A vegetable omelet can work if you're on a diet. If you make it with only one egg and non-stick cooking spray instead of oil, your vegetable omelet can contain as little as 120 calories.
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Most of the calories in a vegetable omelet come from the eggs. One large egg contains 72 calories. To reduce those calories, you can make your omelet from egg whites alone. One egg white contains 17 calories.
Vegetables are low in calories. You can pack your omelet with vegetables and still have a low-calorie meal. One-quarter cup of chopped fresh red tomatoes contains 8 calories, 3 ounces of fresh mushrooms contain 19 calories, 1/4 cup of chopped onions contains 12 calories, 1/4 cup of chopped green bell pepper contains 5 calories and 1/2 cup of raw spinach contains just 3 calories.
Cheese and Oil
You might need a little bit of oil to sauté the vegetables and some more to cook the omelet. If you use canola oil, add 124 calories per tablespoon. If you want to keep the calories down, you can microwave your omelet or try a nonstick cooking spray; a three-second spray contains only 2 calories. If you add a slice of low-fat American cheese to your omelet, it will cost you another 38 calories.