There are many different types of mustard greens. The species Brassica juncea gives us dozens of different varieties which are especially popular in Asian cuisine, such as for stir fries and sautes.
In the United States, they are perhaps most popular in the Southeastern part of the country, where the the name "mustard greens" often refers to the most popular kind in that region; the type "Southern giant curled" which has large bright-green leaves with curled edges. Other popular varieties include the "horned mustard" with darker and flatter leaves, as well as the "old fashioned ragged edge mustard" which is lighter colored and more frilly.
Whichever type you cook with or use in your salads, they add a rather unique pungent and peppery taste to kick up the flavor.
What is mizuna lettuce?
Whether you call it a mizuna greens or lettuce is a matter of semantics. It also goes by the names Japanese mustard greens, California peppergrass, and spider mustard.
There is both green and red/purple mizuna, but this ORAC value represents the green.
Among the Asian varieties of mustard greens, it is one of the most - if not the most - popular, at least in the United States. You will often find it in stir-fries and sautes at restaurants.
Where to buy mizuna leaves? You can sometimes find the green version stocked at the grocery store, assuming it has a respectable produce department. In parts of the country with large Japanese and Chinese populations, such as California, you should have a better chance of being able to buy it at "normal" supermarkets like Ralph's, Von's, Pavilions, etc. In the Midwest and Southeast, a trip to Whole Foods may be necessary to find it and even there, it's by no means guaranteed. On the East Coast it is reasonably available, but not as popular as on the West Coast due to the different demographics. When it's in season - May though November - a large farmers market will be your best bet to find it, wherever it is you happen to you live.
What does mizuna taste like?
Like most mustard greens, it packs more of a zing versus your traditional lettuces. Some describe mizuna as tasting like a sharp watercress. It is peppery and has a hint of bitterness to it, similar to frisee. Though because it is sweeter and less pungent than most mustard greens, its appeal is universal when part of a salad mix.
The antioxidants in most varieties of mustard greens are generally low. For example, the ORAC value of mizuna is roughly 1/10th that of Swiss chard. Testing has not been published for the red colored varieties, such as purple mizuna or other red mustard greens like ruby streaks, scarlet frills, red giant, and red splendor. Even without the test results, we can almost be certain however much antioxidants they have, it is more - significantly higher - than a green mizuna. Why? Because the reds contain anthocyanins, the purely green leaves do not.
The good news is that mizuna has decent amounts of lutein, slightly more than red leaf lettuce. Its zeaxanthin content was comparable. As you are probably aware, both of these phytonutrients offer benefits for your eyes. A wide body of research suggests they may help protect your retina from UV damage.
Even though its overall antioxidant content is low, nutritionally it does shine for one particular category; vitamin K. Measuring out at 2.32 mg per 100 grams, this is exceptionally high. It is 3x that found in Swiss chard and an even higher multiple over many other common lettuces and green leaves we use as food.
Vitamin K is considered "critical" for bone health, as it is needed for retention of calcium. Having adequate vitamin K intake has also been associated with less calcium ending up in the arteries, a condition called arterial calcification, which is something that's definitely not good for you (1). This may be due to the vitamin K's ability to help the calcium stay where it's needed - in the bones - instead of inside your artery walls.
Is it good for you?
The conclusion would be that this variety of mustard greens is not healthy for you in terms of antioxidant content, but in all other regards its extremely nutritious. Most notably its vitamin K levels, which beat the vast majority of salad greens.
Research Support, U.S. Gov't: NASA, Selection of Leafy Green Vegetable Varieties for a Pick-and-Eat Diet Supplement on ISS PDF July 2015