Leafy Greens: Harness the Power of Greens
By Dina Aronson, MS, RDN
Today’s Dietitian
Vol. 25 No. 9 P. 14
Leafy green vegetables are among the most nutrient dense foods in the human diet.1 Calorie per calorie, leafy green vegetables pack more protective vitamins, minerals, and phytochemicals than any other food group—plus they’re a good source of fiber, protein, and omega-3 fatty acids.2-4
The healthfulness of leafy green vegetables is so well established that the Healthy Eating Index, the most validated and widely used index to score diet quality in the United States, includes them as a distinct category (along with beans) separate from the general vegetable category.5 This differentiation allows for a more detailed assessment of dietary quality. The scoring tool reflects the 2020–2025 Dietary Guidelines for Americans, which encourages people to consume more vegetables in general, but with an emphasis on dark leafy greens.6
Nutrition Highlights
Leafy green vegetables are a rich source of most nutrients, particularly vitamins K, A, and E; calcium; potassium; fiber; antioxidants; and folate. They’re low in calories, sodium, saturated fat, total fat, and sugar and contain high concentrations of bioactive compounds such as carotenoids, flavonoids, polyphenols, glucosinolates, saponins, chlorophyll, phytosterols, indoles, and coumarins, all of which have been extensively studied for their roles in health promotion and disease reduction.7 In addition, leafy green vegetables have been used for centuries for their medicinal benefits, including their antioxidant, anti-inflammatory, antiallergic, antibacterial, and antiviral properties. Thousands of bioactive compounds in leafy green vegetables work synergistically to promote overall health.7
Disease Risk Reduction
The nutrients and phytonutrients found in leafy green vegetables provide a broad spectrum of cellular protection. They help maintain DNA integrity, support DNA repair processes, and reduce the risk of oxidative stress and DNA damage.8 A diet rich in leafy green vegetables can contribute to overall cellular health, playing a complex role in decreasing the risk of chronic diseases associated with DNA damage and cellular dysfunction, such as cancer, neurodegenerative diseases, and cardiometabolic conditions. In a recent trial, plasma oxidative DNA damage was significantly reduced during a dietary intervention involving a high intake of leafy green vegetables.8
Leafy green vegetables have an anti-inflammatory effect in the body, protecting against inflammation-driven cardiometabolic diseases, such as heart disease, hypertension, obesity, and type 2 diabetes. Hundreds of studies have demonstrated an inverse association between leafy green vegetable consumption and chronic disease risk. In particular, researchers have focused on the powerful influence of these vegetables to lower the risk of a variety of cancers, CVD, and all-cause mortality.9
In addition, leafy green vegetables may help keep people young and vibrant. A recent small but compelling study found that eating cooked kale for just five consecutive days significantly increased the activity of telomerase (the enzyme responsible for telomere length, which is a marker of aging) in cells.10 Furthermore, leafy green vegetable consumption also has been associated with a reduced risk of eye diseases, such as macular degeneration and glaucoma, diabetes, cognitive decline, and bone health.11-16
The Gut Microbiome
Leafy green vegetables can have a positive impact on the gut microbiome, mainly due to their fiber, which is fermented by beneficial gut bacteria.17 This fermentation process produces short-chain fatty acids, such as butyrate, acetate, and propionate, which serve as an energy source for the cells lining the colon and help maintain a healthy gut environment. Leafy green vegetables also are a source of probiotic fiber such as inulin and fructooligosaccharides, which promote the growth and activity of beneficial gut bacteria. They can stimulate the growth of specific strains of microorganisms, such as Bifidobacteria and Lactobacilli, which contribute to a more diverse and balanced microbiome.
In addition to fiber benefits, leafy green vegetables’ phytonutrient concentration and variety have antioxidant and anti-inflammatory properties that encourage a diverse and balanced gut microbiome.18 They help decrease inflammation in the digestive tract, which helps reduce the risk of leaky gut and related gastrointestinal conditions. Specific compounds in leafy green vegetables, like sulforaphane in broccoli and glucosinolates in kale, have been studied for their potential to strengthen gut barrier function, which helps prevent harmful substances from entering the bloodstream and affecting systemic health.19
Antinutrients
So-called “antinutrients,” present in varying concentrations in different leafy green vegetables, include nitrates, oxalates, phytates, cyanogenic glycosides, and tannins. These compounds, while protective in and of themselves, have the potential to decrease nutrient absorption, particularly that of minerals. Phytates, for example, are antioxidants with antitumor and antidiabetes properties and play a role in osteoporosis prevention, but they’re known to form complexes with minerals like iron and zinc, reducing their bioavailability.20 Balancing these benefits with undesired effects easily can be achieved by consuming a well-planned diet and by employing strategies that maximize nutrient absorption.
