One theme we’re going to return to over and over here is the bad science we see in media every day. Media needs content, so even bad studies with no control groups and self-reporting are trumpeted as news, which is one reason why we are assaulted by supposed proof of a diet or health question one day, then fed “proof” of the opposite the next. It’s all to keep you clicking and getting views to pay for the news site; the study promoters want the world to think their results are meaningful so they’ll get funding to do more. Incentives to hype are great, and so hype is what we get.

You can learn to read the studies for yourself to understand which to pay attention to. Or you can read specialty publications that have a record of understanding how real science works so you can rely on them when they say a study’s conclusions are really valid. That’s what I’m trying to do here — I read all the press releases so you don’t have to.

Today’s example is a large meta-analysis by Paige Miller and Vanessa Perez of the Center for Epidemiology, Biostatistics, and Computational Biology, Exponent Inc., published in The American Journal of Clinical Nutrition. A meta-analysis is a statistical study of a basket of other studies which attempts to discern a better answer to questions raised than the individual studies by combining them; techniques for doing this are complex, but the usual result is to quantify a trend in answers over a much larger examined population. The abstract [with my annotations]:

Background: Replacement of caloric sweeteners with lower- or no-calorie alternatives may facilitate weight loss or weight maintenance by helping to reduce energy intake; however, past research examining low-calorie sweeteners (LCSs) and body weight has produced mixed results. [They will go on to show only unreliable studies show diet drinks induce weight gain.]

Objective: The objective was to systematically review and quantitatively evaluate randomized controlled trials (RCTs) and prospective cohort studies, separately, that examined the relation between LCSs and body weight and composition. [A prospective cohort study identifies a sample, characterizes each element of the sample typically by measuring or questioning, then follows the sample to try to show what initial conditions lead to what outcomes.]

Design: A systematic literature search identified 15 RCTs and 9 prospective cohort studies that examined LCSs from foods or beverages or LCSs consumed as tabletop sweeteners. Meta-analyses generated weighted mean differences in body weight and composition values between the LCS and control groups among RCTs and weighted mean correlations for LCS intake and these parameters among prospective cohort studies.

Results: In RCTs, LCSs modestly but significantly reduced all outcomes examined, including body weight (−0.80 kg; 95% CI: −1.17, −0.43), body mass index [BMI (in kg/m2): −0.24; 95% CI: −0.41, −0.07], fat mass (−1.10 kg; 95% CI: −1.77, −0.44), and waist circumference (−0.83 cm; 95% CI: −1.29, −0.37). Among prospective cohort studies, LCS intake was not associated with body weight or fat mass, but was significantly associated with slightly higher BMI (0.03; 95% CI: 0.01, 0.06). [Emphasis mine: only the unscientific prospective cohort studies showed weight gain for diet drink consumers.]

Conclusions: The current meta-analysis provides a rigorous evaluation of the scientific evidence on LCSs and body weight and composition. Findings from observational studies showed no association between LCS intake and body weight or fat mass and a small positive association with BMI; however, data from RCTs, which provide the highest quality of evidence for examining the potentially causal effects of LCS intake, indicate that substituting LCS options for their regular-calorie versions results in a modest weight loss and may be a useful dietary tool to improve compliance with weight loss or weight maintenance plans.

So what we can see here is that self-promoters who wanted to show low-calorie sweeteners had negative effects on body weight and metabolism were citing poor, uncontrolled studies. They explained this supposed effect by claiming these sweeteners confused metabolism by triggering insulin release through their effect on taste buds, but there was no evidence of that. One plausible explanation for the studies showing weight increases for those who reported drinking diet sodas is that such people are already aware of their tendency toward weight gain, and are more likely to consume diet drinks as a result; other factors about them create the weight gain, and their knowledge of their propensity to gain weight is the cause for their choosing to drink diet sodas. Those other causes of weight gain continue during the study, so naturally this population gains more weight than the unconcerned.

Our meta-takeaway from the meta-analysis: correlation is not causation. If you personally want to change some practice or undergo a procedure to improve something about yourself, pay very little attention to studies which are not randomized, controlled trials or their equivalent. Another good example of this is the constant drumbeat of Conventional Wisdom claiming that going to college increases your future earnings; the figures cited are almost always ignoring the fact that people who complete college have thereby been screened for aptitude for admission and then screened again for the ability to stick to a plan. If you are trying to decide whether or not to go to college, your aptitude and fortitude are already determined, and the effect of going vs. not going for you is much less, especially when you evaluate the opportunity costs: what you could be doing instead of college to educate yourself and grow your earnings capacity. Some people (and you might be one of them) would earn more by starting their own business or going to a high tech startup and skipping higher education. Not that earnings are the most important thing; one common pattern is to gain admission to a prestigious school with excellent networking connections and use those ties to jump into a business without bothering to finish school (the route of Bill Gates and many other entrepreneurs.)

Learn to recognize the lies and distortions in the Conventional Wisdom, and chart your own course.

Now it’s likely true that simply drinking water or tea instead of any soft drink is the healthiest option. But for those who appreciate the carbonation and sweet taste of a diet soda, you should feel free to indulge, in moderation. Diet sodas are far better for your body than sugared sodas, large servings of fruit juice, or jazzy Starbucks or Jamba Juice concoctions.

For more on this topic:

Sugared Soft Drinks: Health Risk? (and What About Diet Soda?)
Another Study on Diet Drinks
Fructose: The True Villain?
More on “Fed Up”, Sugar Subsidies, and Obesity

More on science-based diet for health and fat loss:

Getting to Less Than 10% Body Fat Like the Models – Ask Me How!
Starbucks, Jamba Juice Make You Fat
Fat Doesn’t Make You Fat. Government Guidelines Did!
‘Fed Up’ Asks, Are All Calories Equal?
LeBron James Cut Carbs for Lean Look
Why We’re Fat: In-Depth Studies Under Way
Almonds: Superfood, Eat Them Daily for Heart Health
Fish Oil Supplements Ward Off Dementia
Vani Hari: “Food Babe” and Quack
Cleanses and Detox Diets: Quackery
Sugared Soft Drinks: Health Risk? (and What About Diet Soda?)
Gluten-Free Diets: The Nocebo Effect
Acidic Soft Drinks and Sodas: Demineralization Damages Teeth
Fish and Fish Oil for Better Brain Health
Salt: New Research Says Too Little May Be Unhealthy
Bulletproof Coffee: Coffee, Oil, and Butter for Breakfast?