Cereal comes from a type of grass and is obtained from the edible parts of the plant’s grain. All grains are off the Paleo menu because they tend to be high in carbohydrates and low in nutrients. Refined grains are higher in carbohydrates and lower in nutrients than whole grains, but whole grains, despite what the media would have you believe, are still not healthy. They come loaded with anti-nutrients such as gluten and lectins. These can cause inflammation and leaky gut, compromise the health of your gut bacteria, and spike your blood sugar.
Spikes in blood sugar are problematic because a spike leads to a crash, which leaves you feeling tired, cranky, and craving more carbs to get your blood sugar back up. It can also lead to insulin resistance, which is a dangerous precursor to a number of diseases, including diabetes and cancer.
Mark Sisson has a number of reasons why we should be avoiding grains.