Yes, spice levels can be adjusted to your comfort without significantly impacting the nutritional value of a dish. The primary nutrients come from the main ingredients, not the spices themselves.
While spices offer some micronutrients and antioxidants, they are typically used in small quantities. The bulk of the nutritional content in a meal comes from the vegetables, proteins, and grains used. Reducing the amount of chili powder, cayenne pepper, or other spices to make a dish milder will not drastically change the levels of vitamins, minerals, fiber, or macronutrients (carbohydrates, proteins, and fats). You can also consider using alternative spices that offer flavor without intense heat, such as cumin, coriander, or turmeric. These spices still contribute to the overall flavor profile and may even offer their own unique health benefits. The key is to focus on maintaining a balanced diet rich in whole foods, regardless of the spice level.
Start with a small amount of spice and taste as you go. It's much easier to add more spice than to remove it once it's already incorporated into the dish. This allows you to control the heat level and ensure it's comfortable for your palate.