Edible green leafy vegetables come in two different families:
The brassica family.
Brassicas include broccoli, cabbage, cauliflower and chinese cabbage as well as root vegetables that we rarely eat the leaves from, notably turnip, radish and horse radish. It is the brassicas, sometimes called cruciferous vegetables, that are mostly being referred to when we read about health stories relating to green leafy vegetables.
The amaranth family.
Amaranths include 2 sub-families, the beetroot and chenopod families.
- The beetroot sub-family contains chard, and others, better known for their tubers (underground bulbous roots), beetroot and sugar beet.
- The chenopod family contains spinach, orach and good king henry as well as quinoa which is better known for providing grains.
Some other terms and classifications.
- Cruciferous vegetables are basically the same as brassicas.
- Calabrese is a reference to the type of broccoli most often seen in the UK supermarkets, with large florets and very thick stems. Purple or white sprouting broccoli has smaller florets and stems and is often presented in bunches.
- Bok choi, pak choi and chinese cabbage are all the same thing.
- Chard and sea kale are one and the same.
What are the beneficial substances in green leafy vegetables?
- Vitamins such as vitamin A (from beta carotene), vitamin B6, folic acid, vitamin C and vitamin K.
- Minerals such as potassium and iron (non-heme) in spinach. Small amounts of a number of other minerals.
- Phytochemicals, literally plant chemicals that are not vitamins or minerals. These include the glucosinolates such as indoles and isothiocyanates. Also included here are carotenoids that are not associated with vitamin A formation such as luteins and xanthophylls.
What are the health effects of eating greens?
Prevention of iron deficiency.
The combination of beta-carotene and non-heme iron found in most leafy greens will help prevent iron deficiency. Vitamin A and iron in combination are more effective at relieving anaemia than iron alone1, 2.
Greens get a lot of press regarding their cancer fighting abilities. This is often attributed to their content of the glucosinolates (isothiocyanates including sulphoraphane) and indoles.
H. Pylori prevention.
Sulphoraphane, which is found particularly in young sprouting broccoli florets, has been associated with an inhibition of Helicobacter Pylori infection of the stomach3. The substance seems to kill off the bacteria leading to improved health which can last for a number of weeks after consuming it.
Prevention of cataracts.
The lutein and zeaxanthin found in green leafy vegetables such as spinach and kale have been associated with reduced likelihood of cataracts and have also been found to prevent age related macula degeneration. This is likely to be due to the fact that high concentrations of lutein are found in the macula, which is responsible for our central vision4.
Possible exacerbation of thyroid problems.
Large amounts of the glucosinolates that are found in leafy green vegetables have been associated with goitrogenic activity. That is they reduce the effectiveness of your thyroid gland. This is only an issue for people with malfunctioning thyroids. For these people it is wise to consume green leafy vegetables in moderation. By that I would suggest a maximum of 2 times per week.
From the evidence above it seems that consuming a good amount of greens is likely to improve health for most people. For women there are the possible benefits of increased iron levels as well as a reduced risk of breast cancer. For men and women, the abiliy of greens to reduce H. Pylori infection and reduce age related damage to the eyes must be seen as a positive benefit.
How should I eat greens?
Each method of cooking has its pros and cons.
Steaming is great for taste, especially when the greens have been freshly picked from the garden. If you grow your own or know someone else who does then you can get your greens in premium condition, to give you the maximum in health benefits. By cooking the greens in this way you reduce the goitrogenic effect of the glucosinolates. This is advisable for those with thyroid problems. It is possible to steam using a small amount of water in a pan and then adding in the greens. Just don't immerse all the greens in water as this doesn't improve their taste or health benefits, see "Boiling" below.
Stir frying works well with shredded greens. I advise cooking in olive oil, coconut oil or butter rather than vegetable or sunflower oil as these are healthier options. Cooking in oil makes the fat soluble carotenoids available which are responsible for vitamin A activity and other antioxidant properties. Eating your greens this way should provide beneficial effects for the eyes.
Boiling can leach some of the beneficial phytochemicals, including the ones that reduce cancer risk, out of the greens. If you overcook them, apart from a vaguely unpleasant odour, you may well reduce the health benefits of your greens.