Rates of obesity and metabolic disease have risen sharply over the last 50 years or so, and there’s no question that the causes of this ongoing trend are numerous and multifaceted. While many of the commonly cited contributors—such as decreasing costs of food production and transportation—may be outside of our power to change, others can be avoided entirely through personal lifestyle decisions. Take, for instance, the overconsumption of sugar-sweetened beverages, which can add hundreds of empty calories to daily energy intake. Many individuals attempt to avoid these calories by opting for “diet” versions of the same beverages—swapping real sugar with low- or no-calorie sweeteners.
Still, despite widespread recognition that sugary drinks can contribute to weight gain and metabolic dysregulation, the question of whether switching to diet versions can lead to meaningful weight loss or reductions in risk of obesity remains controversial. I touched on this topic in a recent “Ask Me Anything” episode on sugar and sugar substitutes, noting how factors such as the type of sugar substitute may alter the net impacts on metabolic health. Yet there is one variable that did not receive sufficient attention as part of that discussion: motivation.
Conflicting evidence on metabolic effects
As we explored in depth in a past premium article, studies to date on the effects of sugar versus sugar substitutes on metabolic health and body weight seem to tell different stories depending on where you look.
If we only considered randomized controlled trials (RCTs)—regarded as the “gold standard” for assessing the effects of a given intervention in humans—consumption of artificially sweetened beverages (ASB) appears to have neutral to slightly positive benefits for body weight and body composition relative to sugar-sweetened beverages (SSB). A number of meta-analyses of such studies have shown that individuals randomized to ASB show reductions in body weight equivalent to about 2 lbs, give or take about 0.5 lbs.1–3 This level of weight loss is questionable in terms of clinical significance, but the results at least indicate that ASB have no negative effect on body weight, even if they don’t move the needle substantially in a positive direction.
However, this conclusion seems less solid when we turn to data from animal studies and epidemiology. Investigations in rats have found that animals given food or water sweetened with artificial sweeteners gain more weight than control animals given sugar-sweetened food or water.4,5 Further, most of these studies report that calorie intake between the groups is equivalent, suggesting that animals exposed to artificial sweeteners eat more food than controls such that they experience no net reduction in total energy intake. (This also suggests that artificial sweeteners may decrease energy expenditure, resulting in increased weight gain despite comparable calorie intake.) Results from human epidemiological studies mostly align with the animal data, revealing positive associations between non-caloric sweetener consumption and body mass index (BMI) or obesity risk. For instance, the San Antonio Heart Study found that changes in BMI over the 7- to 8-year follow-up correlated with reported ASB consumption at baseline in a dose-dependent manner (e.g., 3-10 ASBs per week was associated with a 46% greater BMI increase than non-consumption), while no such association was found with SSB consumption.6
A missing variable: motivation
As a general rule, consistent RCT results hold more weight than results from animals (which may not perfectly mirror human physiology) or observational studies (which can be heavily influenced by confounding variables). But in the case of sweetened beverages, randomized trials have an important flaw: they can’t account for the many diverse reasons that various individuals might have for consuming sweetened drinks in the first place. Participants in RCTs consume ASBs or SSBs because they are required to do so by the study protocol, not because they’re thirsty or have a sugar craving or want the hit of caffeine that comes from a Mountain Dew or syrup-laden coffee drink. In other words, randomization—the means by which RCTs avoid the influence of confounds—imposes a level of control that doesn’t accurately recreate real-world consumption of sweet beverages because it overrides the key variable of motivation.
Of course, a Coke is a Coke (and a Diet Coke is a Diet Coke) regardless of whether you’re consuming it out of thirst or a sweetness craving. So why do we care about motivation? Indeed, the intent behind consumption may not affect the direct impact of the beverage itself, but it often does affect other nutritional decisions we make throughout the day.
If you’re drinking a sugary beverage because you crave a shot of sweetness, you will probably end up replacing the calories with other sources of sugar if you switch to alternatives sweetened with low-calorie sugar substitutes. Experimental evidence in humans and animals has demonstrated that nearly all non-caloric sweeteners fail to recreate the neural responses of real sugar—i.e., activating the brain’s reward circuitry and triggering a sense of satisfaction.7,8 Thus, when the specific goal of consuming sweet beverages is to achieve satisfaction of a craving, ASBs simply aren’t going to cut it, and eventually you’ll likely seek out that satisfaction through other sweets. This may not be a bad thing—for instance, you might try to get sugar from a piece of fruit, which is a better source of sugar than soda because it contains vitamins and fiber and doesn’t spike blood glucose as quickly as liquid sugar sources—but you nevertheless aren’t likely to see net benefits on body weight.
