Wednesday, August 27, 2014

Calcium absorption

Calcium is a crucial part of the structure of bones and teeth. You knew that already. So how do you assess what food is a good source of calcium? You may just look for the number of milligrams of calcium there are in a serve, the more the better. Well that doesn’t always work as we shall see.

Or you could read some websites that give you counter-intuitive information, such as those claiming milk, yogurt and cheese being high in calcium, yet being a poor source of calcium. Let’s take a look at the data because Hannah Castledine, university nutrition student, was a bit annoyed when she went to presentation by a chef claiming that dairy just doesn’t cut it as a calcium source and we should rely solely on almonds and leafy vegetables.

Calcium absorption
During digestion the calcium is usually freed-up for absorption, although some calcium complexes can form which are difficult to absorb. That doesn’t mean that every atom of calcium gets absorbed into the blood. We still tend to absorb more calcium as more calcium appears in the diet, independent of its source.

At low amounts of dietary calcium, it is actively transported across the lining of the intestines and into the body. Once this system becomes saturated as we eat more calcium then the extra calcium is absorbed by passive diffusion.

Factors affecting calcium absorption
Vitamin D is needed for effective calcium absorption, and as you know many people are low in vitamin D. On average one in three adults are vitamin D deficient in North America, Australia and New Zealand, even more in the winter months.

Even if you get enough calcium in your food, a low vitamin D level is going to make it difficult to absorb enough. You will still absorb some calcium, but probably not enough for good bone mass.

As we get older our ability to absorb calcium declines, by about 1% every five years, with a slightly bigger drop in women at menopause. Hence, the reason for more calcium recommended for folk as they edge into their 60s.

The lactose in milk seems to give calcium an absorption “leg-up” by augmenting its passage across the intestinal wall. On the other hand, salt and caffeine can increase calcium losses through your pee. This is most likely to be a problem in people eating lots of salty processed food and gallons of coffee. If you eat well, avoid highly salted food and enjoy 3-5 cups of tea or coffee a day, then I wouldn’t change a thing.

Calcium sources
You absorb about 30% of the calcium from milk, yogurt and cheese sources, which is pretty impressive when compared to the 5% absorption from spinach. I mention this specifically because spinach is touted by the pop-nutritionist as a wonderful source of calcium. Much of the calcium in spinach is locked up in the form of calcium oxalate making it just too difficult to absorb. Same with the iron in spinach. Mostly bound to oxalate. As case of being high in calcium and iron but low in bioavailability.

I remember telling that to a men’s health group once, and one bloke did an air punch and said: “That’s going back to the missus”. I had to say that even if the calcium and iron in spinach goes down the loo it doesn’t make spinach a 2nd class veg – it is still high in folate and beta-carotene.

The calcium from Brassica vegetables is much easier to absorb. I’m talking broccoli, kale, bok choy and cabbage, for example. We can absorb about 50% of the calcium from the Brassicas, but beware that 1 cup of broccoli will have about 50 mg calcium (so 25 mg ends up inside you) while a cup of milk will have 300 mg calcium, with you absorbing about 90 mg of that.

If you don’t fancy milk, then I recommend you choose a calcium-fortified beverage in its place, such as a soy milk. How do you know if your milk substitute has added calcium? It will definitely be on the label. If there is no mention of calcium, then assume it is low.

I have previously written about food sources of calcium. Yes, almonds and tahini are other good sources of calcium, with 70 mg calcium in 30g almonds, and 30 mg calcium in a tablespoon of tahini.

The old days
Our ancestors may have got 1500mg of calcium each day through nuts, vegetables, seeds and beans, partly because they would be eating a lot more food than we do. Nowadays we do less activity, therefore need less food, so we need to make sure our food is high quality, and some of us will need to also make use of calcium-fortified foods.

What does it all mean?
I have tried to give you a fair and reasonable summary, free from food politics and bias. Yes, there are many who want to swing your views based personal nutrition evangelism. As you have heard many times from me, you are your own boss and your sources of calcium is your decision. If you are happy with dairy, then continue to make that a major source of calcium. If you are avoiding dairy for philosophical or allergy reasons, then there are other calcium sources.

Essentials of Human Nutrition, 4th edition p 142-145

Modern Nutrition in Health & Disease, 11th edition p 136-140