Leafy greens are undoubtedly healthy! Most everyone is aware of this fact by now. We see it in headlines, in the grocery store produce section, in magazine and book titles. Still, people have questions. What is the healthiest leafy green? Should I eat greens every day? How much? And what about iceberg lettuce? Is it bad for me? Looks like it is time to delve into the details of the nutritional powerhouses known as leafy greens.
Leafy greens are just that – the leafy portion of certain plants. However, they are not just any green leaves but the ones that humans find to be edible, not too bitter and definitely not poisonous! And they are not only green but can also be other colours like red or purple.
As such, they can generally be classified as the leaves found on …
… certain root vegetables (for example, radish leaves, turnip greens, beet greens, carrot tops)
… leafy cruciferous vegetables (for example, kale, arugula, cabbage, bok choy, collard greens,
… lettuces (for example romaine, iceberg, butterhead, Bibb, Boston)
… intensely-flavoured herb greens (for example, parsley, cilantro, basil, oregano, rosemary, thyme,
… other leafy plant groups including chicories (for example, endive, frisée, radicchio, dandelion greens), amaranths (for example, spinach, Swiss chard, amaranth greens) and valerians (for example, mâche),
Why are leafy greens so healthy?
Leafy greens are very high in nutrients and very low in calories. A study from the Centers for Disease Control and Prevention calculated the density of nutrients in almost fifty fruits and vegetables and found that seventeen of the top twenty in healthy nutrients were leafy greens (1).
Leafy greens are packed with important nutrients for well-being such as (2) …
… antioxidants including carotenoids (beta-carotene, lutein and zeaxanthin), flavonoids, and lignans
… vitamins including vitamin A, the vitamin B family (thiamin (B1), riboflavin (B2), niacin (B3), panthothenic acid (B5), pyridoxine (B6), and folate), vitamin C, vitamin E, vitamin K1
… minerals including calcium, iron, magnesium, phosphorus, manganese, potassium, sodium, zinc, copper and selenium
From scientific research, we know that consumption of leafy greens is associated with lower risk of many of the lifestyle diseases that are presently plaguing the world. Consuming greens shows healthful benefits for cardiovascular disease (3), high blood pressure (4), type-2 diabetes (5), cognitive decline (6) and cancer (7). In addition, eating greens has been shown to improve eyesight, immune function and athletic performance (8). Interestingly, studies have even shown that a higher intake of green and yellow vegetables is associated with decreased facial wrinkling (9).
One cup of leafy greens can provide a generous portion of the total Daily Value of many nutrients (2,10,27). (The Daily Value (DV) is the recommended daily amount of a nutrient for best health.)
NOTE: There is no adverse effect from eating too much vitamin K1 (11).
Here are a few examples (2);
1 cup of collard greens can supply 439% of the DV of vitamin K1, 52% of the DV of vitamin A and 10% of the DV of calcium.
1 cup of watercress offers 100% of the DV of vitamin K1.
1 cup of beet greens can provide 32% of the DV of fiber, 70% of the DV of vitamin A and 45% of the DV of potassium and is extremely high in carotenoids.
1 cup of romaine lettuce can supply 83% of the DV of vitamin K1 and 24% of the DV of vitamin A.
1 cup of kale can provide 163% of the DV of vitamin K1, 14% of the DV of vitamin A, 52% of the DV of vitamin C and 13% of the DV of calcium.
1 cup of endive can provide 96% of the DV for vitamin K1, 11% of the DV for vitamin A and 18% of the DV for folate.
1 cup of mâche can supply 21% of the DV of selenium and 12% of the DV of iron.
1 cup of radicchio offers 15% of the DV of copper.
1 cup of chicory greens can provide 6% of the DV of vitamin E.
Note that these amounts are from half of the daily recommended amount for leafy greens so you can see that eating lots of greens will really have you rocking in the nutrient department.
Which Greens are the Healthiest?
Research recently illustrated that those who eat more than thirty different types of plants per week had the most diversity in their gut microbiome. Gut diversity is associated with optimal health (12). This is also true when considering just the family of leafy greens available for humans to consume. All greens are packed with beneficial nutrients. There is no one green that can be labelled as “the healthiest green” although, as a general rule, the darker the green, the more nutrients it contains. Your goal should be to eat as many different greens as you can, in this way providing access to all the health-giving compounds that they offer.
That said, there are some especially healthful greens to be sure to include in your weekly diet. These heavy hitters are kale, arugula, turnip greens, maché (corn salad), endive and watercress. Note that maché (corn salad) is a difficult green to find in North America although it is very common throughout Europe.
Fill out your weekly diet with other green choices such as lettuces, cabbages, bok choy and collard greens.
Spinach, beet greens, Swiss chard, chicory greens and radicchio are also excellent choices but should be eaten in moderation. (See the next section on oxalates.)
