You can count on losing nutrients when you boil vegetables because they leach into the hot water. The amount that's lost varies from one nutrient to the next, but vitamin C and some B vitamins are especially vulnerable. To get the most nutritional bang for your buck, use alternate cooking methods such as steaming and baking.
Boiling poses a double threat to nutrients. High temperatures break down the natural fibers, allowing nutrients to escape, while water dissolves vitamins and they leach out into the cooking water. The amount of nutrients lost depends on the nutrient, the type of vegetable and the amount of water you use during boiling. Vitamin C and the B vitamins are at the highest risk. Minerals and fat-soluble vitamins lose some nutrients due to the high temperature, but they’re less vulnerable to the effect of water.
You’ll lose more vitamin C through boiling than any other nutrient. Since many vegetables, such as sweet peppers, broccoli, peas and Brussels sprouts, are rich sources of vitamin C, this could impact your daily intake. When vegetables are boiled in enough water to cover them, greens lose 45 percent and root vegetables lose 35 percent of their vitamin C. Other types of vegetables lose 25 percent.
The B vitamins are water-soluble, so they dissolve and leach into boiling water. All types of vegetables lose 10 to 15 percent of their vitamin B-6, niacin and riboflavin. Thiamin and folate are more susceptible. Vegetables lose 35 to 40 percent of their total folate and 20 percent of the thiamin they had before being boiled. Losing such a significant amount of folate is a concern considering that green leafy vegetables are one of the best sources.
Minerals are more stable than vitamins and less likely to dissolve in water, but you’ll still lose some as the heat causes the plant to break down. Boiled vegetables lose more potassium than other minerals, according to the U.S. Department of Agriculture. Vegetable greens lose 15 percent of their total potassium, while other types of vegetables lose 10 percent. All of the different types of vegetables lose 5 percent of their calcium, iron, magnesium and zinc.
Tips to Preserve Nutrients
When you’re cleaning vegetables, don’t let them soak in water prior to cooking. Bring the water to a boil before adding the vegetables to limit their time in the water. Instead of draining the cooking water, use it for soups, stews and sauces. Other cooking methods, such as stir-frying, steaming, roasting and microwaving, retain more nutrients, but don’t forget that the length of time after harvesting and exposure to air and light also deplete vitamins and minerals. Store fresh vegetables in the refrigerator and try to buy only the amount you’ll eat within a few days.
- USDA Agricultural Research Service: USDA Table of Nutrient Retention Factors
- University of California Davis: Maximizing the Nutritional Value of Fruits and Vegetables
- University of Kentucky: Vegetable Preparation for the Family
- Cooperative Extension System: Cooking Methods to Preserve Nutrients in Fruits and Vegetables
Sandi Busch received a Bachelor of Arts in psychology, then pursued training in nursing and nutrition. She taught families to plan and prepare special diets, worked as a therapeutic support specialist, and now writes about her favorite topics – nutrition, food, families and parenting – for hospitals and trade magazines.