Nutrition and Bioavailability

Learning objectives

• State the nutrients of attention on a plant-based diet and the practical strategies to cover each.
• Define bioavailability and explain why two foods with the same iron content may deliver very different iron to the body.
• Explain why B12 supplementation (or fortification) is essential on a vegan diet.
• Distinguish ALA, EPA, and DHA omega-3 fatty acids and their plant-based sources.
• Discuss "antinutrients" with appropriate calibration — neither dismissing nor exaggerating their effects.

Protein — for nutrition this time

A well-planned plant-based diet meets and often exceeds protein needs. The classic adult recommendation of 0.8 g per kg body weight is easy to hit with a varied diet of legumes, grains, soy, nuts, and seeds. Athletes, pregnant or lactating people, and the elderly should aim higher (1.2–2.0 g/kg). The math works out comfortably with two to three servings of legume-family foods per day.

Module 2 covered the amino-acid quality side. The short version: eat a variety of plant proteins across the day. The myth of meal-by-meal "complementarity" has been overturned — your body's amino acid pool smooths things out across roughly 24 hours.

Vitamin B12 — the one nutrient with no plant source

Cobalamin (B12) is a complex cobalt-containing molecule made by certain bacteria and archaea. Plants don't make it; reliable B12 in the food supply comes from animal foods — where the animals themselves got it from microbes in their gut or feed — and from fortified foods or supplements.

Deficiency develops slowly (your liver stores 2–5 years' worth) but is serious: irreversible nerve damage if untreated. The current consensus is unambiguous: everyone on a strictly vegan diet must take a B12 supplement or eat reliably fortified foods.

Practical guidance

• A daily supplement of 50–100 µg cyanocobalamin or methylcobalamin, OR
• A weekly supplement of 1000 µg, OR
• Daily fortified foods (yeast flakes, plant milks, breakfast cereals — check the label) totaling at least 6 µg/day across multiple servings.

Ironically, much "nutritional yeast" actually does supply B12 — but only the fortified kind. Read the label every time. Spirulina and some fermented foods contain B12 analogues that can confuse blood tests but do not function biologically. Don't rely on them.

Iron and bioavailability

Plant foods contain plenty of iron — beans, lentils, tofu, dark leafy greens, fortified cereals, dried fruit. The catch is the form: plant iron is non-heme iron, which is absorbed at roughly 2–20% efficiency depending on what else is in the meal. Heme iron (from animal foods) is absorbed at 15–35% with much less variability.

What boosts non-heme iron absorption

• Vitamin C in the same meal can multiply absorption by 3–4×. Squeeze lemon over your lentils, drink orange juice with iron-fortified cereal.
• Cooking in cast iron measurably leaches iron into food, especially acidic preparations.
• Acidic environment in the stomach generally helps absorption — vinegar, citrus, fermented foods.

What inhibits it

• Calcium, especially from supplements or fortified milks consumed with iron-rich meals. Separate by 2 hours.
• Tea and coffee polyphenols (especially within an hour of eating). Tannins bind iron in the gut.
• Phytic acid in unprocessed legumes and grains. Soaking, sprouting, and fermenting (sourdough, tempeh) reduce phytate substantially.

The practical upshot: well-planned plant-based diets typically deliver adequate iron. Population-level studies show vegans have similar iron deficiency rates to omnivores — slightly higher in menstruating women, but the effect is largely manageable with the practices above.

Omega-3 fatty acids — and the algae trick

There are three omega-3s of dietary interest:

• ALA (alpha-linolenic acid): short-chain omega-3, plentiful in flax, chia, hemp, walnuts, and canola/rapeseed oil.
• EPA (eicosapentaenoic acid): long-chain, found mainly in oily fish — but the fish get it from microalgae.
• DHA (docosahexaenoic acid): long-chain, critical for brain and eye function — also from algae upstream of the fish.

The body can convert ALA into EPA and DHA, but inefficiently (typically < 5% conversion to EPA, < 0.5% to DHA in adults). For most vegans, this is fine if ALA intake is generous, but for pregnant people, infants, and the elderly the safest play is to take an algae-derived DHA/EPA supplement — the same upstream source fish use, served directly.

Plant-based EPA and DHA aren't a workaround. Algal oil is the original source. The fish are the middlemen.

Calcium, vitamin D, and iodine

Calcium

Plenty of plant sources: kale, collards, bok choy, almonds, sesame seeds (tahini), calcium-set tofu, fortified plant milks. Note that spinach and Swiss chard are high in oxalates, which bind calcium and reduce absorption. Don't rely on them. Aim for 700–1000 mg/day.

Vitamin D

No reliable food sources, plant or otherwise — most "dietary" vitamin D is fortified or supplemental. Sun exposure synthesizes it in the skin, but latitude, season, sunscreen, and skin tone all affect this dramatically. Many vegans (and many omnivores in northern climates) benefit from a daily supplement of vitamin D₃ (now widely available in vegan, lichen-derived form).

Iodine

Plant foods are generally low in iodine; sea vegetables are highly variable (kelp can be excessive). Iodized salt is the easiest reliable source for most populations. A serving of nori sheets several times a week works too.

Antinutrients in context

"Antinutrients" is a misleadingly scary term for naturally occurring plant compounds that interfere with nutrient absorption. The major ones:

• Phytic acid in whole grains, legumes, nuts, seeds — binds iron, zinc, calcium.
• Oxalates in spinach, chard, beet greens, rhubarb — bind calcium.
• Tannins in tea, coffee, some legumes — bind iron and protein.
• Lectins in raw legumes — interfere with absorption; destroyed by proper cooking.
• Trypsin inhibitors in raw soy — destroyed by cooking.

The honest read: these compounds are real, measurable, and worth knowing about — but they're also dramatically reduced by ordinary food processing (soaking, sprouting, cooking, fermenting), and the health benefits of the foods that contain them vastly outweigh the minor absorption penalties. A varied diet rich in legumes, whole grains, and vegetables is associated with better — not worse — health outcomes across nearly every measured endpoint.

Sourdough fermentation reduces phytate in bread by 50–90%. Tempeh's fermentation reduces it in soy. Cooking destroys lectins and trypsin inhibitors. Most kitchen wisdom around legumes and grains — soak, rinse, cook well — has been quietly solving this problem for millennia.

The science of fortification

Many of the world's biggest public-health wins are quiet acts of fortification: iodized salt eliminated cretinism in vast regions; vitamin D milk wiped out widespread rickets; folic acid in flour has prevented hundreds of thousands of neural tube defects.

Plant-based product designers have a special opportunity — and responsibility — to fortify wisely. A few principles:

• Match form to delivery vehicle. Vitamin D₂ is fine in a beverage; vitamin D₃ (now lichen-derived) is more bioavailable in some uses.
• Watch for interactions. Iron and vitamin C boost each other; iron and calcium fight. A "complete" milk needs to think about both.
• Don't fortify above tolerable upper limits. Some nutrients (zinc, iron, niacin, vitamin A) have real toxicity at high doses.
• Be honest on the label. If you fortify with B12, say so — it lets your customer plan their diet.