The Paleolithic diet, Paleo diet, caveman diet, or stone-age diet requiring the sole or predominant eating of foods presumed to have been available to humans during the Paleolithic era. While there is wide variability in the way the paleo diet is interpreted, the diet typically includes vegetables, fruits, nuts, roots, and meat and typically excludes foods such as dairy products, grains, sugar, legumes, processed oils, salt, alcohol, and coffee. The diet is based on avoiding not just processed foods, but rather the foods that humans began eating after the Neolithic Revolution when humans transitioned from hunter-gatherer lifestyles to settled agriculture. The ideas behind the diet can be traced to Walter L. Voegtlin during the 1970s. In the 21st century, the paleo diet was popularized in the best-selling books of Loren Cordain. The paleo diet is promoted as a way of improving health. There is some evidence that following this diet may lead to improvements in terms of body composition and metabolic effects compared with the typical Western diet or compared with diets recommended by national nutritional guidelines. Following the paleo diet can lead to nutritional deficiencies such as an inadequate calcium intake, and side effects can include weakness, diarrhea, and headaches.