If you are doing your best to eat foods that support your good health, then it makes sense to store, prepare and cook them in ways that result in the highest possible nutrient content. During the February Nutrition Q&A call, we talked about various factors that can lead to loss of nutrients in plant foods during storage and preparation. I recommend having a listen to that to avoid losing nutrients down the drain – literally in some cases.
In this blog, however, we are concentrating on the more positive aspect of how to increase the bioavailability of key nutrients that have known anticancer activity. Specifically, we will look at the best preparation and cooking techniques to maximise sulphoraphane from cruciferous vegetables, allicin from garlic, and lycopene from tomatoes.
Water- and Fat-Soluble Nutrients
Nutrients can broadly be divided into those that are water-soluble and those that are fat-soluble. We can absorb water-soluble nutrients into our bloodstream easily, but we do not store them because any excess gets flushed out in our urine, so we need to eat them regularly. Water-soluble nutrients, such as B vitamins and vitamin C, can easily be lost into cooking water or be destroyed by heat.
By contrast, we need to eat fat-soluble nutrients with a source of dietary fat or else we will not absorb them. Fat-soluble nutrients include:
Vitamin A (eggs, liver, dairy)
Vitamin D (oily fish, egg yolks, mushrooms that have been exposed to sunlight)
Vitamin E (nuts and seeds, avocado)
Carotenoids, including beta-carotene (carrots, melon, squashes, kale), astaxanthin (salmon, prawns) and lycopene (tomatoes, guava, papaya)
Quercetin (red apples, red onions, capers)
Curcumin (turmeric)
When eating any of these foods, make sure that they are accompanied by a source of healthy fat. For example, if you are snacking on carrot sticks, then you will absorb more of the beta-carotene if you dip them in some guacamole or houmous.
Cruciferous vegetables
One reason why cruciferous vegetables such as broccoli, cauliflower, cabbage and kale are so good for our health is that we can obtain a compound called sulphoraphane from them. Sulphoraphane has several potential anticancer benefits, including acting as an antioxidant, having anti-inflammatory activity, and potentially protecting DNA. It also has benefits against cardiovascular disease, neurological conditions and osteoporosis(1).
However, cruciferous vegetables do not actually contain sulphoraphane in a ready-made form. Instead, they contain its precursor which is called glucoraphanin, and this is where your food preparation techniques can work for you (or against you!). An enzyme called myrosinase is needed to convert glucoraphanin to sulphoraphane. Myrosinase is held within the cell walls of cruciferous vegetables and is released when the vegetables are chopped or chewed. If you chop up cruciferous vegetables, the myrosinase enzyme can get to work on the glucoraphanin and convert it into sulphoraphane.
However, like many enzymes, myrosinase is heat-sensitive, and cooking deactivates it. If you chop up your broccoli and then cook it straight away, the myrosinase never gets a chance to convert the glucoraphanin to sulphoraphane, and you will have missed out on the potential health benefits of sulphoraphane.
To avoid this, chop your cruciferous vegetables at least 40 minutes and up to 90 minutes before you cook them(2). I know that it’s not always practical to do this, but sometimes you can do it fairly easily by chopping your veggies first as you start preparing a meal with a fairly long cooking time.
Alternatively, you can sprinkle some powdered mustard seed over your cooked cruciferous vegetables before eating them(3). This works because mustard seeds contain myrosinase.
As sulphoraphane is a water-soluble nutrient, it will be lost into the cooking water if you boil cruciferous vegetables. Steam or stir-fry them instead to retain the most nutrients(4, 5). Finally, if you are eating raw cruciferous vegetables (radishes, watercress or rocket for example), make sure to chew them very well to release plenty of myrosinase.
Garlic
Similarly, it is better to crush garlic ahead of heating it, although less time is needed than for cruciferous vegetables. Crushing garlic releases the enzyme alliinase. Alliinase reacts with compounds in garlic called ASCOs to form new organosulfur compounds, of which the best known is allicin. It is the organosulfur compounds which provide the health benefits. These include anti-platelet, antioxidant, antibacterial and antifungal activity. In terms of cancer, organosulfur compounds from garlic act on many of the hallmarks of cancer, including inhibiting angiogenesis, promoting cancer cell apoptosis and inhibiting metastasis(6).
