The recommended daily intake of calcium for the treatment of hypoparathyroidism is between 1200 – 2000 milligrams (mg)/day. This is in comparison to the recommended amount for the general adult UK population which is 700mg/day. Although calcium is widely available, working out the best sources to get it from can be confusing.
In hypoparathyroidism, this daily calcium requirement should be met through the food we eat rather than depending on too many calcium supplements. Getting calcium through food is easier on our gut and a much healthier choice for our kidneys. Food sources of calcium also contain many other vitamins and minerals which are essential for good health.
Ideally, calcium supplements should be limited to one or two tablets a day and for emergency situations.
Food Sources of Calcium
Milk and other dairy products are the most important sources of calcium in the western world and up to 75% of daily calcium intake can come from them alone. One 250ml glass of milk contains 300mg of calcium and a 30g slice of cheddar cheese contains roughly 220mg so achieving the regular 700mg/day requirement can be reasonably simple.
However, when that requirement rises to 1200-2000mg/day or more, obtaining calcium mainly through dairy is going to result in a diet which is also very high in protein and phosphate, both of which can be detrimental to kidney function in hypoparathyroidism. Fortunately, there are many excellent non-dairy calcium sources available which can contribute to our daily needs.
Fish with soft bones like sardines, tinned salmon and whitebait, leafy greens such as kale and bok choy, broccoli, dried figs, almonds, and calcium fortified foods like soya milk, soya yoghurt, tofu, bread, cereals, and orange juice can all provide substantial amounts of calcium.
The two most common forms of calcium supplements are calcium carbonate and calcium citrate. Calcium carbonate contains a mix of calcium and carbonic acid and is the most used supplement because it contains the highest amount of elemental calcium at 40%. You’ll see this in preparations such as Adcal or Calcichew, which will say for example, 1.25g calcium carbonate equivalent to 500mg calcium or 750mg equivalent to 300mg – the calcium component always being 40% of the total. In comparison, calcium citrate, a mix of calcium and citric acid, contains only 21% elemental calcium. Other preparations also exist but these do not contain enough calcium to be useful as supplements.
To effectively break down in the stomach, so that it can then be absorbed in the small intestine, calcium carbonate should be always taken with food. It needs to be in an acidic environment, and stomach acid increases when you eat a meal. The other benefit of taking calcium carbonate supplements with food, particularly if you’re eating a meaty or phosphate heavy meal, is that they act as phosphate binders, reducing the amount of phosphate available for absorption.
Formulations of calcium also exist which claim to be more beneficial because they are slow-release. What this means is the tablet is covered in a hard outer coating which prevents it from dissolving in the stomach and entering the small intestine quickly. The science behind this is that calcium is absorbed better in smaller amounts and so having it slowly reach the intestine will result in better absorption. While this is correct, the same effect can be achieved by simply taking a normal calcium tablet with a meal. Having a full stomach slows down the rate at which food empties into the small intestine. In general, the only real difference between a standard calcium supplement and the slow-release variety is the length of time it spends in your stomach. Once the calcium empties into the small intestine the amount of time it takes to then be absorbed is the same for both supplements, around 3½ hours. Slow-release calcium is not recommended for use in hypoparathyroidism. Supplements which are broken down quicker in the stomach are preferred since they can provide us with a faster result.
Absorption and Bioavailability
One of the interesting facts about calcium is that eating more does not result in absorbing more. In fact, the larger the serve the less you will absorb.
Studies have shown that we can only absorb serves of up to 500mg of calcium at a time and so our calcium intake should be spread throughout the day.
The 500mg threshold doesn’t mean however that if we were to drink 400ml of milk we would absorb all the 500mg of calcium that it contains.
Regardless of whether calcium is coming from food or supplements we only absorb a small percentage of what we eat. Calcium is a large mineral and our ability to break it down and absorb it depends on a variety of factors. In a mixed diet only 25-35% of the calcium we eat is absorbed. This means that approximately 65-75% passes through our digestive tract and is excreted in our faeces. The amount that a calcium source can be absorbed and then used by the body is called its bioavailability.
Dairy products have a reasonably high bioavailability of around 32% due to the protein, casein, that they contain. In the example above, around 160mg of calcium would be absorbed from the 400ml serving of milk. For supplements, the bioavailability is similar and with a 1.25g calcium carbonate tablet containing 500mg of elemental calcium, you would absorb approximately 150mg.
Some plant foods contain less calcium overall but have a higher bioavailability. For example, 1 cup of cooked bok choy contains around 160mg of calcium and has a bioavailability of 53% resulting in an absorption of 85mg. Others like spinach have a relatively high calcium content but very low bioavailability. One cup of cooked spinach contains approximately 250mg of calcium but with a bioavailability of 5% only 12mg is available for absorption!
