No evidence was obtained indicating that thirst promotes eating and hunger promotes drinking. Thirst was not correlated with energy intake and hunger was not correlated with drinking during the same hour over the 7-d period (r=0.08 and r=0.04, respectively). Further, thirst was not a significant predictor of energy intake and hunger was not a significant predictor of drinking when entered into a linear regression model. Moreover, thirst was not correlated with drinking (r=0.03). Hunger was significantly, albeit moderately, correlated with energy intake during the same hour (r=0.30; p<0.05) and accounted for 14.2% of the variance in total daily energy intake. Using an arbitrary classification of inappropriate ingestive behaviors (i.e., thirsty and hungry, and not drinking or eating; not thirsty and not hungry, but drinking and/or eating; not thirsty but hungry, and drinking but not eating; thirsty but not hungry, and not drinking but eating), participants engaged in inappropriate ingestive events 62% of the time. Participants drank water in response to thirst, in the absence of hunger, 2% of the time, and ate in response to hunger, in the absence of thirst, 68% of the time. The proportion of inappropriate ingestive behaviors did not differ between BMI groups.
These findings raise questions about the fidelity of homeostatic relationships in the current environment as thirst was not predictive of drinking and hunger was only weakly predictive of eating. There are several plausible explanations for these observations. First, changes in the environment or the individual and how the two interact may have degraded the functional relationship between appetitive sensations and intake. The nearly constant availability of foods and beverages and multiple social contexts that encourage eating and drinking in the absence of energy and fluid needs would reduce and confound opportunities to associate eating and drinking with the relief of hunger and thirst. A consequence could be a usurpation of physiological cues to motivate ingestion by environmental cues (26
Second, marked shifts of energy sources have occurred that could also weaken the coupling between appetite and intake. The current low intake of fiber (28
) and high consumption of energy via beverages (30
) are examples that would reduce the satiety value of foods contributing energy. This problem could be exacerbated by the increased consumption of foods with diluted energy content through the substitution of non-nutritive sweeteners for caloric sweeteners and fat replacers for dietary fat. Such manipulations disrupt homeostatic behaviors in rats (31
) and possibly in humans (33
A third explanation for the weak association between thirst, hunger, and intake may entail the influence of non-homeostatic rewarding properties of foods and beverages. The concept of hedonic hunger has recently been described and proposed as a factor in dysregulated feeding (36
). The widespread availability of convenient, inexpensive, highly palatable foods would facilitate the contribution of this mechanism. The concept of hedonic thirst has not been proposed, but may be parallel to hedonic hunger. Properties such as carbonation (37
), color (39
), flavor strength (41
), palatability (42
), sweetness (43
), and temperature (43
) have all been manipulated to encourage drinking that is not necessarily linked to fluid needs.
Another explanation for the lack of association between thirst, hunger, and intake may be more methodological than functional. Current approaches for measurement of appetitive sensations and intake are relatively insensitive (45
). Patterns of thirst and hunger over the week, compared to isolated ratings just prior to or following meals, were used in this study in an attempt to better capture individual differences in appetitive sensations, but the extent to which this improves measurement accuracy and precision requires further evaluation.
Absolute mean daily thirst ratings were higher than absolute mean daily hunger ratings, with less variance being observed in thirst ratings over the day (). Group mean daily thirst was rated at 43±11, while group mean daily hunger was rated at 31±16 on a 100 unit scale. This higher, stable level of thirst over the day would be more permissive to frequent drinking and may pose a challenge to energy balance as energy-yielding beverages were the main contributor to total fluid intake (55%) (). Consumption of energy-yielding beverages is not associated with strong dietary compensation (48
), and thus, may lead to an increase in energy intake and weight gain.
Figure 1 Figure 1A and 1B: Percent contribution of beverage type to peri-prandial i.e. around meals (A) and drink-only (B) fluid intake, according to gender (males vs. females), BMI (normal weight vs. overweight, including obese) and age (less than 30y vs. greater (more ...)
Figure 2A and 2B: Correlation between thirst and total daily energy intake (A) and hunger and total daily energy intake (B) across a 7-day period, from 09:00–21:00 (n = 50).
Habitual drinking patterns in the present population are consistent with previous findings (10
), with 75% of fluid intake occurring peri-prandially. There were no significant differences between gender, BMI, or age groups in the type of beverage consumed during peri-prandial and drink-only events (). However, mean fluid intake from energy-yielding beverages was higher among overweight/obese individuals than normal weight individuals (61% vs. 49%, respectively). Because energy-yielding beverages contribute uncompensated energy and lead to weight gain, overweight/obese individuals may be at particular risk.
Our study failed to find significant differences in appetitive sensations between gender, BMI, or age groups. However, there was a trend towards higher absolute hunger levels in the obese group relative to the normal weight group (p=0.07). This finding warrants further exploration in a larger study population. Additionally, given the overweight/obese group consumed more palatable fluids to meet fluid requirements compared to the normal weight group, possible differential susceptibility to the hedonic properties of beverages demands investigation.
Study limitations include the observational design; thus, no clear conclusions regarding causality can be inferred. Statistical power was also limited, especially for subgroup analyses. Although the telephone administered 24-h dietary recall method used in this study has been validated for collecting dietary information (49
), it is not without error. This technique may yield inaccurate estimates of intake, especially in obese individuals. However, the additional step of identifying and excluding inaccurate reporters with the Goldberg cut-off values (22
) improves the internal validity and minimizes confounding effects of implausible reports.