Kale and other cruciferous vegetables

Who knew a vegetable could be so cool? Although kale has early roots in Greek and Roman culture, it remained a relatively minor commercial crop in the U.S. until recent years. This leafy green reached celebrity status around 2012, appearing on menus of Michelin star restaurants and becoming the choice ingredient of millennial food bloggers.


This vegetable comes in a wide variety—each with its own unique colors, flavors, and textures: redbor is characterized by its deep purple, curly leaves; the blue-green and purple-red leaves of red Russian are known for being semi-sweet; the large green leaves of Siberian are particularly cold weather-hardy; and Chinese kale (Gai Lan), or “Chinese broccoli,” can be used in place of conventional broccoli in many dishes. Like broccoli, kale is part of the Brassica oleracea family, which also includes cabbage, cauliflower, bok choy, collard greens, and Brussels sprouts. Another more commonly recognized name for this vegetable family is Cruciferae or cruciferous, which refers to the shape of its sprouts that resemble a cross.

Rich in

  • Vitamin K
  • Vitamin C
  • Vitamin A
  • Carotenoids lutein
  • Vitamin B6
  • Folate
  • Fiber
  • Manganese

Cruciferous vegetables contain a plant chemical called glucosinolates, sulfur-containing compounds that are broken down into isothiocyanates and indole-3-carbinol after chewing, chopping, or cooking. In nature, glucosinolates act as a first-line defense for plants, protecting them from environmental and biological stresses (insects, fungi, drought conditions). These same substances are being researched for their proposed ability in humans to affect chronic conditions including certain types of cancer and heart disease. Laboratory studies have shown that isothiocyanates and indole-3-carbinol inhibit inflammatory processes, prevent the growth and spread of tumor cells, and protect healthy cells.

Observational studies that follow groups of people over time have sometimes suggested a protective effect of cruciferous vegetables on various cancers and cardiovascular health, but findings have not been consistent. There are several possible reasons for this discrepancy. The use of different study designs and methods, as well as the way in which the vegetables were cooked can change the bioavailability of isothiocyanates and their effects on the disease process. Genes may also play a role, as some people metabolize isothiocyanates more efficiently than others.

Regardless, kale remains a highly nutritious food to include as part of a healthful dietary pattern. The 2015-2020 Dietary Guidelines for Americans recommend that adults eat a variety of vegetables as part of a healthy meal plan, and specifically at least 1½ cups of dark-green vegetables (including cruciferous) per week.

People who are placed on blood thinners or anticoagulant medication to prevent blood clots are sometimes concerned about eating kale and other green leafy vegetables that are rich in vitamin K. Vitamin K has a unique action that assists in clotting blood, and can interfere with the effects of some blood thinners. However, people taking these medicines can safely eat these vegetables with a general precaution: eating a relatively consistent amount from day to day can allow one’s physician to adjust the dose of medication to balance the dietary intake of vitamin K, and should not interfere with the anticoagulant medication’s effectiveness. For those who are on blood thinners or anticoagulant medications, it would be wise to check with their physician and possibly a clinical dietitian.


Another concern has been the goitrogen content of kale and other cruciferous vegetables. Goitrogens are naturally occurring substances that can block iodine from entering the thyroid gland. Iodine is a trace mineral needed by the body to make thyroid hormones that promote normal metabolism. A deficiency of iodine can lead to a condition called goiter, or enlargement of the thyroid. Healthy persons who eat enough iodine and metabolize iodine normally will not be affected by dietary goitrogens. However, if one has an underactive thyroid called hypothyroidism and cannot produce enough thyroid hormone, eating excess goitrogens, especially in raw form, may further suppress thyroid activity and increase the risk of goiter. Those who have hypothyroidism specifically due to an iodine deficiency are at greatest risk. A simple solution is to cook cruciferous vegetables, which deactivates the enzyme responsible for causing the goitrogenic effect. Including a wide variety of vegetables each week other than cruciferous will also protect against eating an excess amount of goitrogens.

Copyright © 2018, Harvard University. For more information about this topic, please see The Nutrition Source, Department of Nutrition, Harvard School of Public Health, www.thenutritionsource.org, and Harvard Health Publications, www.health.harvard.edu.