SIBO

Small intestinal bacterial overgrowth (SIBO) is a pathological condition that Ray Peat understood as a direct consequence of low metabolic rate and hypothyroidism, where bacteria ascend from the colon into the normally sterile small intestine. 2 Peat argued that in very healthy individuals—roughly 15 to 20% of the population—the entire small intestine is sterile, but as health deteriorates and metabolic rate slows with age, bacteria invade farther up the intestinal tract, sometimes even reaching the stomach. 2 This proximal migration occurs because thyroid hormone governs the production of stomach acid, pancreatic enzymes, and peristaltic action; when thyroid function is inadequate, these digestive secretions are underproduced, the pH of the stomach rises, and the slowed motility fails to wash bacteria out of the small intestine before they can replicate. 3

The pathogenesis of SIBO is inseparable from the failure of upper intestinal sterilization. Peat explained that a healthy digestive system creates a streaming fluid from the stomach, pancreas, and liver that should wash the small intestine clean, but hypothyroidism causes a sluggish bowel where food transit time can extend from a healthy 12 hours to 72 hours or more. 2 In extreme cases, the stomach becomes alkaline, allowing alkaline-loving bacteria such as H. pylori to thrive, and some individuals even develop a gastric "brewery" where yeast ferments carbohydrates into alcohol directly in the stomach. 2 The bacteria that colonize the small intestine produce short-chain fatty acids—acetate, butyrate, and propionate—through fermentation of starches and fibers that escape digestion; Peat considered these metabolites to be potentially serious toxins that are absorbed systemically and contribute to inflammation, overlapping with the effects of serotonin, endotoxin, and lactic acid. 2

For treatment, Peat prioritized restoring thyroid function to normalize metabolic rate, digestive secretions, and intestinal motility, which he considered the fundamental resolution. 3 He recommended several direct intestinal antiseptics, most notably a daily raw carrot salad made with grated carrots, olive oil, vinegar, and salt, explaining that carrots possess mild antibiotic functions that selectively reduce bacterial overgrowth without destroying everything. 5 He also noted that cooked mushrooms can gradually disinfect the small intestine. 5 Peat cautioned against the use of metronidazole (Flagyl), describing it as a "pretty toxic antibiotic," and suggested trying safer alternatives first, including germicidal bacterial products like Biosporin. 4 Danny Roddy has extended this perspective by noting that while the carrot salad and mushrooms work rapidly for some individuals, relatively safe, small-intestine-specific antibiotics remain a legitimate option for those who have suffered chronically for years and for whom dietary interventions alone prove insufficient. 10

Dietarily, Peat aligned with those who prefer to avoid fermentable carbohydrates, arguing that sugars are rapidly absorbed high in the small intestine in healthy people and should not provide substrate for bacteria, whereas starches and soluble fibers that resist digestion normally provide the primary nourishment for bowel bacteria. 2 He warned that undercooked starch, starches combined with enzyme inhibitors including polyunsaturated fats, and gums or soluble fibers all support bacterial fermentation. 9 Peat also observed that SIBO can artifactually elevate serum B12 levels because the thriving bacteria produce the vitamin in locations where it should not be, and he noted that a high ratio of calcium to phosphorus in the diet reduces the breakdown of fibers, contributing to a protective effect. 2 The condition reflects a systemic integration of metabolism and digestion: when thyroid and progesterone are deficient, the intestine's ability to absorb sugars quickly is impaired, permitting bacteria to live on sugars as well as starches. 9