Calcium Absorption – it’s not how many nutrients and micronutrients are in a food that ultimately matters, it’s what percentage your body can absorb and make use of. For example, food can be loaded with a particular vitamin or mineral, but if your body can’t take it up and utilize it, it’s worthless from a health standpoint.
An example of a micronutrient that your body doesn’t absorb completely is calcium. Calcium is an essential mineral your body needs in substantial quantities on a daily basis. We think of calcium as being the “bone health” mineral, but you need it on a minute-by-minute basis to sustain life. For example, calcium helps sustain your heart beat, muscle contractions, and even gives your blood the ability to clot. It’s the ultimate multi-tasking mineral!
At one time, it was common for women to take calcium supplements to protect against bone loss, yet, based on recent research, there’s no strong evidence that supplemental calcium preserves bone health, and there may be harms associated with taking calcium supplements. Some studies suggest that calcium supplements raise the risk of heart attacks while others do not. Plus, calcium supplements, but not dietary calcium, increases the risk of kidney stones.
With the safety of calcium supplements under question, most experts recommend getting calcium through diet. But, even if you eat a calcium-rich diet, you don’t absorb all of the calcium from these foods, and the amount of calcium you can absorb varies with the food you eat.
Good Food Sources of Calcium
We think of dairy products as being the ultimate source of calcium and dairy foods are rich in this essential mineral. However, plant-based foods contain a substantial amount of calcium as well. Leafy green vegetables and broccoli are top sources of calcium, while calcium is in a variety of packaged foods. This source of calcium stems from the process of fortification or adding additional vitamins and minerals to make a packaged food more nutritious. Foods fortified with calcium include some fruit juices, breakfast cereals, and plant-based milk. In addition, a variety of nuts and seeds, such as almonds and chia seeds, contain a substantial quantity of calcium. Tofu, too, is rich in calcium, but only if a calcium salt is used to coagulate it.
Yet, even some leafy greens that contain substantial quantities of calcium may not, ultimately, be the best source of this essential mineral when you take into account absorption and bioavailability. For example, some leafy, greens, especially spinach, contains oxalates and phytates, compounds that bind to calcium and block its absorption. Other greens, like kale, contain calcium that’s much better absorbed and utilized by the body. That’s because kale is low in oxalates and phytates.
What Research Shows about Calcium Absorption
While we think of dairy as being a top source of calcium, your body takes up only 30 to 35% of the calcium from milk. So, absorption is far from perfect even when you consume dairy products. In contrast, you can absorb almost 37% of calcium from fortified orange juice and 31% of the calcium from tofu.
Surprisingly, calcium absorption is greater from certain vegetables. For example, calcium absorption is about 61% from broccoli, 54% from Bok choy, and your body can absorb almost 50% of the calcium from kale. In contrast, you only absorb 5% of the calcium from spinach due to its high oxalate content. Rhubarb and Swiss chard are also high enough in oxalates that calcium absorption is greatly reduced. (less than 5%). So, calcium absorption is highly variable even among different types of vegetables.
How about nuts and seeds? You absorb around 20% of the calcium from sources like almonds, beans, and sesame seeds. So, calcium is in a diverse array of foods, but the ability of your body to take up and use that calcium varies.
Age is a Factor in Calcium Absorption Too
Other factors impact how much calcium your body absorbs. Kids take up and use 50%+ of the calcium they take in through diet, whereas adults absorb only 30 to 50%. Vitamin D is another variable with regards to calcium absorption. Vitamin D enhances absorption of calcium from the digestive tract, meaning your body takes up more if you have sufficient vitamin D.
Adequate stomach acid is also important for taking up calcium. As people age, they produce less stomach acid. Therefore, older people can have a harder time absorbing calcium from the digestive tract. Drinking lots of soft drinks may hinder calcium bioavailability as well. Soft drinks contain phosphoric acid, and phosphoric acid can lead to greater loss of calcium in the urine. So, skip the soft drinks if you’re worried about bone health!
Other Factors that Impact Calcium Absorption
What you drink can have an impact on how much calcium your body can take up and use. For example, alcohol reduces the uptake of calcium by the digestive tract and leads to greater calcium loss via the urine. Caffeine also triggers slightly greater urine loss of calcium. So, don’t overdo the caffeine and alcohol if you’re trying to keep your bones healthy. Alcohol can reduce the amount of calcium your body absorbs too. For those reasons, limiting your intake of both alcohol and caffeine makes sense to help keep your bones healthy.
The Bottom Line
Just because a food contains a substantial amount of calcium doesn’t mean you’ll get the benefits of it. If the source also contains factors, like phytates and oxalates, that reduce calcium absorption, you may absorb 5% or less of the calcium in that food. Still, the wisdom of taking calcium supplements is questionable, so it’s best to consult with your physician about this issue. If you have osteoporosis or are at high risk of it, they may recommend a supplement. It’s also important to ensure you’re getting enough vitamin D. More research shows that vitamin K2 and magnesium also play a key role in bone health. So, eat a healthy, balanced diet, make sure you’re getting enough vitamin D, vitamin K2, and magnesium. Then, combine it with exercise to maximize the health of your bones.
Nutrition411.com. “What Percentage of Nutrients Are You Absorbing?”
Mayo Clinic. “Calcium supplements: A risk factor for heart attack?”
Dairy Nutrition. “Calcium and Bioavailability”
LiveStrong.com. “Factors That Affect Calcium Absorption”