Loading…
Story
food source · 6
Canned sardines come with soft, edible bones and have about 300–380 mg of calcium per 100 g, close to a glass of milk, which makes them a rare non-dairy calcium source for people who do not drink milk.
Mechanism · Chew it, then let acid dissolve it
The calcium in soft bones is locked in a crystal called hydroxyapatite. It has to be chewed apart and then soaked in stomach acid before the crystal surface releases calcium ions that can be absorbed; swallowed whole, too little surface is exposed to the acid. The active route that carries calcium across the gut wall is controlled by vitamin D, and sardines carry vitamin D themselves.
Numbers · How much calcium a can holds
Canned sardines have about 300–380 mg of calcium per 100 g, close to a glass of milk. The can is already salty, so do not add much more salt when you serve them.
←Milk
Milk is a dependable source of calcium: a 250 ml glass holds about 300 mg, and about a third of it is absorbed. By the mechanism, phosphopeptides cut from casein during digestion, together with lactose, help keep calcium in an absorbable state, but how much they matter in adults is not settled.
Mechanism · Why calcium has to stay dissolved
Calcium has to be a free ion dissolved in the gut fluid before the carrier proteins in the gut wall recognize it. The farther down the small intestine, the more alkaline it gets, and the more easily calcium comes out of solution and clumps. Milk brings two possible helpers: casein phosphopeptides, cut from casein during digestion, hold calcium ions like small clips and keep them soluble; lactose may make it easier for calcium to seep through the gaps between the cells of the gut wall. Both make sense by the mechanism, but studies disagree on how much extra calcium they bring in for adults. Spinach is the opposite case: its oxalate binds calcium on contact into nearly insoluble calcium oxalate. The calcium content of spinach is real, but little of it is absorbed.
Numbers · How much calcium a glass holds
Milk has about 120 mg of calcium per 100 g, or about 300 mg in a 250 ml glass, close to a third of the US adult Recommended Dietary Allowance (RDA). About 30–35% of it is absorbed, against about 20% from almonds, while the oxalate in spinach locks up nearly all of its calcium. Skim milk loses no calcium: calcium stays in the watery part with the casein, and skimming removes only the fat. Natural milk has little vitamin D; many regions fortify it, which puts calcium and the vitamin that helps it into the blood in the same glass.
Yogurt has about as much calcium as milk, and it is absorbed about as well, because fermentation mainly changes the lactose and leaves the casein. When Greek yogurt is strained, some of the calcium leaves with the whey.
Mechanism · Fermentation leaves the casein alone
During fermentation, lactic acid bacteria eat part of the lactose but leave the casein largely untouched, so the phosphopeptides cut from it during digestion are still there, and yogurt's calcium is absorbed about as well as milk's. Greek yogurt is concentrated by draining off the whey, and some of the minerals dissolved in the whey drain away with it, so it holds somewhat less calcium.
Numbers · Calcium in regular and Greek yogurt
Plain yogurt has about 120 mg of calcium per 100 g and Greek yogurt about 85–110 mg, so both are still good sources; getting vitamin D at the same time helps calcium absorption. Yogurt is not a good source of vitamin K2: its two usual cultures make far less K2 than the lactic acid bacteria in cheese, and MK-7 comes almost only from natto.
When cheese is made, the protein and most of the calcium stay in the curd while water and lactose drain off with the whey, so the calcium ends up highly concentrated. Sodium needs watching too: many cheeses have a lot of salt added for preservation and flavor.
Mechanism · Calcium stays with the casein
When milk curdles, the whey carries away some of the minerals dissolved in water, but most of the calcium is bound to casein and stays in the curd. About 30% of the calcium in cheese is absorbed, similar to milk. Hard cheese holds little water, so it has more calcium by weight.
Numbers · Calcium and sodium in a small piece
Parmesan has about 1,184 mg of calcium per 100 g and Cheddar about 721 mg; 30–40 g a day supplies roughly 20–45% of the adult recommended intake of 1,000 mg. Count the sodium as well: Parmesan has about 1,600 mg per 100 g and Cheddar about 600 mg, which people with high blood pressure or on a low-salt diet should add to their daily total. Cheese also contains vitamin K2, but mainly the MK-8 and MK-9 forms made by lactic acid bacteria, not the MK-7 in natto.
The calcium in tofu mostly comes in with the coagulant: firm tofu set with calcium sulfate (gypsum) can have 150–350 mg of calcium per 100 g, while tofu set with nigari or glucono-delta-lactone carries almost none, so check the coagulant on the label.
Mechanism · The coagulant brings the calcium
Soybeans are not especially rich in calcium, and neither is soy milk; it is the setting step that puts calcium into the curd. Set with calcium sulfate, the calcium goes straight into the tofu; set with nigari (magnesium chloride) or glucono-delta-lactone, it carries almost none. So checking which coagulant is on the label tells you more than the slogan that tofu is good for calcium.
Numbers · Calcium in gypsum-set tofu
Firm tofu set with calcium sulfate can have 150–350 mg of calcium per 100 g, more than the same weight of milk, and it is absorbed about as well as milk's; tofu set with nigari or glucono-delta-lactone has much less. If you are counting on tofu for calcium, check the coagulant first.
Most of the calcium in sesame is in the outer seed coat, the hull, and so is the oxalate that locks calcium up. The calcium in unhulled sesame is real, but its absorption is discounted, and hulling removes a large part of it. Sesame can add some calcium but cannot replace milk.