Cooking garlic before it has been crushed inactivates alliinase and so the production of organosulfur compounds is negligible. If, however, you crush your garlic, leave it for at least 10 minutes to allow the alliinase to work, and then cook it for a short period of time (the research suggests less than 6 minutes), the organosulfur products will be available(7). Similar principles apply to onions.
Tomatoes
Tomatoes are a key part of the Mediterranean diet and consist of several beneficial phytonutrients. The best known of these is lycopene, which is a fat-soluble carotenoid. The anticancer activity of lycopene has been established in relation to prostate cancer in particular. However, in vitro studies indicate that it has activity against a range of other cancer types as well. Lycopene affects cancer signalling pathways including those related to inflammation, proliferation, growth and angiogenesis(8). Consuming lycopene can also be protective against coronary heart disease, cardiovascular diseases in general and type 2 diabetes.
Lycopene is contained in the cell walls of tomatoes and is released during processing and cooking. Cooked tomatoes therefore contain more bioavailable lycopene than raw tomatoes do. Minimally processed tomato products such as tomato purée, paste or juice, also have higher lycopene concentrations than raw tomatoes(9).
Since lycopene is fat-soluble, eating tomato products with extra virgin olive oil (as they would be as part of a Mediterranean diet) furthers optimises absorption.
References
(1) Kim, J.K. and Park, S.U. (2016). ‘Current potential health benefits of sulforaphane’, EXCLI Journal, 15, 571-577. Available at https://pmc.ncbi.nlm.nih.gov/articles/PMC5225737/
(2) Wu. Y., Shen, Y., Wu, X. et al. (2018). ‘Hydrolysis before Stir-Frying Increases the Isothiocyanate Content of Broccoli’, Journal of Agriculture and Food Chemistry, 66(6), 1509-1515. Available at https://pubs.acs.org/doi/abs/10.1021/acs.jafc.7b05913
(3) Okunade, O., Niranjan, K., Ghawi, S.K. et al. (2018). ‘Supplementation of the Diet by Exogenous Myrosinase via Mustard Seeds to Increase the Bioavailability of Sulforaphane in Healthy Human Subjects after the Consumption of Cooked Broccoli’, Molecular Nutrition & Food Research, 62(18), e1700980. Available at https://centaur.reading.ac.uk/77433/1/Okunade%20et%20al%202018%20mnfr201700980%20accepted%20for%20publication%20Sulforaphane%20%20Bioavailability%20Paper.pdf
(4) Song, L. and Thornalley, P.J. (2007). ‘Effect of storage, processing and cooking on glucosinolate content of Brassica vegetables’, Food and Chemical Toxicology, 45(2), pp216-224. Available at https://pubmed.ncbi.nlm.nih.gov/17011103/ (Accessed 1 August 2023).
(5) Sun, J., Wang, Y., Pang, X. (2021). ‘The effect of processing and cooking on glucoraphanin and sulforaphane in brassica vegetables’, Food Chemistry, 360:130007. Available at https://pubmed.ncbi.nlm.nih.gov/33993075/
(6) Talib, W.H., Atawneh, S., Shakhatreh, A.N. et al. (2024). ‘Anticancer potential of garlic bioactive constituents: Allicin, Z-ajoene, and organosulfur compounds’, Pharmacia, 71, 1-23. Available at https://pharmacia.pensoft.net/article/114556/list/9/
(7) Cavagnaro, P.F., Camargo, A., Galmarini, C.R. et al. (2007). ‘Effect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates content’, Journal of Agricultural and Food Chemistry, 55(4), 1280-1288. Available at https://www.researchgate.net/publication/6547222_Effect_of_Cooking_on_Garlic_Allium_sativum_L_Antiplatelet_Activity_and_Thiosulfinates_Content
(8) Collins, E.J., Bowyer, C., Tsouza, A. et al. (2022). ‘Tomatoes: An Extensive Review of the Associated Health Impacts of Tomatoes and Factors That Can Affect Their Cultivation’, Biology, 11(2), 239. Available at https://pmc.ncbi.nlm.nih.gov/articles/PMC8869745/#sec3-biology-11-00239
Gärtner, C., Stahl, W. and Sies, H. (1997). ‘Lycopene is more bioavailable from tomato paste than from fresh tomatoes’, The American Journal of Clinical Nutrition, 66(1), 116-122. Available at https://pubmed.ncbi.nlm.nih.gov/9209178
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