Modern grains as we consume them today are devoid of significant nutrients due to their processing and also contain anti-nutrients that can have harmful effects for people already suffering from health issues. In order to refine grain and improve its shelf stability, it is milled and therefore stripped of nutrients such as proteins, vitamins, lipids, and minerals (Paleo Leap, 2017). In an effort to compensate for the loss of these natural nutrients, manufacturers will try and replace them with supplemental versions - but the reason whole foods can be incredibly nutritious is that their vitamins and minerals are balanced primarily for that specific food. It’s why many of the nutrients in salmon, for example, are fat soluble - because salmon is a fatty fish! Nutrients are not the same when we try and replicate them outside of their natural food form.
Modern grains commonly contain gluten and lectins. Gluten is a protein found in many grain products, and there’s a growing awareness around the amount of gluten intolerance among the population (BioMed Central, 2012). Lectins are a protein and, in plants, are their defense against microorganisms, pests, and insects. They are resistant to human digestion, and this is where they can cause issues depending on the individual. Because we can’t digest them, we’ll often produce an immune response to them. That immune response could result in rashes, inflammation, or joint pain. Also, because they’re indigestible, in some individuals they can do damage to their gut wall, leading to “leaky gut” syndrome and making those folks more susceptible to other food allergies and intolerances (Precision Nutrition).
Examples of grains: anything made with flour or wheat is a good place to start. Beyond that, grains include rice, corn, quinoa, amaranth, buckwheat, spelt, rye, sorghum, oats.