Calcium and protein: friends or foes?

In the last post, we saw that dairy products are a fine source of calcium, thanks to the absorption-enhancing property of lactose and the density of calcium by weight or calorie. So why consider anything else? I think many people have an idea that protein, in which dairy products are rich, interferes with calcium or bone health in some way. In the chapter on bone health in Krause’s Food and Nutrition Therapy, John JB Anderson states that “the relationship between protein and calcium is unsettled” (625).

Let’s take a step back. What is this relationship? On the one hand, protein is considered to have an anabolic (building) effect on bone. After all, around 25% of the bone matrix is protein in the form of collagen fibers. Calcium and phosphorus make up the hydroxyapatite that gives bone its rigidity. On the other hand, a chronically low intake of protein decreases the serum concentration of albumin, a protein that transports nearly everything in the blood, including calcium (Anderson, 625).

So why would high protein intake pose a problem? Here’s where the textbook gets a little fuzzy. First it says that “actions of proteins and their absorbed amino acids… [have] a catabolic [breaking down] effect resulting from the generation of an acid load” (625). But as far as I can tell, extracellular excesses of acid are buffered primarily by bicarbonate - calcium doesn’t have much to do with it.

The second reason given is that urinary losses of calcium increase following meals that contain large amounts of animal protein (625). The exact mechanism is not clear, but it may be because the sulfates generated by the metabolism of sulfur-containing amino acids (cysteine, cystine, methionine) bind to calcium ions in the kidney and prevent them from being reabsorbed (Gallagher, 115). A list of food sources of sulfur follows: meat, poultry, fish, eggs, dried beans, broccoli, and cauliflower. This is a bit misleading, as 3 oz of beef has fourteen times as much methionine as 1 cup of cooked broccoli.

Studies tracking protein consumption and bone fractures or bone mineral density are… numerous. Some come out in favor, some come out a wash. I would say that most people should consider getting more calcium from vegetables, and in the next post, I’ll explain why it’s a great idea.

Photo source: fooducate.com

Can we use calcium from dairy products?

A few weeks ago when I entered Aunt Gayle’s hospice room, the gathered family had a nutrition question for me: can humans absorb the calcium found in milk and other dairy products? I was fairly confident that dairy is a good source of calcium and is well absorbed. If not, the government and the dairy industry’s been doing us a huge disservice for decades.

Then my mother-in-law raised the same question this weekend. Her doctor had recommended leafy greens as a calcium source. Again, I was fairly confident that most leafy greens are a good source of calcium, though not necessarily as well absorbed.

So what's the deal? Should one consume dairy products for the calcium? My newly purchased textbook, Krause’s Food and Nutrition Therapy, by L. Kathleen Mahan and Sylvia Escott-Stump (Saunders-Elsevier, 2008), offers two reasons to do so. First, dairy products are the most concentrated calcium sources, and second, lactose enhances calcium absorption, probably even in those with lactose intolerance (104, 106).

A cursory glance at a Google Scholar search related to lactose-enhanced calcium absorption reassured me that this reason was probably valid.And while "most concentrated" sounded about right, I was curious as to how exactly that played out.

I started calculating the concentration of calcium in some foods, both dairy and non-dairy to get some numbers. I began by calculating milligrams of calcium per calorie, but that didn’t result in dairy being uniformly more concentrated:

Food	Calcium (mg)	Kcals	Mg Ca per kcal
Yogurt, plain, low fat, 1 cup (245g)	448	154	2.9
Milk, 2%, 1 cup (144g)	286	122	2.3
Cheddar cheese, 1 oz (28g)	202	113	1.8
¼ block firm tofu (81g)	163	57	2.9
Kale, 1 cup cooked (130g)	94	36	2.6
Broccoli, 1 cup cooked (156g)	62	54	1.2
Almonds, 1 oz (28g)	75	169	0.4

Tofu stands eye-to-eye with yogurt, a cup of kale beats a cup of milk - what's going on? I switched to milligrams of calcium per gram, which still didn’t yield the results I expected:

Food	Mg Ca per gram
Cheddar cheese, 1 oz	7.2
Yogurt, plain, low fat, 8 oz	1.8
Milk, 2%, 1 cup	1.2
Almonds, 1 oz	2.7
¼ block firm tofu	2.0
Kale, 1 cup cooked	0.7
Broccoli, 1 cup cooked	0.4

Here, water content plays a big role - generally the more water, the heavier the food is. Cheddar is the leader by far, since cheese is basically milk wrung of water. This time, almonds are doing pretty well, and tofu is still a champ. NOTE: These calculations are based on calcium-set tofu, not the silken type, which has about 80% less calcium. Straight-up cooked soybeans have about 0.6 mg Ca per kcal, and 1.0 mg per gram.

Bottom line – the author of the statement "Cow's milk is and dairy products are the most concentrated sources of calcium" has a different understanding of “concentrated sources” than I do.

Next up: the relationship between protein and calcium.

Photo Source: UMN Extension

Childhood (Mis)conceptions

Every morning this week, I've showed up at the U of M's student organic farm. I stick around for a couple of hours, helping out with the tasks at hand: transplanting pansies, harvesting chives and cilantro, weighing the produce, thinning the turnips.

My mind often turns to images formed of farming in my childhood by the Little House on the Prairie books and even The Boxcar Children. Of course, we've got a Prius and more water sources than you can shake a stick at, but I hoof it an awful lot and watering, although it doesn't involve lugging water buckets, can still be tedious.

And then there's thinning plants. You may recall that in the first Boxcar Children novel, back when they were actually living in a boxcar, the Alden children do yard work for Mr. Henry. One task was to thin the carrots. I really had no idea what this meant: I imagined Jessie pulling out a carrot, running it between her fingers to "thin" it, and putting it back in the ground. Why would anyone pull out perfectly good carrots? When Jessie asks to take home the pile of thinned carrots (apparently they didn't all make it back into the ground), I always imagined a heap of pale, pinched roots, ready to wilt at a second glance.

The reality is less laborious. Thinning involves only the removal of the smaller plants, no size-reduction at all. We ended up with a healthy pile of turnip greens that destined for the Campus Club. No humble junkyard stew pot for these organic roots.

Photo source: http://mysticdomestica.blogspot.com/2009/07/boxcar-kid.html