Iceberg lettuce is a green that is often considered inferior, so much so that some believe it to be bad for health. Of course, this is completely unjustified. Like all leafy greens, all lettuces contains a wide variety of advantageous nutrients, just in lower concentrations than in some of the other leafy greens. Think of lettuce as a wholesome green choice and make it a part of the diversity of your weekly green consumption.
What about Oxalates?
Leafy greens, along with many other vegetables, fruits, nuts and seeds, contain a naturally occurring organic compound called oxalic acid. It is present in these plants to provide self-defense for the plant against insect pests and grazing animals by making the plant taste sour. (13). However, oxalic acid can bind with minerals in the colon and urinary tract, forming oxalates. Oxalates can also be produced by our own bodies. In most people, oxalates pass harmlessly out of the body in the stool or urine. Unfortunately, a couple of problems can occur. Oxalate formation and elimination will also remove the mineral that it is bound to. And, in some people, oxalates increase the risk of kidney stones. (14,15)
Greens in general are an excellent source of minerals such as calcium. But a green high in oxalic acid will have a large proportion of its mineral content bound in oxalates. There are three leafy greens that are very high in oxalates. These are spinach, Swiss chard and beet greens with spinach being particularly high. Chicory greens and radicchio are moderately high.
Oxalate formation doesn’t mean that you shouldn’t eat these greens at all but that they should be consumed in moderation. If you are prone to forming calcium oxalate kidney stones (the most common type of kidney stone) you should limit your intake of high-oxalate greens. This is also true if you are consuming greens every day. You don’t want to hamper your mineral intake by eating greens that are removing minerals from your body.
When choosing the foods you eat in your daily diet, concentrate on greens lower in oxalates such as kale, turnip greens, mustard greens, collard greens, arugula, mâche, watercress, endive, cabbages and lettuces to ensure you’ll reap the benefits of the minerals present in greens. Limit your servings of spinach, Swiss chard and beet greens and, to a lesser extent, chicory greens and radicchio, to a couple of servings a week or less. Remember that your goal is to eat a wide variety of plants and, if you follow this plan, any worries about oxalates are unnecessary. (16,17,18,19,20,21)
What About Green Powders?
Green powders claim to be an easy way to obtain a large dose of health-giving foods. Green powders contain dried and ground portions of various plants such as leafy greens, other vegetables, grasses, herbs, fruits, seaweed and algae along with vitamins, minerals and additives such as sugar and dyes to enhance their healthfulness, flavour and colour. Green powders are usually ingested mixed with water or other liquids.
It is important to take a step back here and look carefully at what green powders actually are. Whole foods are not a powdered collection of nutrients but a “package” made up of many different parts, many of which are missing in green powders. In whole foods, vitamins, minerals, and other beneficial nutrients such as antioxidants and phytonutrients are bundled together within a matrix of fiber. The resulting complete package affects the release of and absorption of its nutrients into the body and plays an important role in the benefits that are derived from whole foods.
Research has discovered that supplements cannot replace whole foods. In 2019, a prospective cohort study included data from 30,899 US adults who took part in the NHANES (National Health and Nutrition Examination Survey). Participants were followed for over 6 years. Results showed that use of dietary supplements was not associated with any change in mortality. On the other hand, adequate intake of vitamin A, vitamin K, magnesium, zinc and copper were indeed associated with reduced cardiovascular and all-cause mortality but only when these nutrients originated in whole foods. Excess calcium intake from supplements (more than 1000 mg per day) was associated with increased risk of cancer death. Though they originate from whole foods, green powders are highly processed and are treated like supplements by the body. We now understand that it is best to improve the diet before using supplements (22)
What about microgreens?
Microgreens are the first green leaves that grow after sprouts emerge from the seeds of vegetables and herbs. Microgreens may be small but they are mighty, containing levels of up to forty times more nutrients per weight than their more mature counterparts (23). Microgreens are another excellent way to eat leafy greens. One serving of microgreens is considered to be 25 grams. (24)
How many greens should be eaten daily?
One serving of leafy greens is considered to be 1 cup of raw leaves or ½ cup of cooked greens.
The Canada Food Guide recommends eating dark green vegetables every day, suggesting that kale,
arugula, watercress, turnip greens and mustard greens are good choices (25). The international nutrition expert and physician, Dr. Michael Greger, recommends five servings of vegetables a day including at least 2 servings of leafy greens and 1 serving of cruciferous vegetables (which can be a cruciferous leafy green) (25).
To summarize, try to eat at least five cups of vegetables a day including at least 2 cups of leafy greens.
How to include leafy greens in your diet
Add a couple of large handfuls of leafy greens to your stews, casseroles and soups. They will wilt into a fraction of their original volume while adding a huge nutrient boost to your dish.
Add a layer of greens to your sandwich.
Add a handful of leafy greens to your morning smoothie.
