Everyone loves hazelnuts and they can certainly be included as part of a healthy Paleo lifestyle. These particular nuts are relatively low in carbohydrates and pretty fatty, consisting of almost 50% monounsaturated fats. They are also a good source of protein and trace metals like calcium, magnesium, and potassium.
Because hazelnuts are hard to come by in the wild, remember that hunter-gatherers would not be eating more than a handful at a time. As with all nuts, these are best eaten sparingly.
Paleo and Polyunsaturated Fats
One problem with nuts is that they tend to be high in Omega-6 fats. Omega-6s are a type of polyunsaturated fat with inflammatory properties. Part of living a Paleo lifestyle is trying to avoid inflammation as much as possible, as chronic inflammation is linked to just about everything from heart disease to diabetes. Omega-6s are not inherently bad, but thanks to the development of hydrogenated cooking oils, the average American now consumes Omega-6s in a much greater quantity than our bodies are accustomed to handling. To balance out its effects, we need to be sure to get a healthy dose of Omega-3s, which have an anti-inflammatory effect.
Hazelnuts are a good choice, because they are not as high in Omega-6 fats as a lot of other nuts. Mark Sisson gives the low-down on nuts and Omega-6s in this post.
Paleo and Phytic Acid
Nuts also tend to be high in phytates, an anti-nutrient which binds to nutrients in the seeds to prevent their absorption. This can also cause digestive problems and mineral deficiencies in certain individuals when eating foods that are high in phytic acid.
Chris Kresser has an excellent post on why we should be careful of phytic acid when eating our nuts.