Imagine you are an otherwise healthy middle-aged, overweight-obese woman willing to make lifestyle changes necessary to lose weight. You’re motivated and have demonstrated that you know how to keep an accurate food log, but you have been weight stable for at least the past six months. You show up at a private weight-loss clinic that uses an integrated problem-solving approach towards weight loss. You meet biweekly with clinic staff to discuss behavior change strategies such as goal setting, self-monitoring, and physical activity, and you receive free resources to help on your journey, such as home booklets for recording dietary intake.

The clinic’s goal is to enable weight loss of 7-10% of starting bodyweight over a 24-week period. Your diet program is individualized and designed to introduce a 500-1000 kcal deficit that leads to a weight loss of 1-2 pounds per week. You are also to gradually increase physical activity levels to achieve 60 minutes of moderate-intensity activity 5 days per week.

If this sounds like you, then you may very well have been one of the participants in Madjd et al’s recent study. As it turns out, there were 61 other women just like you attending the clinic in order to lose weight. These women were randomized into one of two groups for the entire 24-week intervention, with the only difference between the groups being the advice given for the type of beverage to be consumed after lunch.

All the women were habitual consumers of DBs, but no further detail on consumption patterns or amounts is provided. One group of women were told to consume 250 mL (~1 cup) of water after eating lunch, while the second group of women were told to consume 250 mL of DB after lunch for 5 days per week. Aside from this, both groups were free to drink water during other times of the day but were not allowed to consume any other DBs. Additionally, both groups were asked not to drink DBs or water during the meal and also not add low-calorie sweeteners such as aspartame or sucralose to beverages such as tea or coffee.

Which group do you think experienced greater weight loss and health improvement?

If you guessed the water group, then you would be correct. Both groups lost a significantly pronounced amount of weight over the 24-week intervention, but the loss in the water group (-8.8 kg; 9.9% initial bodyweight) was significantly greater than the loss in the DB group (-7.6 kg; 8.6%).

Despite the superior weight loss observed in the water group, both groups significantly reduced their waist circumference without difference between one-another. Similarly, significant improvements in all measures of blood lipids and most measures glucose metabolism were not different between groups. The exceptions were fasting insulin, 2-hour postprandial glucose, and HOMA-IR, all of which were significantly reduced in the water group compared to the DB group. However, these benefits towards insulin sensitivity may simply be the result of the additional weight loss rather than owed to the water, per se.

Why did the water group show greater weight loss than the DB group?

Quite simply, the answer is because the water group was eating less; significantly less total calories and carbohydrates, to be specific. This easily explains the study outcomes in that the water group saw greater improvements in insulin sensitivity because of increased weight loss because they were eating less. Simple.

And that is all this study tells us

It is tempting to say that replacing DBs with water benefits weight loss and insulin sensitivity based on these results. That is certainly what the study authors did in the conclusion of the abstract when they wrote,

“Replacement of DBs with water after the main meal may lead to greater weight reduction during a weight-loss program. It may also offer clinical benefits to improve insulin resistance.”

However, given the difference in caloric intake that explains the weight loss that explains the improvements in insulin sensitivity, it may be more appropriate to say that the replacement of DBs with water after the main meal is associated with greater weight reduction during a weight-loss program that may lead to additional improvements in insulin resistance.

The question thus becomes why the water group ate less

The answer to this question is unclear and could be any number of factors. It is possible that the water group experienced greater satiety than the DB group. It has been shown that consumption of both regular and artificial sweeteners activate the primary taste pathway in the brain, and it has been argued that this leads to an interfering with learned relationships between sweet tastes and typical post-meal outcomes, which may result in an impaired ability to compensate for calories when true sweeteners are consumed. However, other research has shown that habitual consumers of artificial sweeteners do show similar brain responses towards non-caloric and caloric sweeteners. Importantly, all the women of the study at hand were habitual DB drinkers.

It is also worth mentioning two previous studies that counter the idea that water is more satiating. The first had overweight-obese adults replace regular soda in their diets with DBs or water with no other dietary intervention or advice for six months. Ultimately this led to a an additional 27 fl oz of water being consumed daily by the water group and an additional 24 fl oz of DBs being consumed daily by the DB group – both amounts at least 3 times as great as the current study intervention. Yet, both groups lost a similar amount of weight (2-2.5% bodyweight).

The second study was a 12-week weight-loss intervention in obese adults. The participants were randomized to consume at least 24 fluid ounces of DBs or water daily. In this study, the DB group lost significantly more weight than the water group. More importantly, the DB group experienced significantly less hunger than the water group, which likely explains the increased weight loss.

Two other, and in my opinion more likely, possibilities for the current study outcomes are the Hawthorne effect and/or subject-expectancy effect. The former describes how study participants may modify their behavior in response to their awareness of being observed, and the latter describes how participants may experience results because they expect them. It is possible that the water group expected to see greater weight loss through replacing all their habitual DBs with water. Thus, they were more compliant with the lifestyle intervention and consequently ate less.

Whatever the reason for the reduced caloric intake in the water group, it is clear that this explains the outcomes observed in Madjd et al’s study. Nonetheless, this study does clearly show that DBs do not interfere with weight loss or the improvements to blood lipids and glucose metabolism that it mediates. One issue with caloric beverages is that they are less satiating than solid foods and can therefore easily lead to weight gain in persons who drink them. It has been shown that both normal-weight and overweight-obese individuals fail to fully compensate for the calories that soda adds into their diet. When considered in conjunction with the current study, it appears that replacing caloric beverages with non-caloric beverages, be it water or DBs, is low-hanging fruit for facilitating weight loss and health improvement.