Combine two or three different types of leafy greens into a salad and top with a homemade oil-free salad dressing. Experiment with mixing different flavours together. For example, arugula and watercress taste peppery; turnip greens and beet greens are robust in flavour; chicory, frisee and endive are slightly bitter; mâché and iceberg lettuce are sweet.
However you eat them, enjoy your leafy greens and the impressive collection of nutrients they provide from their very low-calorie bundle.
1 Di Noia, J. Defining Powerhouse Fruits and Vegetables: A Nutrient Density Approach. Prev Chronic Dis 2014; 11: 130390. Doi: http://dx.doi.org/10.5888/pcd11.130390.
3 Blekkenhorst LC, Sim M, Bondonno CP, Bondonno NP, Ward NC, Prince RL, Devine A, Lewis JR, Hodgson JM. Cardiovascular Health Benefits of Specific Vegetable Types: A Narrative Review. Nutrients. 2018 May 11; 10(5):595. Doi: 10.3390/nu10050595. PMID: 29751617; PMCID: PMC5986475.
4 van der AVoort, C.M.T., Jonvik, K.L., Nyakayiru, J., van Loon, L.J.C., Hopman, M.T.E., Verdijk, L.B. Journal of the Academy of Nutrition and Dietetics. 120(8); 1305-1317. August 1, 2020. A Nitrate-Rich Vegetable Intervention Elevates Plasma Nitrate and Nitrite Concentrations and Reduces Blood Pressure in Healthy Young Adults. Doi:https://doi.org/10.1016/j.jand.2020.02.014.
5 Carter, P., Gray, L. J., Troughton, J., Khunti, K., Davies, M. J. Fruit and vegetable intake and incidence of type 2 diabetes mellitus: systematic review and meta-analysis BMJ 2010; 341 :c4229 . Doi:10.1136/bmj.c4229.
6 Morris, M.C., Wang, Y., Barnes, L.L., Bennett, D.A., Dawson-Hughes, B., Booth, S.L. Nutrients and bioactives in green leafy vegetables and cognitive decline: Prospective study. Neurology. 2018 Jan 16; 90(3):e214-e222.Ddoi: 10.1212/WNL.0000000000004815. PMID: 29263222; PMCID: PMC5772164.
9 Nagata, C., Nakamura, K., Wada, K., Oba, S., Hayashi, M., Takeda, N., Yasuda, K. Association of dietary fat, vegetables and antioxidant micronutrients with skin ageing in Japanese women. Br J Nutr. 2010 May; 103(10): 1493-1498.
12 McDonald, D. et al. American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems, 2018; 3 (3): e00031-200018 Doi: 10.1128/mSystems.00031-18.
13 Prasad, R., Shivay, Y. Oxalic Acid/Oxalates in Plants:From Self-Defence to Phytoremediation. Current Science. 2017; 112. 1665-1667. 10.18520/cs/v112/i08/1665-1667.
14 Heaney, R.P., Weaver, C.M. Oxalate: effect on calcium absorbability. Am J Clin Nutr. 1989 Oct; 50(4):830-832. Doi: 10.1093/ajcn/50.4.830. PMID: 2801588.
15 Mitchell, T., Kumar, P., Reddy, T., Wood, K.D., Knight, J., Assimos, D.G., Holmes, R.P. Dietary oxalate and kidney stone formation. Am J Physiol Renal Physiol. 2019 Mar 1;316(3): F409-F413. Doi: 10.1152/ajprenal.00373.2018. Epub 2018 Dec 19. PMID: 30566003; PMCID: PMC6459305.
19 Chai, W., Liebman, M. Effect of different cooking methods on vegetable oxalate content. J Agric Food Chem. 2005;53(8):3027-3030.
20 Holmes, R.P., Goodman, H.O., Assimos, D.G. Contribution of dietary oxalate to urinary oxalate excretion. Kidney Int. 2001; 59(1):270-276.
21 Zhao, Y., Martin, B.R., Weaver, C.M. Calcium bioavailability of calcium carbonate fortified soymilk is equivalent to cow’s milk in young women. J Nutr. 2005; 135(10): 2379-2382.
22 Chen, F., Du, M., Blumberg, J.B. et al. Association Among Dietary Supplement Use, Nutrient Intake, and Mortality Among U.S. Adults: A Cohort Study. Ann Intern Med.2019; 170: 604-613. May 7, 2019. https://doi.org/10.7326/M18-2478
23 Xiao, Z., Lester, G.E., Luo, Y., Wang, Q. Assessment of vitamin and carotenoid concentrations of emerging food products: edible microgreens. J Agric Food Chem. 2012 Aug 8; 60(31): 7644-7651. Doi: 10.1021/jf300459b. Epub 2012 Jul 30. PMID: 22812633.
26 Greger, M. How Not to Die. Published by Flatiron Books. 2016.