Nutrition in Special States: Pregnancy, the Elderly, Vegans and Bariatric Surgery

The unifying idea: demand and the absorption defect

A person becomes deficient for one of two reasons: they need more, or they absorb less. The individual vitamins and minerals each have their own chapter — this one pulls those threads together and asks a single practical question of every patient: which nutrients are at risk here, and why? The answer always comes from one of two forces. The first is raised physiological demand: pregnancy builds a fetus, so folate, iron and iodine needs climb. The second is an absorption defect: an ageing stomach, a plant-only diet, or a surgically shortened bowel each cut off the supply of a specific nutrient at a specific point. Match the supplement to whichever force is at work, and the rest is detail. Keep that frame in mind as we walk through the four classic states — pregnancy, the elderly, vegans, and bariatric/malabsorptive patients — because it turns a long list of "who takes what" into a small number of understandable rules.

Pregnancy and preconception: building a second body

The single most important nutritional intervention happens before the woman even knows she is pregnant. The headline is folic acid. The neural tube — the future brain and spinal cord — closes in the first four weeks of pregnancy, often before a period is even missed. Folate is essential for that closure, so supplementation must begin before conception and continue through the first trimester to prevent neural-tube defects such as spina bifida. A standard preconception dose covers most women; a much higher dose is advised for those at higher risk — a previous affected pregnancy, diabetes, obesity, or treatment with antiepileptic drugs that deplete folate (valproate and carbamazepine are the classic offenders — cross-linked in the Epilepsy chapter and in the B12/folate chapter that explains the vitamin's role in DNA synthesis). Alongside folate, vitamin D is recommended throughout pregnancy; iodine matters for fetal brain development and thyroid function; and iron is given when the mother is deficient, since the expanding blood volume and the fetus together drain her stores (see the Iron chapter).

Pregnancy is also defined by what to avoid. Vitamin A in high doses is teratogenic: excess retinol and, above all, the retinoid drugs (isotretinoin and acitretin used for acne and psoriasis) can cause severe fetal malformations, which is why women are told to avoid liver and liver products and why retinoid prescribing carries strict pregnancy-prevention programmes — a direct link to the Pregnancy-prescribing and Dermatology chapters. The lesson generalises: more is not safer. The fat-soluble vitamins A and D are the ones that accumulate and turn toxic in excess, whereas the water-soluble vitamins are largely cleared. And there is a caution around herbal and "natural" supplements in pregnancy: many are unstandardised, some are uterotonic or hepatotoxic, and "herbal" does not mean "harmless" — a theme the Herbal-medicines and drug-safety chapters develop in full.

The elderly: undernutrition hiding in plain sight

In older adults the danger is usually too little, not too much. Ageing brings a quiet risk of undernutrition — reduced appetite, altered taste, poor dentition, social isolation, depression, and polypharmacy that dulls hunger or irritates the gut. The result is protein-energy malnutrition that erodes muscle (sarcopenia), slows healing and drives frailty, often in a patient who does not look obviously starved. On top of this sit specific deficiencies. Vitamin D is common because of limited sun exposure and thinning skin synthesis, and it matters here for a reason beyond bone: low vitamin D and calcium together increase the risk of falls and fragility fractures, so combined calcium-and-vitamin-D supplementation is a mainstay of bone protection (linking to the Vitamin D chapter and to the Endocrine/osteoporosis section, where the same pair underpins fracture prevention). Vitamin B12 deficiency is also common, driven by atrophic gastritis that strips the stomach of the acid and intrinsic factor needed to absorb it — and worsened by two drugs the elderly commonly take: metformin and proton-pump inhibitors (PPIs), both of which impair B12 absorption (see the B12/folate chapter).

Because the malnourished older patient is easy to miss, the practical answer is systematic screening rather than intuition. A validated tool such as the Malnutrition Universal Screening Tool (MUST) — combining body-mass index, recent unintentional weight loss and acute illness — flags patients who need dietetic input, oral nutritional supplements, or occasionally artificial feeding. The management principle is stepwise: fix the modifiable causes first (review the drug list, treat depression and constipation, sort out dentures and swallowing), fortify and supplement the ordinary diet, and reserve tube or parenteral feeding for those who cannot meet needs by mouth — territory covered in the Nutrition-support and Refeeding chapters, where reintroducing feeding too fast to a severely malnourished person carries its own danger.

