Sukuma wiki vs spinach: the calcium question

In short. Spinach carries calcium, but it also carries oxalate, which holds on to most of it. Your body takes in a much bigger share of the calcium in greens from the cabbage family, like sukuma wiki, and sukuma wiki is usually the cheaper bunch.

Spinach has a reputation as the calcium green. Thank Popeye, thank supermarket marketing, thank decades of nutrition labels that only ever print total content. But total content is the wrong number to look at. What matters is what your body actually absorbs, and on that measure, sukuma wiki (collard greens) and its close relatives beat spinach by a wide margin.

The number that matters

Spinach does carry a respectable amount of calcium on paper. But spinach is also very high in oxalates, compounds that bind calcium into an insoluble form your gut can't absorb. In studies, only about 5% of spinach's calcium was absorbed. The rest passed through unused, however much spinach was eaten.

Kale, a close cousin of sukuma wiki in the same cabbage family, has little oxalate, and in studies about 49% of its calcium was absorbed, one of the higher figures for greens. That's nearly ten times the share from spinach, gram for gram of calcium. The studies used kale, so we take sukuma wiki to behave much the same way, since the two are so closely related.

Why oxalates matter more than totals

Oxalic acid occurs naturally in many vegetables, but the amount varies enormously by species, and spinach is one of the highest-oxalate foods commonly eaten, alongside foods like rhubarb and beet greens. When oxalate binds calcium in your gut, it forms calcium oxalate, a compound your intestine can't absorb.

Brassica vegetables, the family sukuma wiki belongs to along with cabbage and broccoli, are naturally low in oxalate. That single trait is why a Brassica green with modest total calcium can give your body more than a spinach-family green with more calcium on the label. Content tells you what's on the plate. Absorption tells you what your body takes in. For calcium, the gap between the two is unusually large.

How Kenyan greens compare

Among greens commonly found in Kenyan markets:

• Sukuma wiki (collard greens): low oxalate, and its close cousin kale had about 49% calcium absorption, which makes it the standout choice.
• Cabbage: also low in oxalate, with good absorption, but less calcium than sukuma wiki.
• Managu (African nightshade) and terere (amaranth): their oxalate varies, and there are fewer studies of how much of their calcium the body takes in. They're still good greens to rotate in for everything else they carry.
• Spinach (mchicha wa kizungu): high oxalate, about 5% absorption. It's fine as an occasional green and poor as a calcium plan.

Easy on the budget too

At most Kenyan markets, sukuma wiki is one of the cheapest greens you can buy, and a bunch goes a long way for a family meal. Spinach is less commonly stocked and often sold as a specialty item, so it usually costs more.

Put another way, sukuma wiki is both one of the cheapest greens at the market and one whose calcium your body takes in most easily. That's a rare match, since better nutrition usually costs more. Here, the budget choice and the science-backed choice are the same plate.

The takeaway

Don't chase spinach because a label says it has more calcium, since most of that calcium never reaches your blood. Cook sukuma wiki with your ugali or githeri several times a week, and add a bit of oil to help your body take in the fat-soluble vitamins it also carries. You'll get calcium your body can use, from one of the cheapest greens on the stall.