The Gut and Psychology Syndrome (GAPS) diet was developed by Natasha Campbell-McBride, MD, MMedSci (in neurology and nutrition) to treat her son’s autism spectrum disorder (ASD). She is the author of the 2004 GAPS diet book, which implicates certain foods, dysbiosis (an imbalanced microbiome), and “leaky gut” in the development of diseases and mood disorders. Some people also use the GAPS diet in an attempt to improve their gastrointestinal issues.
The diet consists of an optional introduction phase — (broken into six stages, each adding specific foods that “heal the gut”) followed by the full GAPS diet for 18 to 24 months and then the subsequent reintroduction phase.
Homemade meat stock
Fermented vegetable juice
Water or tea with raw honey
Organic raw egg yolks
Meat that has been roasted or grilled
Cold-pressed olive oil
Homemade carrot juice
Stages 3 and 4 both include GAPS recipes for pancakes, bread, and a milkshake. The pancakes and bread are both made from a nut-butter or nut-flour base with egg and squash, and the milkshake is a combination of an allowed protein, juice, and fat source.
Pressed fruit juice
Baked goods sweetened with dried fruit
Foods allowed on the full GAPS diet include eggs, meat, fresh fruits and vegetables, nuts, unprocessed fats, and garlic. After the full GAPS diet, people can begin reintroducing potatoes, fermented grains, starchy vegetables, beans, and regular grains. The diet also recommends eating fermented food and bone broth at every meal, buying organic food, and avoiding processed foods. Fruit is to be avoided at meals on the diet, while proteins should always be eaten with vegetables; the diet claims these combinations normalize acidity levels in the body.
Despite anecdotal reports of improvement on the diet, it has never been formally researched, and some of the theories — such as dietary treatments for autism and the dietary influence on blood acidity — have been either unsubstantiated or debunked. Altered intestinal permeability (leaky gut) has not been consistently observed in children with ASD. Though the microbiomes of children with ASD differ from those of children without ASD, a causal relationship has not been established between the gut microbiome and ASD.  Importantly, these differences could be due to the significant differences in dietary patterns between children with ASD and others.
Additionally, the introduction phase of the diet is restrictive, which could pose a risk to children with ASD and individuals with bowel diseases, as they are already at risk of malnutrition.  Despite the restrictions of the GAPS diet, it is not a low-FODMAP diet, so it may not be helpful for people with IBS, either.