Lately I've gone on a streak of reviewing studies looking at the effects of low-calorie sweeteners, specifically those in diet soda, on body weight. So far, I've yet to find a smoking gun that indicts diet soda as a cause of weight gain, or a hurdle that can trip up your weight loss efforts.
I came across a paper published last month in Diabetes, Obesity and Metabolism which is just as unconvincing as the previous "diet soda makes you fat!" studies. Let's take a look at the paper and explain why.
This was a clinical trial, which is a step up from previous epidemiological studies that simply compare dietary records to health outcomes over 20 years. But a quick glance at the abstract reveals some important facts about the study. Most importantly, the researchers worked with a group of 81 obese, diabetic women. Since most people don't belong to all three of those categories, the results of the paper don't necessarily apply to the population at large. The authors admit this, too.
When you look at the results, though, this is what you read: "Compared with the [diet beverages] group, the water group had a greater decrease in weight (water, -6.40 ± 2.42 kg; DBs, -5.25 ± 1.60 kg ... in BMI (water, -2.49 ± 0.92 kg/m2 ; DBs, -2.06 ± 0.62 kg/m2 over the 24-week period."
That translates into a difference of just over 3 lbs of weight lost and a BMI reduction of .43 in six months! These are clinically insignificant results. If you're obese, these kinds of changes would likely have an immeasurable affect on your health; they certainly wouldn't get you into that bikini you'd like to fit into, nor drop you into a healthy BMI range. The authors admitted this when they wrote that:
"...the results indicated a significant effect of replacing DBs with water on weight loss during the 24-week period, [but] it seems that the 24-week intervention was not sufficient for significant effects on waist circumference (WC) as a related metabolic variable."
If you're eating a nutritious diet, skipping the bad carbs and exercising appropriately, that extra 3 lbs of weight loss may be a nice cherry on top of your weight loss efforts. But you're likely to experience excellent results by doing the three things I mentioned above, so ridding your pantry of diet soda is probably unnecessary.
But let's dig into the paper and discuss a few other pertinent facts. For one, the study subjects self-reported their dietary intake, the problems with which we've discussed in detail previously. At minimum, self-reported dietary records introduce a degree of uncertainty that can't be accounted for--whatever fancy statistical models you use to compensate. The authors admit this, honestly enough:
"[D]espite weekly follow-up by phone and a fortnightly clinical visit to measure dietary compliance, the present study relied only upon subjective reporting of storing and consuming the water and DBs, which is not as accurate as objective methods for measuring compliance."
The authors also pointed out, interestingly enough, that the water group consumed fewer carbs than the DB group, but they don't know what mechanism may be responsible for this reduction. I'll add that perhaps this is where our emphasis should be--on reducing carb intake--when we encourage people to lose weight. Plenty of studies back up that approach. Moreover, it's possible that you could cut your carbs more drastically, consume diet soda and still lose a lot of weight.
In sum, this is a small study with important limitations. Even if the results are accurate, they're insignificant to most people, especially those of us who need or want to lose a lot of weight.