Cooking to Maximize Nutrient Bioavailability

by Brooklin White MS Candidate, Dietetic Intern Nutrition

Micronutrients, such as vitamins and minerals, are crucial for brain health. In fact, individuals with neurodegenerative diseases have increased needs of these micronutrients. Dr. Terry Wahls, medical doctor, professor and researcher at the University of Iowa, is best known for treating and reversing her own Multiple Sclerosis. Dr. Wahls is convinced that it was her high intake of micronutrients through whole, colorful foods that helped reverse her neurological disease. Micronutrients are of utmost importance when it comes to brain conditions. Green leafy vegetables are important for myelin production (1). Mushrooms help increase Nerve Growth Factors (NGFs), activate natural killer cells and support immunity (2)(3)(4). Deeply pigmented foods, such as beets, blueberries and purple sweet potatoes, are associated with improved cognitive performance and neuroprotection (5). Low micronutrient intake on the other hand, such as zinc, magnesium, biotin, vitamin K, vitamin D and vitamin A, may accelerate the onset of neurodegenerative diseases (6). Dr. Wahls and other experts in the field of neurodegeneration and diet, break down three key micronutrient vegetable groups that individuals susceptible to neurological issues should incorporate:

  1. Green leafy vegetables: Kale, spinach, collards, arugula etc.
  2. Sulfur rich vegetables: Garlic, onions, cabbage, broccoli etc.
  3. Colorful or deeply pigmented vegetables: Beets, purple sweet potatoes, bell peppers, kale… essentially anything with color!

*It’s important to note that some vegetables can fall into different categories

Cooking Methods and Nutrient Bioavailability

Steaming: Steaming is one of the healthiest forms of cooking, especially when eating out as you avoid excess butters and oils. Steaming has been proven to be the one of the best methods for preserving nutrients in vegetables (7)(8).

Sautéing/stir-frying: Sautéing is another common way to cook vegetables. If cooking on medium to low heat, nutrients can be preserved and even enhanced through this method.High temperatures for long periods of time however, increase nutrient loss.

Baking: Baking is a dry-heat method that can be used for vegetables with thick skins, such as potatoes, squash and cruciferous vegetables. Although long cooking times at high temperatures can lead to increased nutrient loss for more sensitive vegetables (broccoli, brussel sprouts, asparagus, etc.) short baking periods at lower temperatures can help maintain nutrient quality. If roasting cruciferous vegetables for instance, try cooking them at 315°F for about 10-12 minutes, until they are slightly tender but still have a slight crunch.

Grilling: Maximal nutritional value can be retained when grilling vegetables for a very short time. Best outcomes occur when using a grilling tray and coating the vegetables with a healthy oil such as avocado or algae. As with any form of cooking, the longer the vegetables are on the grill, the more nutrients will be lost.Avoid charring vegetables, which can create unhealthy byproducts.

Microwaving: Microwaving is considered one of the best ways to retain nutrients due to its short cooking times, especially when it comes to sensitive nutrients, such as vitamin C.

*A note about microwaving-There are two types of radiation: Ionizing and non-ionizing. Ionizing radiation waves are those found in X-Rays and can be dangerous if overexposed due to its ability to alter DNA and create damage to the body. Non-ionizing radiation, such as those found in microwaves, do not have the ability to alter DNA and is thus not considered dangerous to human health (9). Just make sure that you never microwave in plastic bags or containers, which can cause toxic substances to leach into foods.

Boiling: When boiling vegetables, the nutrients seep into the water, so unless you plan on incorporating that liquid into your meal, such as in a soup or gravy, try to avoid cooking your vegetables directly in water. You can often see the nutrient loss as the water will turn green (spinach, broccoli, kale), purple (beets, cabbage) orange (carrots) etc. Think loss of color = loss of nutrients.

Preserving or increasing nutrients:

  • Broccoli: The phytonutrient glucosinolate is best retained in broccoli when it is steamed to al dente (10). However, glucosinolate can be preserved in additional cooking methods such as sautéing, microwaving or baking if the cooking time is limited and the broccoli is al dente (the mushier the broccoli, the less nutrients it contains).
  • Tomatoes: Lycopene is a phytonutrient found in the cell wall of tomatoes. Since the cell wall of plants is broken down when heated, the content of lycopene in tomatoes is actually increased when cooked, especially if cooked in a little bit of EVOO (11). When compared ounce by ounce, processed tomato products such as tomato paste, sauce, or juice contain as much as 2-10 times as much available lycopene as fresh, uncooked tomatoes (12).
  • Carrots: Beta carotene, a popular phytonutrient found in carrots and member of the vitamin A family, becomes more bioavailable when the vegetable is cooked or processed (chopped, pureed).
  • Fruit: Incorporating plenty of servings of vegetables and fruit will help ensure you are eating the required nutrients.Since fruits are often eaten raw, you can make sure to get essential vitamins such as vitamin C to supplement those you may have lost through cooking. Luckily, limes, lemons, strawberries, kiwi, mango, and papaya, in addition to oranges, are filled with vitamin C. Berries are also high in vitamin C and contain increased levels of phytonutrients and antioxidants, due to their dark color.

