Is Yogurt a Diet Food?

Maybe, but probiotics are not as simple as they’re touted to be.

Americans once thought of yogurt as a spartan breakfast or lunch suitable for a dieting teenage girl or older woman. More recently, yogurt has become the best-known probiotic food, the lead player in a huge and fast-growing business, with global sales of food and supplements projected to reach $32 billion this year. Fans claim a variety of benefits for probiotics, including weight loss. The problem is that the word “probiotics” covers a wide variety of bacteria, with different effects. Saying “probiotic” is like saying “European” as if Danes were just like Greeks.

The probiotic argument runs like this: you need “friendly” bacteria to rebalance the normal inhabitants of your large intestine or colon, which much science shows have been influenced in bad ways by the standard Western diet. Probiotic supplements or food may contain a single strain of bacteria or many, in daily doses of 1 to more than 250 billion organisms.

Lactobacillus bulgaricus and Streptococcus thermophiles are the two bacteria that turn milk (including cow, sheep, and goat milk) into yogurt. The voluntary “Live & Active Cultures” seal from the National Yogurt Association (NYA) is a promise that a refrigerated yogurt contains at least 100 million cultures per gram of those two bacteria at the time of manufacture (10 million for frozen yogurt) — but many may have perished since then. The Food and Drug Administration has not approved the seal, and the NYA doesn’t monitor products.

Some people see yogurt as a diet food because it contains calcium. There’s evidence that consuming too little calcium can contribute to obesity, but eating extra dairy and calcium doesn’t seem to aid dieting.

The best evidence tying yogurt and healthy weight comes from a 2011 analysis of data collected over 20 years from more than 120,000 U.S. men and women in their 30s, 40s, and 50s. This study added to the growing body of evidence that standard dieting advice, “Eat less and exercise more,” Is too simple. What you eat counts, too, as well as other factors like how much you sleep. Although the researchers couldn’t explain why, they concluded that people who ate yogurt gained less weight over the two decades. These yogurt-eaters picked up the habit when it was a sign of special health awareness and in the years before the probiotic commercial boom.

That’s important, because some kinds of Lactobacillus seem to promote weight gain. Farmers have been giving animals probiotics to promote weight gain for decades, most commonly Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus fermentum, and Lactobacillus reuteri. Infants and people with diseases causing weight loss have also received various lactobacillus supplements to promote weight gain. A review of 17 randomized control trials in humans, 51 studies on farm animals, and 14 experimental models concluded that Lactobacillus acidophilus in particular caused significant weight gain in humans. Acidophilus is often included in yogurts touted as especially pro-biotic, and Greek yogurt, non-dairy yogurt, and kefir, a fermented milk drink.

The same review — and others — have found that a strain of Lactobacillus gasseri can aid weight loss. Gasseri may show up in traditional fermented foods, but it’s currently not on labels in your supermarket — researchers isolated it from breast milk. You may see reliable products emerge as the science develops. In the meantime, if you want to eat yogurt while trying to keep your weight down or lose pounds, it makes sense to stick to ordinary yogurts, if you can find them — or experiment a bit to see what seems to work for you. Or to hedge your bets, try a variety of traditional fermented foods rather than eat the same yogurt every day: get more than one culture in your guts.