More nutrition experts are yelling unexpected advice: eating diet food is making us fatter.

Partly, this may be biology, partly psychology. But the evidence mounts that when we consume foods labeled diet we may be eating exactly the wrong stuff.

This gets ever more important as obesity emerges as a global health issue. Now, in fact, new research said that - for the first time ever - there are more obese people on the planet than there are malnourished. So the hunt is on for who to blame.

An article in the Journal for the Association of Consumer Research out of the University of Chicago tackled the psychology issue involved in why diet foods may not produce weight loss and, wrote the authors, “People hold an implicit belief that healthy foods are less filling than unhealthy foods, an effect we label the ‘healthy = less filling’ intuition. The consumption studies provide evidence that people order greater quantities of food, consume more of it, and are less full after consuming a food portrayed as more versus less healthy.”

Order a veggie burger in other words and you may go for a double just because it seems, well, less filling than an Angus beef burger.  

“From a psychological perspective, we tend not to get very much emotional satisfaction out of eating food we label as ‘healthy’ or ‘diet,’" said registered dietitian Abbey Sharp. "It's just not very enticing or sexy, and also gives us license to eat as much as we want of it because it possess the health halo of being ‘low fat.’” Call that strike one.

Strike two may be what the experts call the satiety issue. Food with fats fill us up and keep us full. Low-fat diet foods leave us hungry and so we eat more. And when we eat enough to no longer feel hungry, we just may have consumed a lot of calories...despite eating “diet foods.”

“Fat takes a long time to digest when compared with carbohydrates, so higher fat foods tend to help us feel fuller longer," Sharp said. "Eat a lower fat version, and you're simply going to need to eat sooner after.”

But the bigger issue just may be that many “healthy” foods are in fact bad for our waistlines. Their “goodness” as such may in fact be questionable. That’s strike three in the eyes of many experts.

How can that be? Colette Heimowitz, nutritionist at Atkins Nutritionals, said that in many foods labeled “low fat,” fat has indeed been removed but it has been replaced with sugar. Meaning low fat foods may in fact put pounds on you.

Why do manufacturers do this? Fat is flavor, so when it's removed, they throw in sugar to make the food more palatable.  

Said Heimowitz: “Low fat foods are not the optimal choice.”

“These products do nothing for weight management because when we eat foods high in carbs especially white refined ones, our bodies digest them more quickly," said registered dietitian Rene Ficek at meal delivery service Seattle Sutton's Healthy Eating. "This can lead to blood sugar swings and cravings making it more difficult to control our overall calorie intake, ultimately making it more difficult to manage weight.”

A study by the Telegraph newspaper in London found that it is commonplace for a food labeled “low fat” to have much more sugar than its untampered with version, in some cases up to five times more.

How bad is sugar for you? The World Health Organization - WHO - recently cut its daily maximum sugar intake to six teaspoons daily, or at most 12 teaspoons. WHO noted that the devilish issue with sugar is that for most of us it comes on the sly: “Much of the sugars consumed today are ‘hidden’ in processed foods that are not usually seen as sweets.”

Low-fat foods are said by some experts to be prime culprits in delivering this unexpected sugar.

How can this be that so much seems foggy and uncertain with dieting? “Nutrition is a relatively new science," Ficek said. "Therefore, we are continually learning new things within this science daily.”

For a long time, saturated fat was seen as the prime culprit in obesity. Now - increasingly - scientists are eyeballing sugars and simple refined starches. Is today’s science better? We’ll know in a decade and the size of our waists will be the answer.

This article is commentary by an independent contributor. At the time of publication, the author held TK positions in the stocks mentioned.