Vegetables: Raw Vs. Cooked!

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Ever wonder whether vegetables are healthier when they are raw versus cooked?   Some claim that vegetables are better raw because vitamins are lost during cooking. However, is that always true?

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What Nutrients Are in Vegetables?

Before digging deeper, we should know what main nutrients are present in vegetables and why  they are healthy. Vegetables are known to contain phytonutrients where“phyto” means plants and these nutrients have anti-oxidizing effects (2). Therefore, they are commonly known as antioxidants and many of them are involved in protecting our cells from reacting with oxygen as well as fighting off  free radicals that are damaging them (1).

Antioxidants: Water Soluble Vs. Fat Soluble

Antioxidants slow down the deterioration of our cells and reduce our risks of getting chronic diseases like cancer and heart disease (1). In general, there are 2 main types of antioxidants, water-soluble and lipid-soluble. Ascorbic acid (vitamin C) and phenols are common water-soluble antioxidants in vegetables. Phenols are derived from the plant’s own metabolism that produce anti-oxidizing properties (1). Lipid-soluble types like carotenoids and vitamin E are in fruits and vegetables. Carotenoids are the red and orange pigments like carrots and red peppers and are required to produce vitamin A (1). These are more stable in cooking and some are actually absorbed better after cooking. Many people may over-generalize all nutrients in vegetables as water-soluble types and therefore believed that they can be lost while cooking especially with methods that involve water.

 

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Some Facts:

It is indeed true that water-soluble antioxidants can deteriorate after cooking. Boiling and microwaving have detrimental effects on vitamin C levels in vegetables causing a 30-50% loss of antioxidant activity, depending on the types of vegetables (3, 4). However, for some antioxidants, heat is required to increase the availability of them to us. For example, heat can help soften the cell walls of the vegetables and extract the carotenoids that are trapped inside (5). Heat can help release the antioxidants from vegetables and increase their levels substantially.

 

Let’s discuss the effect of cooking on several common types of vegetables !

  1. Broccoli

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Research suggested that steaming is the best method amongst microwaving, boiling and frying to cook broccoli since it retains the highest levels of ascorbic acid (vitamin C) in them (3,6,7). Broccoli contains high levels of carotenoids and many other phenols (3). Cooking broccoli help release the carotenoid’s availability and increase the overall antioxidant activity in it. Even though cooking broccolis may decrease the level of vitamin C, eating them cooked is still recommended due to the enhanced availability of the other antioxidants.

 

  1. Cauliflower

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Similar to broccoli, steaming would be the best method to cook cauliflower to keep the vitamin C  and other phytonutrients intact (3). However, cauliflower does not contain that many carotenoids and they won’t be extracted from breaking down the cell walls, therefore, cooking them mean decreasing the level of total antioxidants(3). In this case, eating cauliflower raw is preferred over cooking them.

 

  1. Tomato/Red peppers

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Both tomato and red peppers have ample amounts of lycopene, a type of pigment that gives them their bright red as well as providing antioxidizing power (8). Apart from lycopene, they also have high levels of carotenoids. Therefore, it is better to consume them cooked since heat increases the availability of both the lycopene and the carotenoids. Research suggests that boiling and grilling these veggies will extract their antioxidants (5). Microwaving is not suggested since it decreases the total antioxidant levels of both of these vegetables (5).

 

  1. Carrots

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Carrots are full of carotenoids and you can tell because they are bright orange (6)! As mentioned, carotenoid availability can be enhanced with several cooking methods,  but in the case of carrots, carotenoids levels are enhanced only slightly (~5%) when they are  boiled (6). When carrots are steamed, fried or microwaved, carotenoids levels decreased by 20-40% (6). In fact for carrots, any kind of cooking decreases the levels of vitamin C by 10-40%, with the highest decrease noted when carrots are fried (4). Even though carrots do not contain very high levels of vitamin C, it is still wise to consume carrots raw. Since boiling carrots can only increase carotenoids levels slightly, it cannot offset the loss of vitamin C in the process.

As seen above, different types of vegetables react differently with cooking. We should not jump to a conclusion when deciding whether we should eat them raw or cooked. It is important for us to consume a variety of different vegetables and have them both raw and cooked in our diet. We need a balance the amount of water-soluble and fat-soluble antioxidants in our body.

 

Written by Stephanie Yu in collaboration with Renée Chan.

Edited by Renée Y. Chan, MBA, MS, RD, RDN, CDN

 

References:

  1. Podsędek, A. (2007). Natural antioxidants and antioxidant capacity of Brassica vegetables: A review. LWT – Food Science and Technology, 40(1), 1–11. doi:10.1016/j.lwt.2005.07.023
  2. Phytonutrients – Nature’s Natural Defense. Retrieved December 30, 2016, from EatRight Ontario, https://www.eatrightontario.ca/en/Articles/Vitamins-and-Minerals/Phytonutrients-–-Nature’s-Natural-Defense.aspx
  3. Pellegrini, N., Chiavaro, E., Gardana, C., Mazzeo, T., Contino, D., Gallo, M., … Porrini, M. (2010). Effect of different cooking methods on color, Phytochemical concentration, and Antioxidant capacity of raw and frozen Brassica vegetables. Journal of Agricultural and Food Chemistry, 58(7), 4310–4321. doi:10.1021/jf904306r
  4. Jiménez-Monreal, A. M., García-Diz, L., Martínez-Tomé, M., Mariscal, M., & Murcia, M. A. (2009). Influence of cooking methods on Antioxidant activity of vegetables. Journal of Food Science, 74(3), H97–H103. doi:10.1111/j.1750-3841.2009.01091.x
  5. Ryan, L., O’Connell, O., O’Sullivan, L., Aherne, S. A., & O’Brien, N. M. (2008). Micellarisation of Carotenoids from raw and cooked vegetables. Plant Foods for Human Nutrition, 63(3), 127–133. doi:10.1007/s11130-008-0081-0
  6. Miglio, C., Chiavaro, E., Visconti, A., Fogliano, V., & Pellegrini, N. (2008). Effects of different cooking methods on nutritional and Physicochemical characteristics of selected vegetables. Journal of Agricultural and Food Chemistry, 56(1), 139–147. doi:10.1021/jf072304b
  7. Turkmen, N., Sari, F., & Velioglu, Y. (2005). The effect of cooking methods on total phenolics and antioxidant activity of selected green vegetables. Food Chemistry, 93(4), 713–718. doi:10.1016/j.foodchem.2004.12.038
  8. Bramley, P. M. (2000). Is lycopene beneficial to human health? Phytochemistry, 54(3), 233–236. doi:10.1016/s0031-9422(00)00103-5

 

About the author:

Stephanie Yu is a student studying nutritional sciences at UBC.

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