Calcium is an essential nutrient that needs to be included in the diet because our bodies cannot manufacture it. Some examples of food sources that are rich in calcium include:
- Milk and dairy products
- Fish with edible bones such as canned sardines and anchovies
- Beans, tofu, soy beans, and vegetables such as spinach, watercress and broccoli
Some foods are also fortified with calcium such as milk, yogurt, breakfast cereals, rice, and even biscuits.
When assessing the calcium that can be obtained from foods, usually the calcium content is more important than bioavailability as the calcium absorption efficiency is similar between most foods. However, the bioavailability of calcium can be affected by oxalate and phytate present in plant-based foods. These bind to or chelate with the calcium and prevent its absorption. The oxalic acid content is high in foods such as spinach, chocolate or cocoa products and lower in beans, sweet potato, soybean products, kale and okra. Phytic acid is found in seeds and in soybeans.
Factors affecting calcium absorption
Several factors influence how calcium in the diet is absorbed, as well the amount of calcium the body requires. Some examples of these factors include:
- When the calcium needed in the body is low, the calcium absorption also declines.
- As a person ages, the absorption of calcium declines and their requirement for calcium intake increases.
- Factors that affect absorption include calcium intake, estrogen status, vitamin D status and the transit time within the intestine.
- Phytic acid in cereals and pulses inhibit the absorption of calcium from food, as do oxalates present in some types of vegetables.
- Other foods such as lactose and non-digestible polysaccharides increase calcium absorption.
- Phosphorus exists as an important structural component of bone in the form of calcium phosphate or hydroxyapatite. When blood calcium levels are low, parathyroid hormone is secreted, which stimulates the conversion of vitamin D to its biologically active form, calcitrol. This then activates the vitamin D-dependent absorption of dietary calcium and phosphorus from the gut. Parathyroid hormone also increases phosphate excretion from the urine and decreases calcium excretion from urine.
- Sodium competes with calcium in the kidneys for re-absorption because these minerals use the same transport system. A high intake of salt (sodium chloride) increases the amount of sodium absorbed while increasing the loss of calcium.
- Excessive caffeine intake sharply increases urinary calcium losses, especially in adults and postmenopausal women.
- Calcium absorption may differ between ethnic groups as Chinese women have been reported to absorb more calcium than white women. However, this may represent adaptation to a lower availability and intake of calcium in the Chinese.
For normal growth of bones and the skeleton, infants need around 120 mg of calcium each day. However, the proportion of calcium absorbed from breast milk is only about 55% to 60% and for infant formula, it is about 40%. Therefore, the estimated daily recommended calcium intake for babies are as follows:
- Breast fed babies aged 0 – 5 months - 300 mg/day
- Formula fed babies aged 0 – 5 months - 400 mg/day
Children & adolescents
For children and adults, the recommended amount is:
- Children aged 1 to 3 years - 500 mg/day
- Children aged 4 to 6 years - 600 mg/day
- Children aged 7 to 9 years - 700 mg/day
- Boys aged 10 to 18 years - 1,000 mg/day
- Girls aged 10 to 18 years - 1,000 mg/day
Among adults, the recommended amount is:
- Men aged 19 to 65 years - 800 mg/day
- Men aged more than 65 years - 1,000 mg/day
- Women aged 19 to 50 years - 800 mg/day
- Women aged more than 51 years - 1,000 mg/day
Pregnant and lactating women
Among pregnant women, the recommended amount is:
- First trimester - 1,000 mg/day
- Second trimester - 1,000 mg/day
- Third trimester – 1000 mg/day
For lactating women, the recommended daily intake of calcium is also 1,000 mg/day.
Dietary calcium supplements
For those who do not take in adequate amounts of calcium in the diet, supplements are recommended. Altogether, the diet should contain no more than 2500 mg of calcium per day. Several different types of supplements are available but calcium carbonate and calcium citrate are the most common types of supplement. Calcium carbonate should be taken after food, whereas calcium citrate may be taken at any time of day. Calcium supplements should not be taken with high fibre meals and an adequate fluid intake is advised to prevent kidney stones.
Reviewed by Sally Robertson, BSc