste. Zinc has a page of its own.
Iodine
In many countries the main sources are dairy and seafood, so removing both removes most of the supply. Iodised salt is the reliable route where available; seaweed is not, varying by orders of magnitude between harvests.
Calcium
Not simply a dairy question — absorbability varies enormously between plant sources. Oxalate in spinach binds calcium so tightly that only a few per cent is available, while kale and bok choy give up around half. Fortified plant milks are the straightforward answer, provided the carton is shaken.
Diagram to come
Two parallel routes from a plate to the gut wall, drawn as one diagram. Upper route: haem iron, an iron atom held inside a flat ring structure, passing through the gut wall by its own dedicated door — a wide, mostly unobstructed arrow, noted as barely changed by the rest of the meal. Lower route: non-haem iron, a bare charged ion arriving at a narrow door that accepts only one of its two charge states. Beside that lower door, two arrows push in opposite directions: one for vitamin C, widening the flow by converting the ion to the accepted state; one for phytate, tea polyphenols and calcium, binding the ion so it never arrives. The asymmetry is the point — the upper route is steady, the lower one is a valve the meal controls.
Iron reaches the gut in two chemically different states, and treating them as one nutrient is the commonest mistake made about plant-based eating.
Haem iron is an iron atom held at the centre of a flat nitrogen-containing ring, the structure that sits in haemoglobin and myoglobin — which is why it comes from meat and nothing else. The ring keeps it soluble and carries it across the intestinal wall by a route of its own. Published absorption figures put it between fifteen and thirty-five per cent, and the rest of the meal makes little difference.
Non-haem iron arrives as a bare ion, at the mercy of its surroundings: absorption ranges from around two per cent to twenty, depending almost entirely on what else is on the plate. The transporter accepts iron in its ferrous state, carrying two positive charges — but at the near-neutral pH of the small intestine it drifts into the ferric state, carrying three, and ferric iron forms insoluble compounds that pass through.
That is the gap vitamin C fills, doing two jobs at once: it donates an electron, converting ferric iron back to the ferrous form the transporter accepts, and it forms a soluble complex that keeps the ion from precipitating on the way. The effect is large and dose-dependent — enough that a squeeze of lemon over the lentils is a nutritional act rather than a garnish.
Pushing the other way are phytate in whole grains and legumes, polyphenols in tea and coffee, and calcium — each binding non-haem iron and holding it unavailable. Tea with a meal surprises people most; moving the cup an hour either side removes most of the effect. That argues for arranging those foods, not avoiding them.
The Lifelong Vitality Pack exists in a vegan version, with the marine omega complex replaced by one built from algae and seed oils.
Everything above describes individuals. Almost nobody buys supplements for one. The question in most homes is how a single shopping decision covers four requirements.
Two adults differ from each other far less than either differs from a seven-year-old. Buy for the divergences, share everything else.
| Item | Separate purchase? | Reason |
|---|---|---|
| A children’s multivitamin | Usually yes | Amounts and format both differ. |
| An omega-3 | Often no | One bottle can serve several people, where the label covers those ages. |
| A multivitamin for the adults | No | One product covers both adults in most homes; iron is the exception to check. |
| Bone nutrients | Sometimes | Only where the house multivitamin does not carry them. |
| Anything for pregnancy or nursing | The practitioner decides | Not a shelf decision. |
One last thing, easy to lose in a course about products. Every requirement here is met first by food. A supplement covers a gap; it replaces neither a diet nor professional medical care.
Keep supplements out of reach of children, and iron-containing products particularly well out of reach. If you are pregnant, nursing, under medical care or taking medication, speak to your healthcare practitioner before use. Product availability, formulation and label reference figures differ from market to market. Nothing here is intended to diagnose, treat, cure or prevent any disease.
The requirement moves. Which products end up in the house is the next lesson.
Where to go from here
Lesson 10 of 12 · Nutrition