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THE idea that we can improve our gut health by eating foods containing live “friendly” bacteria, or probiotics, dates back to the early 20th century. Ilya Mechnikov, a Russian biologist whose work on immunity led to a Nobel prize, postulated that consuming soured milk fostered beneficial bacteria in the intestines. He claimed that people in Bulgaria who ate yogurt lived longer as a result, and his ideas helped to popularise yogurt in western Europe and North America.

The main types of bacteria found in commercial yogurt are Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus thermophilus. Several studies have found that…