Karen O’Hare, student dietian, Queen Margaret Uiniversity.

 

This green, leafy vegetable has grown in popularity in recent years and has become one of the ‘trendy’ foods. So, what is all the fuss about?

Kale is a member of the cabbage family alongside other cruciferous vegetables such as broccoli, cauliflower and brussel sprouts. It is available all year-round, but is at its best from September to February1. It is best known for its distinctively bitter taste, which may put some people off, but when prepared correctly and combined with the right dishes it can be a tasty and nutritious addition. There are a number of different types of kale; two of which are most commonly available in supermarkets in the UK. Curly kale which, as its name suggests, has crinkly leaves is the most common2. The second type is black kale or ‘cavolo nero’2. This form of kale is a darker, purplish green, and has a sweeter, spicier taste. Both types either come shredded and ready to cook, or in larger leaves that need to be torn.  When buying kale, it is important to look out for crisp leaves that aren’t browning or wilting to ensure that you are getting the freshest produce. Kale is best stored in the refrigerator and eaten within 2-3 days of purchase as after this period, it will begin to get bitterer and less appealing3.

One portion of raw kale is approximately 80g4. The full nutritional composition of this portion and the corresponding reference nutrient intakes (RNI) can be seen in Table 1.

Table 1: Nutritional composition of a portion of raw kale (80g) and UK RNI’s

Nutrient Amount RNI
Children Teens Adults
Macronutrients  
Energy (kcals) 26
Carbohydrate (g)

Of which sugars (g)

Fibre (NSP) (g)*

1.1

1.0

3.1

15g (2-5y)

20g (5-11y)

25g (11-16y)

25g (16-18y)

30g

Protein (g) 2.7
Fat (g) 1.3
Micronutrients
Vitamin A (retinol equivalent) (µg) 420 400µg (1-6y)

500µg (7-10y)

600µg (11-14y) 600-700µg
Vitamin K (µg) 498.4 1µg/kg
Vitamin C (mg) 88 30mg (1-10y) 35mg (11-14y)

40mg (15-18y)

40mg
Folate (µg) 96 70µg (1-3y)

100µg (4-6y)

150µg (7-10y)

200µg (11-19y) 200µg
Calcium (mg) 104 350mg (1-3y)

450mg (4-6y)

550mg (7-10y)

1000mg (11-18y males)

800mg (11-18y females)

700mg

*AOAC value unavailable5; Nutritional composition of raw kale5; Reference Nutrient Intakes6

Macronutrients

Kale is a low-energy dense food, only containing 26 calories per portion5. It provides bulk to a meal, and therefore can be a useful addition to someone’s diet if they are trying to lose weight. Selecting low energy-dense foods, like kale and other fruit and vegetables, has been recommended to prevent overweight or obesity in adults7. It is also rich in insoluble fibre and therefore aids digestion. Diets high in insoluble fibre have been found to reduce the risk of cardiovascular disease and type 2 diabetes8.

Micronutrients

Although kale may be a low-energy food, in terms of vitamins and minerals it carries a lot of bang for its buck! Kale is an excellent source of beta-carotene which is a pre-cursor to vitamin A5.Vitamin A is a fat-soluble vitamin that plays an essential role in the body. It is vital for good vision and ensuring that the immune system functions effectively9. It also plays a role in keeping skin and the mucous membranes that line the nose, mouth and sinuses healthy9. Another fat soluble vitamin that kale is rich in is vitamin K. Vitamin K is essential for blood clotting and therefore plays a role in wound healing10. It also aids in helping to keep bones healthy, particularly in the elderly11. The Department of Health recommends eating a varied, balanced diet in order to reach the RNI’s for vitamins A and K.

Kale is also a great source of vitamin C, containing 88mg of vitamin C per 80g portion5. Unlike vitamin A, vitamin C is water-soluble meaning that we do not store it in our bodies. Therefore it is essential that we get adequate amounts from our diet. Vitamin C is important for growth and repair of body tissues, wound healing and maintaining the health of bones and teeth12. It also aids in absorbing iron from foods and is a powerful antioxidant, limiting some of the damage caused by free radicals. Free radicals are substances in the body that can cause cellular and DNA damage and contribute to the ageing process and the development of various health conditions such as cancer and heart disease12.

Kale also contains a substantial amount of folate. Folate or folic acid is a member of the B-vitamin family, and is a water-soluble vitamin. It is vital for the formation of red blood cells and it also plays a role in the formation of DNA13. Folic acid also works alongside vitamin B12 to help nerves to function properly13. Deficiency of folic acid can lead to a form of anaemia called macrocytic anaemia. Women who are considering pregnancy or who are pregnant or lactating should take a folic acid supplement of 400µg/day, as well as eating a diet rich in folate13. This is to reduce the risk of neural tube defects occurring in the foetus.

