Eating leafy green vegetables is no doubt good for you, but there is such a thing as too much of a good thing. With any food or medicine, herbal or otherwise comes the potential for contraindications or overdose – something I find Ted Manzer addresses well in his blogging. The question also arises as to whether you need the helpful properties in the first place. When I looked into the peculiar absence of records of Australian Aboriginal people eating native spinach (Tetragonia tetragonioides, also known as warrigal greens, New Zealand spinach and Botany Bay greens), something that I have used a lot of in recent years, the evidence stacked up in favour of them actually being better off without it. The text below is largely taken straight from an academic style abstract (for an archaeology conference last year) and might be a bit dry (and academically referenced rather than hyperlinked), but I’ll leave it as written anyway. Many of the conclusions could be applied to a number of other oxalate-rich greens commonly in forager’s salad bowls. If there is any foraging take-home from it, I would say it is that while we should utilise tetragonia and other oxalate-rich greens, they should not be staples, but rather always with diversity in the dish. I don’t think that this observation is new – few on food ever are – and in fact it seems very much in line with traditional approaches to weed-eating that I have looked into (like those of Crete and elsewhere in the Mediterranean).
When the British off the First Fleet struggled with the challenges of the Australian landscape in Sydney, among the first native foods they embraced were the wild greens for their ability to combat scurvy from the bleak diet of ship stores (Low 1989, Mann 1811). The best known of these was tetragonia (Tetragonia tetragonioides). At the time, David Dickinson Mann (1811) noted that ‘Botany Bay greens’ were gathered in Sydney in large quantities and ‘esteemed a very good dish’ by Europeans ‘but despised by the natives’. In fact, there is almost no documented use among Aboriginal people anywhere in Australia (Low 1988).
This same plant was eaten across the Tasman by Maori and its use has spread to many parts of the world as the only widely grown Australian vegetable. In the process it has been assumed to have good nutritional qualities just as other spinach-like plants do (but see Kawashima and Valente Soares 2005). That Aboriginal people did not apparently eat it may at first seem either a dietary mistake on their part, or a failure on the part of ethnohistorians to record that they actually did. This is however reliant on an assumption that because it is believed to be good for some, it should be good for all.
The conclusion of our research was that tetragonia, although known to be edible by at least some Aboriginal people, was adaptively avoided because any benefits were outweighed by the potential costs. Tetragonia offers few calories and only some important micronutrients in minimal amounts. The leaves must be cooked due to the high oxalic acid content, but in removing this some nutrients are lost as well (Kawashima and Valente Soares 2005). What little remains is otherwise adequately provided in traditional Aboriginal foraging diets with their great diversity of roots, fruits, seeds and often lightly cooked meat and seafood (see also Kuhnlein et al. 2004). The remaining oxalic acid further prevents the absorption of other nutrients such as iron and niacin (B3) in the leaves and would extend this effect to other foods eaten at the same time (see Anderson 2005:47). Oxalic acid is also associated with kidney stones and the risk would be exacerbated by dehydration – a presumably recurrent condition for many Aboriginal people.
Overall, eating tetragonia may be beneficial if you are a scurvy struck convict, starchy-dieted horticulturalist (e.g. some traditional Maori) and probably most people on modern western diets. But, for a person on a diverse traditional Aboriginal diet exposed to common dehydration, you are almost certainly better off without Tetragonia in your diet; and this, somehow, seems to have been known throughout Aboriginal Australia. The same might be said of a number of other greens not eaten by Aboriginal people, especially Portulaca oleraceae – eaten almost globally as a vegetable but largely only used for seeds by Aboriginal people (Latz 1995).
References: Anderson, E.N. Everybody Eats: Understanding Food and Culture. New York: NYU Press. Kawashima, LM and LM Valente Soare. 2005. Effect of blanching time on selective mineral extraction from the spinach substitute commonly used in Brazil (Tetragonia expansa). Ciência e Tecnologia de Alimentos 25(3): 419-424. Kuhnlein HV, Receveur O, Soueida R, Egeland GM. 2004. Arctic indigenous peoples experience the nutrition transition with changing dietary patterns and obesity. J Nutr. 134 (6): 1447–53. Latz, P. 1995. Bushfires and Bushtucker: Aboriginal Plant Use in Central Australia. Alice Springs: IAD Press. Low, T. 1988. Wild Food Plants of Australia. Sydney: Angus and Robertson. Mann, D.D. 1811. The Present Picture of New South Wales. London: John Booth. Symonds, M. 1984. One Continuous Picnic: A History of Eating in Australia. Adelaide : Duck Press.