"Baby spinach" is a term typically used to describe spinach that has been harvested during a fairly early stage of plant growth, usually between 15-35 days after planting. We're usually familiar with baby spinach in the grocery store because of its small leaves, tender texture, and sweet taste in comparison with mature, fully formed spinach leaves. (For these mature spinach leaves, the harvest dates are usually between 40-65 days.)
Research has shown that the concentration of nutrients in spinach may vary. Some studies show that baby spinach to be more concentrated than mature spinach in nutrients like vitamin C, carotenoids, and flavonoids while others show the opposite. What seems to impact this variation is the soil, the season, and the climate in which the spinach grows. So, we can't say that either baby spinach or mature spinach is categorically richer in nutrients than the other.
There had been research showing that baby spinach had lower levels of oxalic acid. Yet, other studies have shown the opposite, that in fact some samples have higher levels. Therefore, like with the nutrients, we can't categorically say that baby spinach is higher or lower in oxalates. The levels of oxalates formed depend upon a variety of factors.
While the research currently doesn't allow for conclusions, here's what we know. We see all types of spinach as nutrient-rich additions to your Healthiest Way of Eating. We also prefer to use the delicate leaves of baby spinach in salads while using mature spinach when we quick cook this delightful vegetable. We feel that this is the best way to enjoy the unique tastes and features of these different variations of spinach.
Franceschi VR, Nakata PA. Calcium Oxalate in Plants: Formation and Function. Annual Review of Plant Biology. 2005; 56: 41-71
Grazyna Jaworska G, Kmiecik W. Content of Selected Mineral Compounds, Nitrates III and V, and Oxalates in Spinach (Spinacia Oleracea L.) and New Zealand Spinach (Tetragnoia Expansia Murr.) from Spring and Autumn Growing Seasons. Electronic Journal of Polish Agricultural Universities, 1999; 2(2).
Kitchen JW, Burns EE. The Effect of Maturity on the Oxalate Content of Spinach (Spinacia oleraceae L.). Journal of Food Science 1965 July; 30(4): 589.