For example, heat is a simple and common way to overcome the antinutrient properties of most leafy green vegetables. In addition, cooking methods such as blanching, boiling, and steaming are generally more effective at reducing antinutrient levels compared with methods like frying or microwaving. However, because cooking greens also may cause some loss of vitamins and other nutrients, steaming and quick blanching are recommended rather than boiling, as it can lead to the greatest nutrient loss. For most people, the benefits of cooking foods that contain antinutrients generally outweigh the potential nutrient loss.
Nutrient pairing is another strategy RDs and their clients can employ. Iron absorption, for example, is regulated at the gut level, so consuming foods high in vitamin C with iron-rich greens offsets the inhibitory effects of phytate by increasing absorption of nonheme iron.21 Spinach with tomatoes, kale with citrus dressing, and bok choy stir-fried with bell peppers are some simple meal ideas to maximize iron absorption. Copper also is important for the absorption of iron from leafy green vegetables, so including copper-rich foods like nuts, seeds, and whole grains can enhance iron absorption.
Avoiding leafy green vegetables for fear of their antinutrient properties is, for most individuals, missing the forest for the trees. The overall nutritional benefits that greens provide far outweigh any potential drawbacks associated with antinutrients and can be largely overcome with simple planning.22
Optimizing Nutrient Benefits
The general bioavailability of protective nutrients from leafy green vegetables can be maximized with some simple dietary tweaks. For example, the inclusion of healthful fats (eg, nuts, seeds, olive oil, avocados) with the greens helps increase the absorption of vitamin K, other fat-soluble vitamins, and fat-soluble compounds like carotenoids and other antioxidants.23 Spinach sauteed in olive oil, a green salad with tahini dressing, or chopped nuts added to a green stir-fry are all tasty ideas.
Vitamin D is known to enhance the absorption of calcium from leafy green vegetables, but the effect isn’t due to the timing of consumption; the body must have a healthy vitamin D status to affect calcium absorption.
Overconsumption of Leafy Greens
Overconsumption of leafy green vegetables is possible but rare. The most significant risks appear to be overexposure to oxalate or glucosinolate.
In one case, a woman suffered oxalate nephropathy and ultimately end-stage renal disease.24 The combination of her physiological vulnerability (ie, history of gastric bypass) and dose (ie, about 2 cups of raw spinach juice per day) were the culprits.
Cruciferous greens like kale and cabbage (not all leafy greens are cruciferous, and not all cruciferous vegetables are leafy greens) contain glucosinolates, natural sulfur-containing goitrogenic compounds that can interfere with thyroid function if consumed raw and in excess. Cooking these crucifers deactivates the enzyme that releases the compounds, rendering the effect harmless. One would need to eat several cups of raw cruciferous greens every day (ie, dose depends on the type of green) to have an adverse response.25
Counseling Recommendations
Given the myriad of health benefits of leafy green vegetables, RDs should encourage their consumption. Fortunately, leafy green vegetables are accessible, affordable, and available in dozens of varieties. They support nearly all health goals and are appropriate for many cultural eating patterns. Some delicious examples include South Asian spinach curry, Italian escarole and beans, Jamaican callaloo with hot peppers, Chinese stir-fried bok choy, Middle Eastern sautéed dandelion greens, and American kale salad.
For those who value convenience, precut, prewashed leafy green vegetables are an excellent choice that saves time in the kitchen. Frozen greens, which are flash frozen shortly after harvesting, are as nutritious as fresh (or more so) and also may offer cost savings.
RDs can help clients get excited about leafy green vegetables by suggesting recipes that fit their preferences and lifestyle or by focusing on foods they already enjoy. Leafy green vegetables easily can boost the nutritional profile of soups, stews, egg dishes, grain dishes, pizzas, sandwiches, wraps, stir-fries, potato dishes, burritos, and more. Green herbs such as parsley, cilantro, and mint are considered leafy green vegetables as well and are used to enhance the flavor and nutritional profiles of a variety of meals.
It’s important to note that increased leafy green vegetable consumption is contraindicated in patients taking anticoagulant medications (eg, coumadin or warfarin), because of the high levels of vitamin K in greens. Sudden increases in vitamin K intake can cause these drugs to become less effective, leading to a higher risk of clots. The general recommendation is to maintain a consistent daily intake of vitamin K that includes leafy greens.26 If patients taking anticoagulants wish to incorporate more leafy green vegetables to boost health, they need to do so under the care of their prescribing physician who can adjust the medication dose accordingly.
— Dina Aronson, MS, RDN, is director of nutrition content for Tangelo in Bloomfield, New Jersey.
References
1. Drewnowski A. New metrics of affordable nutrition: which vegetables provide most nutrients for least cost? J Acad Nutr Diet. 2013;9:1182-1187.
2. FoodData Central results: kale, raw. U.S. Department of Agriculture website. https://fdc.nal.usda.gov/fdc-app.html#/food-details/323505/nutrients. Published April 1, 2019.
3. FoodData Central results: beef, ground, 80% lean meat/20% fat, raw. U.S. Department of Agriculture website. https://fdc.nal.usda.gov/fdc-app.html#/food-details/2514744/nutrients. Published April 20, 2023.
