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Oxalate and Calcium Absorption

You try to get as much calcium as you need from your food. But are you absorbing all of this mineral? What about oxalate and calcium absorption?

These are common questions amongst my clients who are trying to eat enough calcium to keep their bones healthy. Let’s break it down.

How does oxalate affect calcium absorption?

What is the relationship between calcium and oxalate? Calcium is a mineral, and oxalate is an acidic compound that naturally forms in plants. Oxalate has an attraction for calcium. The two substances bind together to make a calcium oxalate salt.

Calcium is the most powerful food that can change the absorption of oxalates in your gut (1). This is a good thing if you are prone to calcium oxalate kidney stones. You will absorb less oxalates if you increase calcium in your meals.

What happens to calcium once it is bound up with oxalate? Calcium oxalate is too big to absorb, and your body eliminates it. The calcium is (literally) flushed away along with the oxalate.

If you are trying to eat enough calcium to build up your bones and keep them healthy, this might not be a good thing. Oxalate can prevent calcium from getting into your body. 

How do you know how much calcium you are absorbing?

Bioavailability is the extent that a nutrient can be absorbed by your body and enter your circulation to be used however the body decides is best. Researchers have measured the calcium bioavailability of many different types of food (2).

Oxalate reduces calcium bioavailability. It binds first with the calcium in the oxalate-containing food. For example, spinach is high in both calcium and oxalate, so in the end you don’t get as much calcium from this vegetable.

Free oxalate binds with calcium from other food in the gut. So, high oxalate food reduces calcium absorption from calcium-rich food that is eaten in the same meal.

For instance, beets are high in oxalate but don’t contain a lot of calcium. If you eat a meal heavy in beets along with milk or cheese, you absorb less calcium from those dairy products.

What is the bioavailability of calcium in high oxalate food?

Milk is the gold standard as for calcium bioavailability. It is high in calcium and low in oxalate. However, some non-dairy food also has an excellent calcium bioavailability (3).  

Kale, broccoli, bok choy, turnip greens, kai choy and choy sum are low oxalate vegetables and a good source of calcium.

Find out about other low oxalate greens.

Soybeans are high in oxalate, but soaked and fermented soy products like tofu are low oxalate, and have a high calcium bioavailability. I will talk more about ways to lower oxalate later in this article.

Soy Milk fortified with calcium carbonate has a bioavailability that is equal to cows milk (4).  

Sweet potatoes are moderately high in oxalate and so have moderate calcium bioavailability.

Spinach and rhubarb are very high in oxalate and have a very low calcium bioavailability. You do not absorb much calcium from these vegetables.

Other high oxalate food includes Swiss chard, sorrel, taro leaves, beets, amaranth, licorice root, almonds (and almond products), buckwheat and cocoa. This is not a complete list of high oxalate food.

Harvard T. H. Chan School of Public Health provides an oxalate table.

Is there a formula I can use to figure out how much calcium I am getting from food?

Calcium and oxalate absorption is a complicated process. Researchers have proposed algorithms, but there is no tried and true formula right now that would be practical to use and apply to a wide population (5).

Your best bet is to avoid very high oxalate food at the same time you eat your high calcium food. 

If you love a high oxalate food, separate it from high calcium food by 3 or 4 hours. After a meal has passed through the stomach and small intestine, those oxalates will no longer bind with the next incoming food. 

Does the recommended intake of calcium need to be adjusted based on calcium bioavailability? 

You do not need to adjust your calcium intake based on bioavailability. If a food contains 400 mg of calcium, this is the amount you can calculate in your daily calcium intake. 

The exceptions are very high oxalate foods like spinach, rhubarb and Swiss chard. Do not count these as good sources of calcium in your diet. 

Can you lower oxalate in food?

Oxalates dissolve in water, so boiling and steaming can lower the oxalate content of food (6). Boiling is more effective than steaming, and this method works better when the food is in smaller pieces.

It is even better to soak food before you cook it. Cooking in a pressure cooker or instapot destroys more oxalate.

Roasting, grilling or baking food does not decrease oxalate, and may even cause it to increase.

What else can I do to absorb more calcium?

Calcium needs an acidic environment in the gut. This converts it into the best form for absorption. If you have good stomach acid, you can absorb more of this mineral.

Find out how to measure your stomach acid at home.  

Gastric transit time makes a difference in how much calcium you absorb. When food moves too quickly through your digestive system, such as less than 12 hours from start to finish, there is less time to absorb all of the calcium.

Eat or drink your calcium with a meal. It will stay longer in your digestive tract and have time to be absorbed.

Your vitamin D status is super important for calcium absorption. Vitamin D sends messages to your gut telling it to bring more calcium into your body (3).  

Final Thoughts

Calcium and oxalate bind together to make a large compound that can’t be absorbed into your body. When you eat a high oxalate food that contains calcium, you don’t absorb much calcium from that food.

High oxalate food can also reach out to other food from the same meal and absorb some of that calcium while it is in your gut.

Your best bet is to separate very high oxalate food from your main dietary sources of calcium. After 3 or 4 hours those oxalates will no longer affect the next food that you eat.

So, if you have almonds at breakfast, by lunchtime you no longer need to worry about oxalate interfering with your lunch.

You do not need to start counting the oxalates in every single food! Only very high oxalate food has a significant impact on the amount of calcium you absorb.

You can decrease the oxalate content of food by soaking, steaming and boiling before eating.

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