Gulp by Mary Roach: review Horace Fletcher was the great prophet of extreme chewing. The self-styled "nutritional economist" advised mastication until solid food was completely liquid, which in the case of a shallot took 10 minutes. Unsurprisingly, mealtimes for Fletcher followers - who included Henry James and Franz Kafka - were extended affairs. One composed a Chewing Song, to be performed while - as Mary Roach puts it - "diners grimly toiled." It was noted that Fletcher's tongue "was thickly coated and his breath was highly malodorous." Inevitably the study of the digestive process has always attracted cranks and fanatics. William Beaumont spent years extracting gastric acid through a hole in a patient's stomach, to show how it could dissolve meat, bones and pretty much anything else. John Harvey Kellogg, "the archbishop of roughage," in Roach's droll phrase - urged "Nature's Plan" which meant that the "healthy colon" should empty itself four times a day, assisted by platefuls of Kellogg's Corn Flakes. The lesson from Roach's exhaustive investigation into what happens after we put food in our mouths is that the great majority of us need no help from dietary or digestive fads. Chewing helps ease the passage of what we eat, but such is the efficiency of the small intestine and colon that we can easily process food that is entirely unchewed. But we are not all so lucky. There is a harrowing section about the sufferings of poor Elvis Presley, who did indeed die (of acute heart arrhythmia) while striving in vain to have a bowel movement. Presley was tormented by constipation because of "colonic inertia." The autopsy revealed his distended colon packed with undigested material, including barium administered four months before his death. No aspect of the digestive journey is too revolting for Roach. She explains vivaciously why rats eat their own excrement, how President Garfield was fed through his bottom, what happened when Rosa Montoya de Hernandez was held for 16 hours at Los Angeles International airport with 88 bags of cocaine in her gastrointestinal tract. Roach gnaws on narwhal skin, plunges her arm up to her neck in a cow's stomach, and criss-crosses the globe to find experts on spit, regurgitation, dyspepsia, roughage, faecal bacteriotherapy and allied subjects. Don't be put off by her book's silly title or her publishers" attempts to portray her as a bodily functions comic turn. In fact she is a good journalist: hard-working, endlessly curious, irreverent, attentive, sceptical. She illuminates a tricky but hugely fascinating subject while rarely blundering across the borders of reasonable taste. Her book is - dare I say it? - the best kind of lavatory reading. Gulp Mary Roach Oneworld, £11.99, 340pp