Should You Kick the Diet Soda Habit?

Author: Dr. Stephen Chaney

We are consuming ever increasing amounts of diet sodas to combat the obesity epidemic. In 1960 14% of the U.S. population was obese and 3.3% of us consumed diet sodas. By 2010 41% of the U.S. population was obese and 20% of us were consuming diet sodas. It’s pretty clear that diet sodas aren’t helping us solve the obesity epidemic, but are they actually part of the problem?

You’ve probably seen the headlines questioning whether diet sodas actually help you lose weight. In fact many of the headlines imply the diet sodas will cause you to gain weight. Two of the more sensational headlines I came across said “Think diet sodas help you lose weight? Not so, Purdue study finds”, and “Can diet sodas actually cause more weight gain than regular sodas?”

Let me start with the first headline. The Purdue publication referred to in the headline (Swithers, Trends in Endocrin. & Metab., 24: 431-441) wasn’t really a study, it was an opinion piece. That simply means that it was a review where the references were selected on the basis of the author’s opinion. That’s OK if you clearly label it as an opinion piece, which Dr. Swithers did.

Now for the second headline: There is no good evidence that diet sodas will cause you to gain more weight than regular sodas. However, a number of published studies suggest that consumption of diet sodas is associated with weight gain – sometimes just as much weight gain as consumption of the sugar sweetened sodas they replace.

Do Diet Sodas Make You Fat?

The evidence that Dr. Swithers (Trends in Endocrin. & Metab., 24: 431-441) cited was pretty impressive.

For example, the San Antonio Heart Study recorded consumption of diet sodas and regular sugar sweetened sodas in 3,862 adults (average age 44) and measured the increase in BMI (a measure of obesity) over the next 7-8 years. That study found:

• Individuals consuming >21 diet sodas/week were almost 2-fold more likely to become overweight or obese than individuals consuming no sodas.

• There was a clear dose response effect, with a 41% increased risk of becoming overweight or obese for each can or bottle of diet soda consumed/day.

• The increase in weight associated with diet soda consumption was just as great for those who were at normal weight at the beginning of the study as it was for those who were obese at the beginning of the study.

• In this study the increase in weight associated with soda consumption was greater for diet sodas than it was for regular sodas.

Another major study (Circulation, 116: 480-488, 2007) recorded diet and regular soda consumption in 6039 participants in the Framingham Heart Study (average age 53) and measured the increase in obesity (along with other parameters associated with metabolic syndrome or pre-diabetes) over the next 4 years. This study found:

• Individuals consuming one or more sodas/day had a 48% increased risk of becoming obese compared to people with infrequent soda consumption.

• In this study the weight increase associated with soda consumption was virtually the same for diet sodas and regular sodas.

Are These Studies True?

These, and similar studies have been criticized because they are looking at associations, which do not prove cause-and-effect. For example, it’s not always clear whether the people in those studies gained weight because they were consuming diet sodas or consumed diet sodas because they were overweight.

That argument is less persuasive for the San Antonio Heart Study, because the weight gain associated with diet soda consumption was also seen with people who were at normal weight at the beginning of the study. Still there is a need for good double blind, placebo controlled intervention studies.

There have been very few intervention studies in which one group of subjects were told to drink only diet sodas and the other group only regular sodas. Unfortunately, in those studies the total caloric intake of the diet soda group was also restricted. So while the diet soda group did lose weight, it’s not clear whether that weight loss was due to the diet sodas or the overall caloric restriction of the diet.

You may have also seen the recent headlines from a study showing that people consuming diet sodas gained no more weight than people consuming water (Obesity, 22: 1415-1421, 2014). But once again, both groups were given detailed instructions on how to restrict total calories. Almost any diet will work if you have a dietitian looking over your shoulder and telling you how to restrict calories.

So what is the average consumer to think? On the one hand, dietitians and health professionals are telling you to drink diet sodas if you want to lose weight. On the other hand, you keep seeing these headlines saying the diet sodas may not help you lose weight or may even cause you to gain weight.

Of all the recent blogs and online articles on the topic, the only one I actually recommend reading is from WebMD (http://www.webmd.com/diet/features/diet-sodas-and-weight-gain-not-so-fast).

WebMD often adheres to the AMA line, but I found this to be a very balanced analysis of the science behind the question of whether diet sodas help or hinder weight loss.

How Could Diet Sodas Possibly Cause Weight Gain?

The million dollar question is: How could diet sodas possibly cause weight gain? After all, they contain no calories. I think the most useful perspective from the Web MD article is that it’s probably not the diet sodas themselves that cause weight gain. It’s what we eat with the diet sodas that cause the weight gain. Here are a couple of quotes I found particularly enlightening.

Dr. Barry Popkin, a colleague from the University of North Carolina, calls it the “Big Mac and Diet Coke” mentality. He says: “Especially in America, we have a lot of people who eat high-fat, high-sugar diets, but also drink diet sodas.”

Why is that? Dr. David Katz from Yale University has research suggesting that artificial sweeteners may condition people to want to eat more sweet foods. He says: “Our taste buds don’t really differentiate between sweet in sugar and sweet from, say, aspartame. The evidence that this sweet taste is addictive is pretty clear. What I have seen in my patients is that those who drink diet soda are more vulnerable to processed foods with added sugars.”

There is some independent evidence to back up that hypothesis. For example, one recent study showed that rats given artificially sweetened yoghurt with their rat chow ate more rat chow and gained more weight than rats fed sugar-sweetened yoghurt with their rat chow (Behavioral Neuroscience, 122: 161-173, 2008). Another study in humans showed that consumption of artificial sweeteners activates a portion of the brain associated with cravings for sweets (Physiology & Behavior, 107: 560-567, 2012).

However, this viewpoint is controversial. Some experts think that the association between diet sodas and weight gain is psychological rather than physiological. Simply put, when people consume diet drinks they feel that they can splurge elsewhere.

The Bottom Line

• Once again there is no magic bullet. There is no good evidence that diet sodas will help you lose weight unless you carefully control the calories in everything else you eat. And, diet sodas may just cause you to gain weight because they make you crave the very foods that are worst for your waistline.

• In addition, there may be other good reasons not to consume diet sodas. For example, recent studies have shown that consumption of diet sodas may be linked to increased risk of metabolic syndrome or pre-diabetes (Circulation, 116: 480-488, 2007) and heart disease (see Does Sugar Cause Heart Disease? and Can Soft Drinks Cause Heart Disease?

• My recommendations are to drink water, herbal teas, unsweetened tea & coffee or unsweetened mineral water or seltzer – perhaps with a splash of fruit juice.

• Finally, there is no substitute for a healthy, calorie controlled diet; exercise; and lifestyle change if you want to lose weight and keep it off.

These statements have not been evaluated by the Food and Drug Administration. This information is not intended to diagnose, treat, cure or prevent any disease.