Grain-based foods are alleged in the popular media to cause various neurodegenerative conditions, such as mild cognitive impairment (MCI), Alzheimer's disease, and Parkinsons disease. The scientific literature shows that diets containing the right balance of foods, including grain-based foods with an optimal mix of whole and enriched (or fortified) grains, are associated with lower risk for developing a number of neurological conditions. This article, the first of a two-part review, examines the literature on the role of macronutrients, particularly carbohydrates (CHOs), dietary fiber, and vitamins provided by grain-based foods, in the development of various dementias. Studies suggest that grain-based foods and their CHOs and vitamins when incorporated in diets that meet calorie and nutrient needs are important for maintaining brain health and reducing the risk for various dementias. In contrast, excess CHO, fat, or calorie intake from any source, including grain-based foods, may lead to impairment of glucose tolerance, increased inflammation, and production of advanced glycation end products, which are all risk factors for these dementias. The dietary fiber and vitamins found in whole and enriched grain-based foods are associated with normal cognitive functioning. Despite the fact that intake of many B vitamins is low in many elderly people and MCI, Alzheimer's disease, and Parkinsons disease sufferers, the benefits of high-dose supplements, much less lower intakes from grain-based foods, are unclear. It is known, however, that these components play key roles in reactions that reduce oxidation in the brain and promote healthy brain function and that grain-based foods provide one-Third to one-half of the required intake of many B vitamins. Although the contribution of these foods to meeting dietary requirements for these vitamins is without question, their role in preventing or treating these disorders is not clear. However, it is clear that grains and the macronutrients and vitamins they contain, when consumed in recommended amounts, do not increase the risk for MCI, Alzheimer's disease, or Parkinsons disease. The companion review will assess the roles of minerals and phytonutrients from grain-based foods, specific grain-based foods, and dietary patterns that contain recommended amounts of grains (e.g., Mediterranean, DASH, and Mind diets), compared to unbalanced patterns, in altering the risk and course of MCI, Alzheimer's disease, and Parkinson's disease.