Calcium for Women: Are There Any Particularities

Calcium is considered a "women's" mineral because of osteoporosis, which affects women far more often. Yet the real female particularities of calcium metabolism are connected not only with menopause: they begin with the formation of bone mass in youth, concern female athletes with an energy deficit, pregnancy, and even premenstrual syndrome. The editorial team examines what of this is confirmed by research.

Calcium and the female skeleton

More than 99% of the calcium in the body is stored in the bones and teeth in the form of hydroxyapatite. The rest is in the blood and cells, where it governs muscle contraction, the transmission of nerve impulses, and blood clotting. The level of calcium in the blood is maintained very strictly: if its intake from food is insufficient, parathyroid hormone "borrows" the mineral from the bones.

In women, bones are on average smaller and thinner than in men, and peak bone mass is lower. In addition, estrogens play a key role in restraining bone breakdown. So any conditions that lower estrogens - menopause, amenorrhea in female athletes, some medical interventions - accelerate the loss of bone tissue.

Peak bone mass is formed by the end of the third decade of life. The higher it is, the greater the "reserve" a woman has at the time of menopause. So for the prevention of osteoporosis, what matters is not only what a woman does at 55, but also how she ate and moved at 15-25.

Calcium is only one of the factors of bone strength. No less important are vitamin D, sufficient intake of protein and energy, strength and impact-loading exercise, and the absence of smoking. A calcium supplement without these components gives a limited effect.

Norms at different stages of life

In its 2011 report, the US Institute of Medicine set calcium norms taking age and sex into account. For women aged 19-50 the recommended norm is 1000 mg per day, the same as for men. The difference appears after 50: for women the norm rises to 1200 mg, whereas for men this happens only after 70. The reason is the accelerated loss of bone mass after menopause.

During pregnancy and lactation the norm does not increase. The body adapts on its own: in pregnant women the absorption of calcium in the intestine is substantially enhanced, and during breastfeeding part of the calcium is temporarily mobilized from the bones and is restored after lactation ends.

The main food sources are dairy products (milk, yogurt, cheese), fortified plant drinks, tofu made with calcium salts, canned fish with bones, almonds, and some green vegetables. Women who do not consume dairy products should deliberately plan their diet or discuss a supplement.

Female athletes and energy deficit

For female athletes the main threat to the bones is not so much a shortage of calcium as low energy availability, when calories are not enough to cover the expenditure on training and the body's basic functions. The International Olympic Committee consensus on RED-S syndrome (Mountjoy et al., 2018) describes how such a deficit disrupts hormonal regulation, in particular menstrual function.

Amenorrhea or infrequent menstruation in a female athlete means a low level of estrogens, and therefore accelerated loss of bone tissue and an increased risk of stress fractures. In such a situation, extra calcium tablets do not compensate for the hormonal deficiency.

The IOC consensus emphasizes that the basis of recovery is increasing energy availability and normalizing menstrual function, while calcium and vitamin D should be sufficient as part of the overall plan. Female athletes with cycle disturbances need examination by a doctor, not only a correction of the diet.

In practice: endurance female athletes, those in weight-class sports (gymnastics, combat sports, running), and those on a plant-based diet should check whether they meet the calcium norm, and also track the regularity of menstruation as a marker of energy status.

Pregnancy, lactation, and PMS

During pregnancy calcium has one more proven role. In populations with low calcium intake, its additional intake reduces the risk of preeclampsia - a dangerous complication involving elevated blood pressure. On this basis the WHO, in its 2013 guideline, recommends calcium supplements for pregnant women precisely in populations with low dietary calcium intake. For women who already get enough calcium, such an effect is not expected.

Lactation is accompanied by a temporary decrease in bone mineral density, which cannot be fully prevented with additional calcium. Studies show that after breastfeeding ends, density usually recovers. So for a breastfeeding woman it is enough to maintain the usual norm.

An interesting, though less well-known, topic is premenstrual syndrome. In a multicenter randomized study by Thys-Jacobs and co-authors (1998), taking 1200 mg of calcium as carbonate per day reduced the severity of PMS symptoms (mood swings, fluid retention, food cravings, pain) compared with placebo. This is one of the few interventions for PMS with a randomized evidence base, though it does not suit everyone.

• Pregnancy: the norm does not increase, but in populations with low intake, supplements reduce the risk of preeclampsia.
• Lactation: the loss of bone mass is temporary, mega-doses are not needed.
• PMS: there are randomized data in favor of calcium; the decision should be discussed with a gynecologist.

Supplements: forms, doses, risks

The most common forms are calcium carbonate and citrate. Carbonate contains about 40% elemental calcium, is cheap, but requires stomach acid for absorption, so it is better taken with food. Citrate contains about 21% calcium, is absorbed independently of acidity (Recker, 1985), and is better suited to people taking drugs that reduce stomach acidity.

The efficiency of absorption decreases as the single dose increases, so large amounts are usually split into intakes of 500 mg or less. The main principle is that a supplement should only top up the diet to the norm, not be added to already sufficient intake.

Supplements carry risks. In the large Women's Health Initiative study (Jackson et al., 2006), postmenopausal women taking calcium with vitamin D more often developed kidney stones. The meta-analysis by Bolland and co-authors (2010) raised the question of a possible increase in the risk of myocardial infarction with calcium supplements without vitamin D. These data are debatable, but they are an argument in favor of getting calcium mainly from food.

Calcium also reduces the absorption of iron, levothyroxine, some antibiotics, and drugs for treating osteoporosis, so these are taken at an interval. Digestive side effects, in particular constipation and bloating, more often occur with carbonate.

Editorial conclusions

The calcium norm for women under 50 is the same as for men - 1000 mg per day - and after 50 it rises to 1200 mg because of the accelerated loss of bone mass after menopause.

For young women and female athletes the priority is the formation of peak bone mass and sufficient energy availability; calcium does not compensate for a hormonal deficiency in cases of cycle disturbances.

During pregnancy calcium is useful for preventing preeclampsia in women with low intake, and for PMS it has randomized confirmation of an effect. Supplements are safer to use as a complement to the diet rather than a replacement for food.

The editorial team also recommends reading the materials "Calcium After 40," "Vitamin K2 for Women: Are There Any Particularities," and "Iron for Women: Are There Any Particularities."

References

1. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
2. Mountjoy M, Sundgot-Borgen JK, Burke LM, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. Br J Sports Med. 2018;52(11):687–697.
3. World Health Organization. Guideline: Calcium supplementation in pregnant women. Geneva: WHO; 2013.
4. Thys-Jacobs S, Starkey P, Bernstein D, Tian J. Calcium carbonate and the premenstrual syndrome: effects on premenstrual and menstrual symptoms. Am J Obstet Gynecol. 1998;179(2):444–452.
5. Jackson RD, LaCroix AZ, Gass M, et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med. 2006;354(7):669–683.
6. Bolland MJ, Avenell A, Baron JA, et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. BMJ. 2010;341:c3691.
7. Recker RR. Calcium absorption and achlorhydria. N Engl J Med. 1985;313(2):70–73.