As we know, milk and dairy products are rich in calcium. What is perhaps a little less well known is that dairy calcium has a good bioavailability, i.e. it is easily assimilated by the body. In contrast, most vegetable sources contain less calcium or offer calcium with lower bioavailability.
According to the National Institutes of Health (NIH), “calcium is necessary for vascular contraction and vasodilation, muscle function, nerve transmission, intracellular signaling, and hormonal secretion.
In Canada, nutrition recommendations (Dietary Reference Intakes (DRIs)) are developed in collaboration with the United States by expert committees of the Institute of Medicine (IOM), which base their decisions on a thorough review of the available scientific evidence. For calcium, recommended intakes are defined as those necessary to maintain calcium balance and bone health.
The calcium requirements specific to the different age categories are presented in the table below. Although recommendations may differ from country to country, the DRIs are relatively consistent with most national and international recommendations. For example, according to the IOM, adults need 1000 mg of calcium per day3, a recommendation similar to that of the World Health Organization (WHO)
Milk and dairy products, such as yogurt and cheese, provide good amounts of calcium. In addition, cow’s milk has good bioavailability, with a calcium absorption rate of about 30-35%.
While some dark green vegetables, such as broccoli and kale, have a higher absorption rate, they contain less calcium. To get the same amount of calcium found in a cup (1 serving) of milk, more than two cups (4 servings) of broccoli must be consumed.
In addition, the bioavailability of calcium from plant sources may be compromised by the presence of certain compounds. Some foods contain oxalic acid and phytic acid, which bind to calcium to form insoluble complexes, interfering with calcium absorption. These foods are considered to be low sources of calcium.
Examples of foods containing high levels of oxalic acid include spinach and rhubarb.
- Eating spinach and milk at the same time reduces the absorption of calcium from milk;
- When assessing calcium intake, PEN (Practice-Based Evidence in Nutrition), the Dietitians of Canada’s knowledge transfer tool, indicates that calcium in spinach and rhubarb should not be considered.
- Foods with high levels of phytic acid include wheat bran and whole-grain products, which are high in fiber.
- However, since the Canadian population’s diet is generally low in dietary fiber, it is not recommended to limit phytic acid intake.
Since the bioavailability of calcium from plant sources may be lower, these sources can be a barrier to adequate calcium intake. Therefore, bioavailability is a parameter to be assessed when the diet is composed solely of vegetable sources of calcium.
In conclusion, although milk and dairy products are not the only sources of calcium, consuming these foods as part of a healthy diet helps to meet calcium needs more easily.