4. Johnson M, Pace RD, McElhenney WH. Green leafy vegetables in diets with a 25:1 omega-6/omega-3 fatty acid ratio modify the erythrocyte fatty acid profile of spontaneously hypertensive rats. Lipids Health Dis. 2018;17(1):140.
5. Developing the Healthy Eating Index. National Institutes of Health, National Cancer Institute, Division of Cancer Control and Population Sciences website. https://epi.grants.cancer.gov/hei/developing.html. Updated August 1, 2023. Accessed August 30, 2023.
6. Scientific report of the 2020 dietary guidelines advisory committee. Dietary Guidelines for Americans website. https://www.dietaryguidelines.gov/2020-advisory-committee-report. Published 2020.
7. Barba FJ, Esteve MJ, Frígola A. Bioactive components from leaf vegetable products. In: Atta-ur-Rahman, ed. Studies in Natural Products Chemistry. Elsevier; 2014:321-346.
8. Frugé AD, Smith KS, Riviere AJ, et al. A dietary intervention high in green leafy vegetables reduces oxidative DNA damage in adults at increased risk of colorectal cancer: biological outcomes of the randomized controlled meat and three greens (M3G) feasibility trial. Nutrients. 2021;13:1220.
9. Aune D, Giovannucci E, Boffetta P, et al. Fruit and vegetable intake and the risk of cardiovascular disease, total cancer and all-cause mortality-a systematic review and dose-response meta-analysis of prospective studies. Int J Epidemiol. 2017;46(3):1029-1056.
10. Tran HTT, Schreiner M, Schlotz N, Lamy E. Short-term dietary intervention with cooked but not raw brassica leafy vegetables increases telomerase activity in CD8+ lymphocytes in a randomized human trial. Nutrients. 2019;11:786.
11. Francisco SG, Smith KM, Aragonès G, et al. Dietary patterns, carbohydrates, and age-related eye diseases. Nutrients. 2020;12(9):2862.
12. Kang JH, Willett WC, Rosner BA, et al. Association of dietary nitrate intake with primary open-angle glaucoma: a prospective analysis from the nurses’ health study and health professionals follow-up study. JAMA Ophthalmol. 2016;134(3):294-303.
13. Carter P, Gray L J, Troughton J, et al. Fruit and vegetable intake and incidence of type 2 diabetes mellitus: systematic review and meta-analysis. BMJ. 2010;341:c4229.
14. Morris MC, Wang Y, Barnes LL, et al. Nutrients and bioactives in green leafy vegetables and cognitive decline: prospective study. Neurology. 2018;90(3):e214-e222.
15. Kang JH, Ascherio A, Grodstein F. Fruit and vegetable consumption and cognitive decline in aging women. Ann Neurol. 2005;57:713-720.
16. Sim M, Lewis JR, Prince RL, et al. The effects of vitamin K-rich green leafy vegetables on bone metabolism: a 4-week randomised controlled trial in middle-aged and older individuals. Bone Rep. 2020;12:100274.
17. Usuda H, Okamoto T, Wada K. Leaky gut: effect of dietary fiber and fats on microbiome and intestinal barrier. Int J Mol Sci. 2021;22(14):7613.
18. Kan J, Wu F, Wang F, et al. Phytonutrients: sources, bioavailability, interaction with gut microbiota, and their impacts on human health. Front Nutr. 2022;9:960309.
19. Wei L, Zhang J, Zheng L, et al. The functional role of sulforaphane in intestinal inflammation: a review. Food Funct. 2022;13:514-529.
20. Pires SMG, Reis RS, Cardoso SM, et al. Phytates as a natural source for health promotion: a critical evaluation of clinical trials. Front Chem. 2023:1174109.
21. Piskin E, Cianciosi D, Gulec S, Tomas M, Capanoglu E. Iron absorption: factors, limitations, and improvement methods. ACS Omega. 2022;7(24):20441-20456.
22. López-Moreno M, Garcés-Rimón, Miguel M. Antinutrients: lectins, goitrogens, phytates and oxalates, friends or foe? JFF. 2022;89:104938.
23. Moran NE, Johnson EJ. Closer to clarity on the effect of lipid consumption on fat-soluble vitamin and carotenoid absorption: do we need to close in further? Am J Clin Nutr. 2017;106(4):969-970.
24. Makkapati S, D'Agati VD, Balsam L. "Green smoothie cleanse" causing acute oxalate nephropathy. Am J Kidney Dis. 2018;71:281-286.
25. Chu M, Seltzer TF. Myxedema coma induced by ingestion of raw bok choy. Letter to the editor. NEJM. 2010;362:1945-1946.
26. Chang CH, Wang YW, Yeh Liu PY, Kao Yang YH. A practical approach to minimize the interaction of dietary vitamin K with warfarin. J Clin Pharm Ther. 2014;39(1):56-60.