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We all know that sugar-sweetened sodas and soft drinks, especially sodas and soft drinks sweetened with high-fructose corn syrup, pack on the belly fat. But are the diet gurus right when they tell us zero-calorie drinks do the same thing?

In the United States, sweetened carbonated beverages account for about one-fourth of all fluid consumption, more than juice, more than coffee, more than tea, and more than milk. Only water is drunk more often than sweetened soft drinks, and some people don't drink water at all.

There's a Lot of Sugar in Regular Sodas

When soft drinks are sweetened with high-fructose corn syrup, cane sugar, or honey, it's a no-brainer that they contribute to weight gain. A 12-ounce (365 ml) can of regular Coca Cola, for example, contains 39 grams of sugar that pack 140 calories. The more frequently consumed 20-ounce (590 ml) bottle of Coca Cola contains 65 grams of sugar worth 240 calories. A one liter bottle of Coca Cola, which is a little larger than a 7-11 Big Gulp, contains 108 grams of sugar for 400 calories. And if you drink 2 or 3 or 4 Cokes or similarly sugary soft drinks every day, the wonder isn't that you don't gain a couple pounds (a kilo or so) every week.

Diet gurus often tell us that sugar-sweetened soda pop has no nutritional value whatsoever. That's not quite true. We actually do need glucose, and if the choice is between drinking Dr. Pepper and death from hypoglycemia, the better choice is to drink the Dr. Pepper.

Most of the time, however, our bodies already are getting enough glucose from other carbohydrate foods, and extra sugar has to be burned in the liver or stored or fat. Since the liver can only process about 10 grams (40 calories) worth of fructose at a time, most of the sugar from soda water becomes fat. But what if the beverage is sweetened with aspartame (Nutrasweet) or sucralose (Splenda)?

Diet Soda Drinkers Gain Weight Even Faster

In the 1990's and the first decade of the 2000's, researchers at the University of Texas Health Sciences Center in San Antonio followed 474 people aged 65 to 74 in the  San Antonio Longitudinal Study of Aging, or SALSA. The researchers took measurements of height, weight, waistline, and diet soda consumption when volunteers were enrolled in the study and about every 2-1/2 years thereafter. 

The researchers also recorded  diabetes status, leisure-time physical activity level, neighborhood of residence, age and smoking status at the beginning of each interval, as well as sex, ethnicity, and years of education.

The results of this study are widely quoted as:

  • Diet soda drinkers experienced 70% greater increase to waistline measurements than non-diet soda drinkers and
  • People who drank two or more diet sodas per day had an average 500% greater increase to waistline measurements than those who didn't drink any diet sodas at all.

Actually, the results of the study were a bit more nuanced than that. Among people who had diabetes, drinking diet sodas did not increase belly fat. Among people who made an effort to get into shape by increasing the amount of exercise they go, drinking diet sodas did not increase belly fat. And among people who were already obese, that is, they had a BMI over 30, drinking diet sodas did not increase belly fat. Only people who were in the best shape suffered a detrimental effect--measured in terms of increased abdominal waistline--when they drank more diet soft drinks.

Continue reading after recommendations

  • Fowler SP, Williams K, Resendez RG, Hunt KJ, Hazuda HP, Stern MP. Fueling the obesity epidemic? Artificially sweetened beverage use and long-term weight gain. Obesity (Silver Spring). 2008 Aug. 16(8):1894-900. doi: 10.1038/oby.2008.284. Epub 2008 Jun 5.
  • Peters JC, Wyatt HR, Foster GD, Pan Z, Wojtanowski AC, Vander Veur SS, Herring SJ, Brill C, Hill JO. The effects of water and non-nutritive sweetened beverages on weight loss during a 12-week weight loss treatment program. Obesity (Silver Spring). 2014 Jun. 22(6):1415-21. doi: 10.1002/oby.20737. PMID: 24862170.
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