Vegans and vegetarians: the nutrients plants can't supply

A well-planned plant diet is healthy — but one vitamin is simply not there. A well-constructed vegetarian or vegan diet supports health at every life stage, but it demands attention to a short list of at-risk nutrients. The absolute one is vitamin B12: it is made by bacteria and reaches humans through animal foods, so a strict plant diet contains essentially none. B12 must be supplemented (or taken as reliably fortified foods) — there is no plant workaround, and because the liver stores years' worth, deficiency creeps in slowly and can cause irreversible neurological damage before the anaemia appears (a warning developed in the B12/folate chapter). Beyond B12, iron and zinc from plants are the non-haem forms, which are less bioavailable and partly blocked by phytates, so intake must be higher and is helped by vitamin C at meals (see the Iron and Trace-elements chapters). Calcium, iodine, vitamin D and the long-chain omega-3 fatty acids round out the list — each obtainable from a thoughtfully planned diet plus targeted fortified foods or supplements. The message is not that plant diets are deficient by nature, but that a few nutrients must be deliberately planned rather than assumed.

Bariatric surgery and malabsorption: a lifelong supplement bill

Bypass the bowel that absorbs a nutrient, and you must replace that nutrient forever. Bariatric surgery works partly by two mechanisms that also cause its nutritional cost: it restricts how much can be eaten, and — in bypass operations — it reroutes food past the stomach and duodenum, the very sites where several nutrients are absorbed. The result is a lifelong risk of deficiency that maps precisely onto anatomy. B12 needs stomach acid and intrinsic factor, and iron and calcium are absorbed mainly in the duodenum and upper jejunum, so a bypass predictably threatens all three. Where fat absorption is reduced, the fat-soluble vitamins A, D, E and K fall too. And one deficiency deserves special fear: thiamine (vitamin B1). Its body stores last only weeks, so a bariatric patient who develops persistent vomiting can slide into acute thiamine deficiency within days — "bariatric beriberi" or Wernicke's encephalopathy — a link straight to the Thiamine and Refeeding chapters. Folate, copper and zinc round out the list. The standard of care is therefore a lifelong multivitamin plus specific supplements (B12, iron, calcium with vitamin D) and periodic monitoring of levels, not a course that ends when the wound heals.

The same anatomy-driven logic applies to chronic malabsorption from disease rather than surgery. Coeliac disease damages the duodenal and jejunal lining where iron, folate and calcium are absorbed; inflammatory bowel disease — especially Crohn's involving the terminal ileum — knocks out B12 and bile-salt (hence fat and fat-soluble vitamin) absorption; pancreatic insufficiency removes the enzymes needed to digest fat, causing fat-soluble vitamin deficiency; and short-bowel syndrome after resection can compromise nearly everything. These conditions live in the Gastrointestinal section, but the nutritional principle is identical to bariatric surgery: identify which absorptive segment is lost, and replace what that segment used to deliver — often for life, and often by a route that bypasses the broken gut.

Two more states: critical illness and obesity

Two further states deserve a mention. Critical illness (sepsis, major trauma, burns) is a catabolic state: the body breaks down its own protein and energy needs rise sharply, so intensive-care nutrition aims to support — not overfeed — a patient whose metabolism is in overdrive. But feeding the severely depleted patient carries the refeeding trap: giving carbohydrate to someone who has been starved triggers an insulin surge that drives phosphate, potassium and magnesium into cells and can cause dangerous hypophosphataemia — which is why thiamine is given and feeding is started low and slow (the Refeeding chapter covers this in full). At the other extreme, obesity is best understood as malnutrition-with-excess: an over-supply of energy that can coexist with genuine micronutrient deficiency, and a state in its own right rather than simply "too much food." Its pharmacological management — notably the GLP-1 receptor agonists that have transformed weight-loss therapy — is a distinct topic developed in the Endocrine section, and sits outside this nutritional chapter.