*If heating up frozen berries in the microwave – add that extra juice to your smoothie or sparkling water.

  • Legumes: Legumes do contain a high number of lectins when in a raw state, but lectins are nearly all destroyed when soaked and cooked. After legumes are cooked or processed, they are loaded with protein, iron zinc, fiber, folate and potassium.
  • Spinach: Cooking spinach slightly increases the bioavailability of vitamin A, vitamin E, protein, fiber, zinc, calcium and iron (13).
  • In general, fat soluble vitamins (ADEK) are more stable when cooked.
  • Aside from the increased nutrients seen from cooking vegetables like tomatoes and carrots, eating raw vegetables is one of the best ways to maximize nutrient intake. Vitamin C for instance, which is found in vegetables like yellow peppers, kale and parsley, is very sensitive to heat and is thus better consumed in the raw state. This is also true for B vitamins such as thiamin, biotin, niacin and riboflavin, which are found in dark leafy vegetables. Garlic is an example of a sulfuric vegetable that contains B and C vitamins and thus has more benefits when eaten raw. Crushing or chopping garlic releases an enzyme called alliinase, which causes the formation of allicin which has shown anti-inflammatory and antioxidant benefits. Alliinase is broken down by heat, however, so it is recommended to let garlic stand after crushing for 10 minutes before cooking it to maximize these beneficial properties (14). An interesting study published in the British Journal of Nutrition showed that individuals who were on a raw food diet for a long period of time had higher levels of beta-carotene than controls, but their total levels of vitamin A were the same and they had lower levels of lycopene, which is the powerful antioxidant found in tomatoes (15). This emphasizes the importance of incorporating a variety of raw and cooked foods in the diet.

Loss of nutrients:

  • Boiling: As noted above, boiling tends to be the worst method for retaining any type of vegetable nutrients, unless you plan on using the cooking liquid as a part of a soup or stew.
  • Olive oil: Olive oil has a smoke point of 350°F which means all of the nutrients will be cooked off at temperatures higher than 350°. Try to keep cooking temperatures low when using olive oil. When using higher temperatures, use a healthy cooking oil with a higher smoke point.
  • High Temperatures + Long Cooking Times: Overall, the quickest way to zap nutrients is to cook them in high temperatures for too long. Especially when it comes to dark leafy greens (collards, kale, spinach), make sure to heat them up only until they start to wilt.
  • Water soluble vitamins such as vitamin B and C tend to be the most vulnerable to degradation when cooking.

Take away tips

The truth of the matter is that any type of heat or processing will reduce the number of vitamins in fruits and vegetables. Heat can break down and destroy 15 to 20 percent of certain nutrients, such as vitamin C, folate and potassium (7). When it comes to preserving nutrients, focus on minimizing the amount of water you use (unless you are incorporating it into a soup or consuming as a part of the meal) limit the time you cook vegetables (al dente broccoli and brussel sprouts for example helps preserve the amount of the phytonutrient glucosinolate) and keeping the cooking temperatures as low as possible, whenever possible.

  • Fruits and vegetables start to lose their nutrient content as soon as they are picked. It is best to purchase produce from farmers markets, or at least locally (most produce labels will note where they were grown).
  • If you are looking to make a soup or stew with a variety of vegetables, opting for a few frozen bags will maximize the soup's nutrient content. Frozen vegetables are often picked at their peak and then washed and frozen fairly quickly after.
  • Wait to chop or wash your vegetables until you’re ready to eat them as nutrients can become lost after any type of processing (However, keep washing and chopping as a way to meal prep as eating these vegetables slightly processed is much better than not at all).
  • If possible, don’t peel vegetables such as carrots, cucumbers, beets or potatoes. Most nutrients are held in the cell wall of these plants (aka the skin), and thus are often thrown away when peeled. Washing thoroughly is sufficient to remove harmful microorganisms. You can even cook these plants in their skins and peel them after – leaving the plant intact helps preserve more nutrients.
  • In general, most plants should be cooked until they are barely tender, the brighter the color and crisper the crunch tends to offer the most amount of nutrients.

