If you're eating clean and still struggling with health or weight issues, there could be a food sensitivity at play. Maybe you've already eliminated the things that humans are known to be sensitive to: dairy, grains, sugar, soy, food chemicals, toxic load... Yet, you're still having troubles.
Enter, the nightshade.
Nightshades are a family of plants that flower in the night. They contain chemical compounds known as saponins, to which some people (randomly) are sensitive.
If you are sensitive to saponins:
You will immediately feel relief upon eliminating nightshades from your diet; and
You should definitely avoid the nightshades forever.
Some nightshades tend to be peoples' favourite snack vegetables - bell peppers and tomatoes, most notably. So if a nightshade sensitivity emerges, it's a bit of a tough pill to swallow. In my experience, as soon as I've uncovered a nightshade sensitivity in a client, they are more than willing to forego their red-peppers-and-hummus snack habit forever, simply because they feel so much better.
Just like every other food sensitivity, an intolerance to nightshades sets off an immune cascade that can become autoimmune in no time flat. That means your immune system starts attacking your own body tissues, most notably the intestinal lining. This sets the stage for systemic inflammation: the root cause of many diseases of the modern age.
If you're trying to hack your health and happiness and still feel like you're not quite there yet, try a 4-6 week nightshade elimination and see how you feel.
Here is a handy list of nightshade vegetables from the fine folks at Paleo Magazine. (Note that quite a few chile- and pepper-based spices are also nightshades!)
Paleo Mag's accompanying nightshade article can be found here.