What to eat during the changing seasons
◆ Most vegetables have higher nutritional value in summer and autumn than in winter and spring.
Among them, pumpkin is a typical example. Pumpkin available in autumn has much higher vitamin C content than that in spring; its carotene content is 3.4 times higher; its sugar content is 27%—89% higher; and the content of trace elements such as potassium, sodium, calcium, phosphorus, and zinc is also significantly higher than in spring. Besides pumpkin, other vegetables with higher nutritional value in autumn than in spring include lettuce and kale. The content of two important nutrients—glucosinolates and flavonoids—is also richer in autumn than in spring.
◆ Some vegetables available in summer have higher nutritional value than in winter.
The most typical examples are the commonly eaten tomatoes, cucumbers, and carrots. Research shows that the vitamin C content in summer tomatoes and cucumbers is about twice that of winter ones; the carotene content in carrots in June is 1.5 times that in the depths of winter.
◆ Why does the nutritional value of vegetables change with the seasons?
This is mainly because most vegetables in winter and spring are grown in greenhouses or glass greenhouses, while in summer and autumn they are mostly grown outdoors, leading to significant differences in light. The nutritional level of vegetables is closely related to photosynthesis. Vegetables grown in summer have ample sunlight, which enhances photosynthesis, promoting the accumulation and conversion of chlorophyll, vitamins, and other nutrients. At the same time, outdoor planting helps promote plant metabolism and nutrient absorption from the soil, increasing the mineral content of vegetables.
In contrast, vegetables grown in greenhouses or glass greenhouses in winter and spring have poor ventilation, which reduces the evaporation of surface water from the vegetables. Consequently, the minerals absorbed from the soil also decrease. On the other hand, vegetables grown in greenhouses in winter and spring have higher pesticide residues than those in summer and autumn. This is mainly because the higher temperature and humidity in greenhouses lead to more serious pests and diseases, resulting in increased pesticide use. Furthermore, the weak light and poor ventilation in greenhouses are not conducive to the degradation of pesticides, causing most of them to remain on the vegetables.
The change in the nutritional value of vegetables is related not only to the season but also to the type of vegetable, making it difficult to explain each case individually. However, summer and autumn are the harvest seasons for most vegetables. This is an excellent time to supplement nutrition by eating more seasonal vegetables. In winter and spring, when the nutritional value of most vegetables is relatively low, it is even more important to eat more to ensure sufficient nutrient intake.