There is unfortunately no one cooking or preparing method that retains 100% of plant nutrients. In order to maximize nutrient consumption, the key is to incorporate fruits and vegetables in a variety of ways: raw, roasted, steamed, boiled (as soup) grilled, etc. In fact, the best way to eat your fruits and vegetables is in the ways that you enjoy. Although there is merit to these nutrient loss concerns, the most important factor is to make sure you are actually incorporating these fruits and vegetables into your diet. Focusing on maintaining a colorful diet will ensure you are receiving adequate vitamins and minerals. Since micronutrients are imperative for brain health, we suggest you aim for 10 servings of vegetables per day. We’ve provided you with two examples of how you can incorporate these servings into your diet:

Day 1:

Breakfast: Veggie + Berry Smoothie

  • 1 cup unsweetened almond milk, 2 cups of spinach, 1 cup frozen cauliflower rice, packet of unsweetened acai, ¼ cup mixed berries, with walnuts and flaxseed

Lunch: Veggie Pita

  • ¼ cup cucumbers, ½ an avocado,1 cup kale,1 small tomato,½ orange bell pepper, lemon juice, and hummus, wrapped in a collard green leaf

Snack:

  • 2 stalks celery (1 cup) with almond butter

Dinner: Roasted Salmon with Veggies

  • 3 oz Salmon with 1 cup sautéed mushrooms, 2 garlic cloves (2 tsp) and 1 cup broccoli and 1 purple sweet potato, baked and drizzled in olive oil, garlic, salt and pepper

Day 2:

Breakfast: Veggie Egg Scramble

  • 2 sliced and sautéed small tomatoes, ¼ cup onions and 1 cup mushrooms in olive oil, scrambled with pasture raised eggs. 1 cup raw arugula on the side with a squeeze of lemon juice, salt and pepper.

Lunch: Mexican Burrito Bowl

  • 2 cups of spinach, ½ cup black beans, 2 tbsp cilantro, ¼ cup red onion, ½ red bell pepper, ¼ cup cucumber, 1 small tomato, ½ an avocado, and topped with your favorite salsa.

Snack:

  • ½ a green bell pepper, 2 large carrots (about 1 cup), 2 celery sticks (1 cup) with hummus

Dinner: Thai Curry Bowl

  • Your favorite homemade Thai curry with 2 celery stalks (1 cup), 1 cup mushrooms, ¼ cup white onion, 2 cloves garlic (2 tsp), ½ a red pepper and 1 cup of spinach cooked within the soup….½ a cup of fresh kale on top as a bonus!

References

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  2. Mori K, Obara Y, Hirota M, Azumi Y, Kinugasa S, Inatomi S, Nakahata N. Nerve growth factor-inducing activity of Hericium erinaceus in 1321N1 human astrocytoma cells. Biol Pharm Bull. 2008 Sep;31(9):1727-32.
  3. Lee DH, Kim HW. Innate immunity induced by fungal β-glucans via dectin-1 signaling pathway. Int J Med Mushrooms. 2014;16(1):1-16.
  4. Akramiene D, Kondrotas A, Didziapetriene J, Kevelaitis E Effects of beta-glucans on the immune system. Medicina (Kaunas).2007;43(8):597-606.
  5. Krikorian R, Shidler MD, Nash TA, Kalt W, Vinqvist-Tymchuk MR, Shukitt-Hale B, Joseph JA. Blueberry supplementation improves memory in older adults. J Agric Food Chem.2010 Apr 14;58(7):3996-4000.
  6. Ames, B. N. (2006). Low micronutrient intake may accelerate the degenerative diseases of aging through allocation of scarce micronutrients by triage.Proceedings of the National Academy of Sciences,103(47), 17589–17594.https://doi.org/10.1073/pnas.0608757103
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  9. Centers for Disease Control and Prevention. (2019, February 14).Non-ionizing radiation.https://www.cdc.gov/nceh/radiation/nonionizing_radiation.html
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  11. Dewanto, V., Wu, X., Adom, K. K., & Liu, R. H. (2002). Thermal Processing Enhances the Nutritional Value of Tomatoes by Increasing Total Antioxidant Activity.Journal of Agricultural and Food Chemistry,50(10), 3010–3014.https://doi.org/10.1021/jf0115589
  12. University of California, Berkeley. (2014, January 29).60+ healthy ways to cook vegetables. @berkeleywellness.https://www.berkeleywellness.com/healthy-eating/recipes/article/60-healthy-ways-cook-vegetables
  13. Etcheverry, P., Grusak, M. A., & Fleige, L. E. (2012). Application of in vitro bioaccessibility and bioavailability methods for calcium, carotenoids, folate, iron, magnesium, polyphenols, zinc, and vitamins B6, B12, D, and E.Frontiers in Physiology,3.https://doi.org/10.3389/fphys.2012.00317
  14. Song, K., & Milner, J. A. (2001). The Influence of Heating on the Anticancer Properties of Garlic.The Journal of Nutrition,131(3), 1054S-1057S.https://doi.org/10.1093/jn/131.3.1054S
  15. Garcia, A. L., Koebnick, C., Dagnelie, P. C., Strassner, C., Elmadfa, I., Katz, N., Leitzmann, C., & Hoffmann, I. (2008). Long-term strict raw food diet is associated with favourable plasma β-carotene and low plasma lycopene concentrations in Germans.British Journal of Nutrition,99(6), 1293–1300.https://doi.org/10.1017/S0007114507868486