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.
References:
Essentials of Human
Nutrition, 4th edition p 142-145
Modern Nutrition in
Health & Disease, 11th edition p 136-140