However, sweetness isn’t the only motivation for consuming sweet drinks. Many people drink Coke simply because they’re thirsty and need something to wash down lunch or because they like the caffeine. In such cases, a sweet taste may be desirable to improve palatability, but sweetness is not the objective in itself (for instance, many people add sugar or syrup to their morning coffee because they want caffeine but find black coffee too bitter, not because they crave sugar). When satisfaction of a sweet tooth isn’t the goal, the absence of sweet-related reward signaling doesn’t undermine the intended outcome from beverage consumption—diet drinks satisfy thirst and provide caffeine just the same as their sugar-heavy counterparts. Thus, swapping for diet options in these instances is not likely to create a strong drive to seek out more food and sugar from other sources, and the switch could indeed lead to reduced total calorie consumption.
Motivation matters
All research on the long-term metabolic health effects of sugar and sugar substitutes is beset with shortcomings—a common problem in nutrition, which is so intricately tied to countless other variables of daily life and culture. These variables may substantially influence the relationship between a given exposure (e.g., non-calorie sweeteners) and a given outcome. We’ve seen on numerous occasions how this can compromise epidemiology studies, which are subject to the impact of confounding, but the problem also affects RCTs, as overly controlled study conditions can eliminate critical modifiers of a given exposure–outcome relationship.
With respect to sweeteners and their impacts on health, the list of modifiers is long. In the recent AMA, we discussed several such variables—from the type of sugar or sugar substitute to the timing of intake—for which we have fairly clear data from research. But although motivation may be more difficult to evaluate in a direct, empirical manner, the indirect insights we have thus far support the idea that it can have a profound influence on the relationship between sweetened drinks and weight changes. For those seeking sweetness per se, switching from SSBs to ASBs will likely result in compensatory sugar intake from other sources, yielding no net change in body weight or even possibly a slight increase. But when sweetness isn’t the explicit goal, the same switch may indeed result in the modest weight loss reported in many RCTs. In other words, why one chooses to switch to artificially sweetened beverages may be even more important than whether one chooses to switch when it comes to body weight and metabolic effects.
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References
1. Laviada-Molina H, Molina-Segui F, Pérez-Gaxiola G, et al. Effects of nonnutritive sweeteners on body weight and BMI in diverse clinical contexts: Systematic review and meta-analysis. Obes Rev. 2020;21(7):e13020. doi:10.1111/obr.13020
2. Tan HE, Sisti AC, Jin H, et al. The gut-brain axis mediates sugar preference. Nature. 2020;580(7804):511-516. doi:10.1038/s41586-020-2199-7
3. Tobiassen PAS, Køster-Rasmussen R. Substitution of sugar-sweetened beverages with non-caloric alternatives and weight change: A systematic review of randomized trials and meta-analysis. Obes Rev. 2024;25(2):e13652. doi:10.1111/obr.13652
4. Miller PE, Perez V. Low-calorie sweeteners and body weight and composition: a meta-analysis of randomized controlled trials and prospective cohort studies. Am J Clin Nutr. 2014;100(3):765-777. doi:10.3945/ajcn.113.082826
5. Martinez C, Gonzalez E, Garcia RS, et al. Effects on body mass of laboratory rats after ingestion of drinking water with sucrose, fructose, aspartame, and sucralose additives. Open Obes J. 2010;2(1):116-124. doi:10.2174/1876823701002010116
6. Feijó F de M, Ballard CR, Foletto KC, et al. Saccharin and aspartame, compared with sucrose, induce greater weight gain in adult Wistar rats, at similar total caloric intake levels. Appetite. 2013;60(1):203-207. doi:10.1016/j.appet.2012.10.009
7. 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;16(8):1894-1900. doi:10.1038/oby.2008.284
8. Crézé C, Candal L, Cros J, et al. The impact of caloric and non-caloric sweeteners on food intake and brain responses to food: A randomized crossover controlled trial in healthy humans. Nutrients. 2018;10(5). doi:10.3390/nu10050615
