A current investigation by the Food and Drug Administration (FDA) is studying the effect of grain-free dog food on a condition called Canine Dilated Cardiomyopathy (DCM). According to the FDA:
"In July 2018, the FDA announced that it had begun investigating reports of canine dilated cardiomyopathy (DCM) in dogs eating certain pet foods, many labeled as "grain-free," which contained a high proportion of peas, lentils, other legume seeds (pulses), and/or potatoes in various forms (whole, flour, protein, etc.) as main ingredients (listed within the first 10 ingredients in the ingredient list, before vitamins and minerals). Many of these case reports included breeds of dogs not previously known to have a genetic predisposition to the disease. The FDA’s Center for Veterinary Medicine (CVM) and the Veterinary Laboratory Investigation and Response Network (Vet-LIRN), a collaboration of government and veterinary diagnostic laboratories, continue to investigate this potential association. Based on the data collected and analyzed thus far, the agency believes that the potential association between diet and DCM in dogs is a complex scientific issue that may involve multiple factors.
"We understand the concern that pet owners have about these reports: the illnesses can be severe, even fatal, and many cases report eating “grain-free” labeled pet food. The FDA is using a range of science-based investigative tools as it strives to learn more about this emergence of DCM and its potential link to certain diets or ingredients."
Certain brands of grain-free dog food seem to be more suspect than others. There is a lot more information about this from the FDA here: https://www.fda.gov/animal-veterinary/news-events/fda-investigation-potential-link-between-certain-diets-and-canine-dilated-cardiomyopathy