Mechanism · Calcium and oxalate share a layer
Calcium has to stay a dissolved ion to cross the gut wall. Most of the calcium in sesame is in the outer seed coat, the oxalate sits in the same layer, and a good share of the calcium is already bound to oxalate as insoluble calcium oxalate; the phytate in sesame locks up part of the rest. Hulled sesame loses that whole layer and a large part of the calcium with it; only unhulled sesame is worth counting for calcium, and even then its absorption is discounted.
In practice · Unhulled, ground, and only a side source
Tahini ground from unhulled seeds keeps much more calcium than tahini from hulled seeds, and whole sesame seeds often pass through undigested, so ground sesame is better. In the amounts people actually eat, a spoonful of sesame brings far less calcium than a glass of milk, so sesame is better treated as a supplementary calcium source. The calories in tahini come from its oil, not from the little calcium that gets absorbed.
synergy · 5
Calcium triggers a muscle contraction; relaxing means pumping the calcium back into storage, and that pump runs on ATP bound to magnesium. So both calcium and magnesium need to be adequate, and a large dose of one does not make up for a shortage of the other.
The mineral in bone is mainly a crystal of calcium and phosphate (hydroxyapatite). Healthy bone takes more than calcium: it also needs phosphorus, vitamin D and exercise that puts load on the bones.
Counterintuitively, more calcium in the diet goes with fewer calcium-oxalate stones. In the gut, calcium binds oxalate and both leave in the stool, so less oxalate is absorbed into the blood and ends up in the urine. In Curhan 1993's cohort of men, those in the top fifth of dietary calcium had about 0.56 times the stone risk of the bottom fifth (an association). A calcium supplement taken alone, away from meals, cannot do this binding, and Curhan 1997 found it linked with a slightly higher stone risk in women. What matters is calcium that comes with food.
Fluoride acts mainly on teeth: when acid dissolves a little tooth enamel, calcium and phosphate settle back in, and fluoride takes part in this repair, making the rebuilt enamel more resistant to acid.
A potassium-rich diet, especially the potassium that comes as potassium citrate and similar forms in fruit and vegetables, reduces the calcium lost in urine, which the mechanism suggests may indirectly help bone. It is the opposite of high sodium, which increases calcium in the urine.
cofactor · 5
In severe magnesium deficiency, the parathyroid glands release less parathyroid hormone and the body responds to it less, so blood calcium falls and calcium supplements alone cannot correct it. In the clinic, magnesium has to be replaced first, and blood calcium then rises with it.
→Bone
99% of your calcium is stored in bone as hydroxyapatite crystals grown along collagen fibers. When blood calcium dips, parathyroid hormone sends osteoclasts to draw calcium back out of bone into the blood. Bone is calcium's checking account, with money going in and out, not a vault that only takes deposits.
The moment a nerve signal arrives, the sarcoplasmic reticulum releases its stored calcium into the cell; calcium binds to troponin, and only then is myosin's binding site exposed. Without calcium, no cross-bridges form between the filaments and the muscle cannot contract.
Calcium is the building material for bone, but absorption has a ceiling: above roughly 500 mg at once, the share absorbed falls, so supplements should be taken with meals and in divided doses. Food comes first and supplements only fill the gap; take too much at once and little of the extra is absorbed.
Otoconia, the inner ear's "ear stones", are calcium carbonate crystals that are denser than the fluid around them. When you tilt your head, gravity pulls on the gel-like membrane they sit on, the membrane drags the hair cells' bundles to one side, and the brain learns how your head is oriented relative to gravity.
antagonism · 3
↮Iron
Calcium lowers the absorption of iron eaten in the same meal, and both non-heme and heme iron are affected. People who need more iron can take calcium tablets or large servings of dairy at a different time from their iron supplement or iron-rich meal.
When you eat a lot of sodium, the kidneys excrete more calcium along with the sodium, so urinary calcium rises. Years of high sodium, combined with too little calcium and vitamin D, may quietly speed up bone loss.
↮Zinc
A diet very high in calcium slightly lowers zinc absorption. People who get enough zinc need not worry; it is worth attention only for those whose zinc intake is already borderline.
depletes · 1
Caffeine does slightly depress calcium absorption in the gut, but only very slightly, and it has no effect on the total calcium passed in urine over 24 hours (Heaney 2002). How slight? One or two tablespoons of milk fully offset it. In people who meet their calcium needs there is no evidence that caffeine harms bone. It is worth knowing precisely because it is small: a real interaction that marketing has inflated.
regulates · 4
Active vitamin D (calcitriol) makes gut cells build the channels and carrier proteins that actively absorb calcium. When vitamin D is low, this active route stays barely open, and much less of the calcium you eat gets absorbed.
K2 influences where absorbed calcium ends up: it activates osteocalcin, which helps lay calcium into bone, and matrix Gla protein, which helps keep calcium from settling in artery walls. In the Rotterdam Study, people who ate more K2 had a lower risk of coronary heart disease; this is an association.
Estrogen has been holding back osteoclasts, the cells that break bone down. After menopause estrogen falls sharply, the brake comes off, and bone calcium is lost faster than it is laid down; the loss is concentrated in the years around the final menstrual period and then settles into a slower, steady decline.
←After birth and breastfeeding
During breastfeeding most of the calcium in milk is borrowed from the mother's bones: spine bone density falls 5% to 10% on average over 3 to 6 months of exclusive breastfeeding. In a randomized trial, calcium supplements did not stop the fall or raise milk calcium; bone is rebuilt after weaning.