Lastly, kale is a good source of plant-based calcium. It is also low in oxalates, which are substances that can inhibit the absorption of calcium, and therefore has excellent absorbability14. Calcium is well-known for its importance in ensuring strong bones and teeth, but it is also required for muscle function and regulating muscle contractions15. Inadequate intake of calcium can lead to rickets (in children) or osteoporosis in later life, and therefore it is very important to ensure that you are getting enough. The Department of Health advises that having a balanced, varied diet will ensure that you reach the RNI.

Phytochemicals

Phytochemicals are not actually nutrients themselves but are naturally occurring compounds in many plant-based foods16.They provide plants with their colour, odour and flavour and are believed to be largely responsible for the many protective health benefits of these foods. They are also believed to play a role in disease prevention, such as cardiovascular disease, type 2 diabetes and cancers16. Kale contains an abundance of various phytochemicals, such as glucosinates and carotenoids (beta-carotene), which have been shown to have cancer-preventative and anti-inflammatory properties16.

Incorporating kale in your diet

There are a number of different ways in which kale can be prepared and it is a very versatile vegetable.

  • Can add it to a salad or a stir-fry
  • Pan-fry with some garlic and chilli and have it as a side dish
  • Add to dishes such as spaghetti bolognese or a chicken curry
  • Add to soups, e.g. kale and green lentil soup
  • Add to a green smoothie
  • Make homemade kale crisps by adding a bit of oil and then baking for 10-15 minutes

It is important to be aware of the different cooking methods and to try to avoid over-cooking as this will reduce the vitamin and mineral content of kale and other vegetables. As kale contains a number of water-soluble vitamins these can be lost when boiling, and therefore steaming is a much better cooking method for nutrient preservation.

References

  1. Bord Bia: Irish Food Board. (2017). Available: http://www.bestinseason.ie/a-z/kale/. Last accessed 23rd Feb 2017.
  2. (2017).Kale. Available: https://realfood.tesco.com/glossary/kale.html. Last accessed 16th Feb 2017.
  3. Diabetes UK. (2017).What’s in season: kale. Available: https://www.diabetes.org.uk/Guide-to-diabetes/Enjoy-food/Cooking-for-people-with-diabetes/Seasonal-cooking/whats-in-season-kale/. Last accessed 20th Feb 2017.
  4. Public Health England. (2015).5 a day portion sizes. Available: http://www.nhs.uk/Livewell/5ADAY/Pages/Portionsizes.aspx. Last accessed 17th Feb 2017.
  5. McCance, R. (2002).McCance and Widdowson’s The composition of foods. 6th ed. Cambridge: Royal Society of Chemistry.
  6. British Nutrition Foundation. (2016). Nutrition Requirements.Available: https://www.nutrition.org.uk/attachments/article/234/Nutrition%20Requirements_Revised%20Oct%202016.pdf. Last accessed 20th Feb 2017.
  7. Health Improvement Scotland. (2010). Management of Obesity.Available: http://www.sign.ac.uk/guidelines/fulltext/115/. Last accessed 21st Feb 2017.
  8. Public Health England. (2015). SACN Carbohydrates and Health Report.Available: https://www.gov.uk/government/publications/sacn-carbohydrates-and-health-report. Last accessed 21st Feb 2017.
  9. Ehrlich, S.D. (2015). Vitamin A (Retinol).Available: https://umm.edu/health/medical/altmed/supplement/vitamin-a-retinol. Last accessed 22nd Feb 2017.
  10. Public Health England. (2015). Vitamin K.Available: http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Vitamin-K.aspx. Last accessed 18th Feb 2017.
  11. Ehrlich, S.D. (2013). Vitamin K.Available: http://umm.edu/health/medical/altmed/supplement/vitamin-k. Last accessed 17th Feb 2017.
  12. Ehrlich, S.D. (2013). Vitamin C (Ascorbic acid).Available: http://umm.edu/health/medical/altmed/supplement/vitamin-c-ascorbic-acid. Last accessed 17th Feb 2017.
  13. British Dietetic Association. (2016). Folic acid.Available: https://www.bda.uk.com/foodfacts/FolicAcid.pdf. Last accessed 19th Feb 2017.
  14. Heaney, R.P. & Weaver, C.M. (1990). Calcium absorption from kale. The American Journal of Clinical Nutrition. 51 (4), p656-657.
  15. Public Health England. (2015). Available: http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Calcium.aspx. Last accessed 16th Feb 2017.
  16. Webb, D. (2013). Phytochemicals’ Role in Good Health.Available: http://www.todaysdietitian.com/newarchives/090313p70.shtml. Last accessed 23rd Feb 2017.