How Preparation Affects The Nutritional Composition of Vegetables
By Fiona Riddle
At this point, most people are well aware of the numerous health benefits of a diet rich in fresh fruits and vegetables. Including a diverse range of different types (and colors) provides your body with fiber, phytonutrients, polyphenols and essential vitamins and minerals. What many people might not know, however, is that the ways in which vegetables are grown and how they are prepared can impact their nutritional composition – either making them more or less nutritious.
Vegetables can be cooked and prepared in numerous different ways: raw, steamed, seared, boiled, roasted, grilled, etc. and each cooking method can impart changes to the overall nutritional composition of the food. When vegetables (and other foods like meat) are cooked, our bodies are able to better digest and absorb their nutrients. Cooking has been the earliest form of food processing with enormous benefits for humans. There are some instances, however, in which several key nutrients are reduced during cooking.
Boiling, for example, is a popular method of cooking many vegetables such as broccoli and cauliflower since it’s quick and fairly hands-off. Simply add cut up pieces of your vegetable of choice to a pot of boiling water, cook until tender and drain. The issue with this method, however, is that many of the beneficial nutrients are actually released into the water, which is then dumped down the drain after cooking.
This method can result in the loss of nearly 50% of water-soluble vitamins. Vitamin C, specifically, is sensitive to heat and will leach out of vegetables when they’re immersed in hot water. Consequently, one would need to consume all of the cooking liquid in order to retain 100% of the nutrients. You can also try peeling your vegetables after they are cooked in order to preserve more nutrients, as the skin provides a barrier between the nutrients and water.
On top of reducing available nutrients, boiling (and other cooking methods including steaming) reduces much of the soluble fiber content of vegetables. This may contribute to improvement in digestibility, as your body has a harder time breaking down fiber. Once fiber is broken down into smaller components which can easily be absorbed in the small intestine, it will no longer provide food (prebiotics) for your gut microbes which live further downstream. This way, different types of cooking will impact how much of the fiber will be metabolized into short chain fatty acids, which have a range of beneficial effects in particular local and systemic anti-inflammatory effects throughout the body.
If you are looking to maximize your fiber intake for optimal gut health, it is important to include some raw vegetables in your diet, such as in a salad, since they will contain the most microbe accessible carbohydrates. And while pressure cooking is touted for making vegetables and legumes more easily digestible, it drastically reduces their dietary fiber content when compared with other cooking methods.
Microwaving is another very popular cooking method in the Western world, made easier by the prevalence of “microwave in bag” vegetables at most grocery stores. While studies have shown mixed results when examining the nutritional composition of microwaved food, there is evidence suggesting a shorter cooking time will preserve the nutrient content as vegetables have less time to break down. It appears that overall, microwaving food has a minimal impact on its nutritional composition.
Sautéing and stir-frying involve cooking food at a high temperature for a short amount of time in a little fat or oil. Since food is cooked quickly and without water, there is little change in the water-soluble vitamin contents. One study also showed the highest retention of antioxidants amongst bell peppers that were stir-fried or roasted compared to boiled or steamed. These findings might suggest that dry-heat cooking is optimal for antioxidant preservation. Additionally, cooking with a small amount of fat actually helps to optimize the absorption of many nutrients as it becomes more bioavailable, further highlighting the benefits of these methods.
Additionally, research has highlighted the impact that different cooking methods have on chlorophyll levels in broccoli specifically. Boiling, stir-frying and microwaving broccoli all reduced its chlorophyll content by 16-27%, yet broccoli that was steamed was virtually unchanged. Steaming also allowed for the highest retention of vitamin C when compared to boiling, stir-frying, and microwaving.
Consequently, steaming may be one of the best methods for preparing vegetables in order to preserve the most nutrients, possibly due to the use of only minimal water at a lower temperature for a short period of time. This same explanation may be attributed to microwave cooking as well. Since the vegetables do not spend time submerged in water, there is less of a chance of the water-soluble vitamins escaping.
In comparison, cooking vegetables at a very high heat for extended periods of time with a lot of water seems to result in the most nutrient loss (like boiling your veggies until they become soggy). And while many of the nutrients leach into the liquid when making soup, consuming the broth along with the veggies allows you to absorb all of the nutrients (which is one of the main reasons broth is so nourishing)
To summarize, for optimal nutritional composition, cook your vegetables at a low temperature, for a short amount of time with minimal water. If you are going to use high heat (such as stir-frying), cook them for as little time as possible. Pairing them with some high-quality fat like olive oil or grass-fed butter will also ensure optimal nutrient absorption.
Fiona Riddle is a Certified Health Coach with a degree in Psychology from UCLA. She is passionate about a holistic approach to health when working with her private coaching clients. She is an avid cook, constantly creating and sharing new recipes on her Instagram (@feelgoodwithfi) to showcase simple clean home cooking. She has helped clients take their health into their own hands and successfully boost their energy and confidence through sustainable lifestyle changes. www.feelgoodwithfi.com