It's the opposite really ...
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Boston Lettuce (one 163g serving) has only 3.5 grams. That is a very low amount of carbohydrate considering it's high volume.
Boston lettuce also has 2 grams of fiber per serving and since fiber counts as a carb that means that it only has 1.5 grams of carbs per serving - fiber doesn't go into the bloodstream and affect your insulin or get stored as fat.
It doesn't matter what diet you're on, in my book that counts as a low-carb food which means you can eat as much boston lettuce as you like and not worry about it.
You would need to eat 12 heads of the lettuce just to reach your carb limit on a Keto diet, that's how little carbs it has.
Controlling insulin is key when it comes to losing weight and good health and adding in boston lettuce ensures that you are as safe as can be with that food group.
Eat and be healthy.
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