For most healthy people, who are eating a diet consisting of dairy products and a variety of other foods, it’s usually not necessary to worry too much about bioavailability. For these people, their varied diet will provide enough calcium. For those, though, who eat no or very few dairy products or for those with compromised gut absorption, being aware of bioavailability and the factors that can both improve and reduce your calcium absorption can be very helpful.
Table 1. Food sources of bioavailable calcium
|Food source||Serving size (g) *||Calcium content (mg/serving)||Absorption (%)||Estimated calcium absorbed (mg)||Servings needed to equal 1 cup of milk|
|Orange juice (calcium added)||240||300||36.3||109||0.8|
|Soy milk (calcium added)||240||300||21.1||66.3||1.0|
|Tofu (calcium added)||126||258||31.0||80.0||1.2|
|Whole wheat bread||28||20||82.0||16.6||5.8|
|Wheat bran cereal||28||20||38.0||7.5||12.8|
*Based on one-half cup serving size (approx. 85g for green leafy vegetables), except for milk (1 cup or 240ml) and cheese (42.5g)
Factors that can reduce calcium absorption
There are several factors which can lead to a decrease in the amount of calcium we absorb. These include:
- Increasing age
- Decreased stomach acid
- Menopause (due to a decrease in oestrogen)
- Vitamin D deficiency
- Some diseases including inflammatory bowel disease, coeliac disease, renal failure, and lactose intolerance
- Eating large volumes of calcium in a single serve
- Taking calcium tablets without a meal
- Vegan diets
- High salt diets (due to increased urinary calcium excretion)
Some foods also contain naturally occurring compounds which can decrease the bioavailability of calcium. These compounds are called antinutrients and are designed to protect the plants they occur in from bacterial infection and from being eaten by insects. The two antinutrients which are of importance when it comes to calcium absorption are called oxalates and phytates.
Oxalates: Oxalates are strong blockers of calcium absorption. The oxalates in plant foods bind to calcium and because we don’t have any enzymes in our gut to break this bond, the calcium is unable to be absorbed. Food preparation techniques such as boiling (rather than steaming or baking) can reduce the oxalate content of these foods and improve the calcium bioavailability.
Oxalates occur in high amounts in spinach, rhubarb, soy products, almonds, potatoes and sweet potatoes, beets, white beans, raspberries, and tea.
Phytates: Like oxalates, phytates are naturally occurring compounds found in many plant foods. While they are not as potent as oxalates, the presence of phytates can still inhibit calcium absorption. Cooking, soaking, and fermenting these foods can all help to reduce their phytic acid content.
Phytates are largely found in whole grain cereals, bran, oats, black beans, kidney beans, soy products, almonds, peanuts, lentils, and seeds.
The effects of oxalates and phytates aren’t cumulative, they only effect the meal they are being eaten at. They can however affect the absorption of calcium from other foods or supplements eaten at the same meal. For example, in a meal containing both spinach and cheese, the oxalates from the spinach will bind with the calcium in the cheese and reduce its bioavailability during that meal.
Because oxalates and phytates are found in so many foods which are considered healthy and nutritious, it is not recommended that you avoid or limit them. Instead, be aware that they may not provide significant amounts of calcium to your diet (even if they contain large amounts) and try to schedule your meals so that any high oxalate and phytate foods are not eaten at the same time as calcium rich ones.
Factors that can improve calcium absorption
While there are many factors that can reduce our ability to absorb calcium, fortunately there are also a few which can help to increase it.
- Maintain your vitamin D levels (recommended level for hypoparathyroidism is 75mmol/L) through supplements and food sources such as salmon, tuna and swordfish, cheese, mushrooms, and egg yolks
- Divide your calcium intake throughout the day by including some in each meal and snack
- Always take your calcium supplements with food
- Keep the salt content of your diet low
- Eat foods rich in Vitamin C with your calcium meal or supplement. Vitamin C is an important antioxidant and studies have shown that it can enhance the amount of calcium absorbed in the intestine
- Include some protein with your calcium containing meal or supplement. Protein binds with calcium making it more soluble and therefore more available for absorption
There is also evidence that prebiotics can be beneficial when trying to increase calcium absorption. While most calcium absorption occurs in the small intestine, a small amount (around 10%) also happens in the large intestine. Prebiotics containing galacto-oligosaccharides are fermented by bacteria in the large intestine, this lowers the pH and improves our ability to absorb more calcium there.
By Tania Gelmetti