The public is confronted with conflicting messages about weight management. Beverage guidelines are no exception. Not drinking caloric beverages is, for example, described as essential for weight loss,1 while drinking noncaloric beverages, such as water, is described as ridiculous for weight loss.2 Moreover, the scientific literature recommends drinking water and other beverages including low-fat milk and small quantities of fruit juice,3-5 at the same time as criticizing beverage recommendations as lacking an adequate evidence base.6-9
The paragraphs that follow describe a strong evidence base for recommending drinking water for weight management. Accumulated data from the most rigorous of study types, crossover experiments, and randomized and controlled trials indicate that drinking water results in lower total energy intake and higher rates of fat oxidation than other beverages. Advice to drink water is associated with reduced weight gain in children and weight loss in dieting adults.
Drinking Water Results in Lower Total Energy Intake
In crossover experiments, drinking water consistently results in lower total energy intake than caloric beverages.10-17 The effect is attributed to a lack of compensation for the calories in beverages. Individuals do not eat less food to compensate for beverage calories (Figure). The excess energy intake associated with caloric beverages is approximately equivalent to the calorie content of the beverage.18-23 Although drinking water contains 0 cal, caloric beverages contain on average 200 cal (about 838 kJ) per serving. Except in milk, calories from beverages mainly come from carbohydrates.
Although some very young children (<4 years) compensate for beverage calories by eating less food,24,25 the ability to compensate decreases with age,26 perhaps because of family environment, restrictive child-feeding practices, parent role models, or social cues about food portion size.26 Long-term crossover studies furthermore indicate that compensation for calories in beverages does not improve over time.27-29
Although some data suggest that noncaloric "diet" beverages can limit total energy intake like drinking water,14 other data indicate less caloric benefit.27,30 Aspartame-sweetened drinks may increase food intake.31
Drinking Water Results in Greater Rates of Fat Oxidation
Fat oxidation is maximal when blood insulin levels are low. Insulin inhibits the rate-limiting enzymes (eg, hormone-sensitive lipase, acylcarnitine transferase, and pyruvate carboxylase) that breakdown triglyceride into free fatty acids, transport free fatty acids into the mitochondria, and commit them to oxidation by the tricarboxylic acid/Krebs cycle.32,33
Drinking water does not contain macronutrients like other beverages and so does not trigger insulin like other beverages. The glycemic index of drinking water is 0, whereas that of milk is 30 to 40, that of juices is 40 to 60, and that of sugar-containing soft drinks and sports drinks is 50 to 80.34 The lower glycemic response to drinking water translates into greater rates of fat oxidation, because even small increases in insulin, such as those observed after fructose intake,35 inhibit body fat breakdown and fat oxidation.
Decades of crossover experiments consistently demonstrate greater rates of fat oxidation when drinking water is consumed instead of a caloric beverage, before or during low-to-moderate intensity exercise36-51 (Table). Across studies, fat oxidation is approximately 40% greater after water than after a caloric beverage.52 Under some study conditions, such as jogging after an overnight fast, reported rates of fat oxidation are as much as 0.5 g/min higher after drinking water than after a caloric beverage.
Fat oxidation is depressed after food intake by postprandial increases in insulin. The choice of beverage paired with a meal, however, can have a major impact on the length of time that fat oxidation is depressed. Drinking water with a meal can restore blood insulin levels and rates of fat oxidation to premeal values approximately 2 hours earlier than intake of a caloric beverage with the same meal.53-55 Intake of 500 to 600 kcal (about 2000-2500 kJ) of carbohydrate can depress fat oxidation for 6 hours after ingestion.56 Noncaloric, "diet" drinks may not restore postprandial fat oxidation like water, because sweet taste can stimulate insulin independent of calories.57,58
In children, interventions that promote drinking water prevent weight gain
Six intervention trials have reported the effects of drinking water promotion on weight change in children59-63 and adolescents.64 Five of 6 interventions combined advice to drink water with other beverage change recommendations.59,60,62,63 One intervention combined advice to drink water with installation of water fountains, distribution of water bottles, instruction about the water needs of the body, and teacher-sanctioned water-bottle filling times during the school day.61
Four of 6 studies randomized either schools59,60,62 or individuals64 to the beverage change intervention or control. Two studies compared change in schools in an intervention community versus change in schools in a control community.61,63 The school-based studies recruited representative samples of school-aged children generally (ie, all children attending the schools). The randomized study designs and representative study samples allow inferences to be drawn about exposure versus nonexposure to the intervention for weight management in the general pediatric population.
The results of 5 of 6 studies indicated that interventions that promote drinking water reduce excess weight gain. In the study by Ebbeling et al,64 body mass index (BMI) increased by 0.21 kg/m2 in the control group, but only 0.07 kg/m2 in the intervention group. James et al60 reported an increase in the prevalence of overweight of 7.5% in the control group, and no change in the intervention group (−0.2%). Taylor et al63 reported an increase in BMI z score in normal-weight children in the control group, no change in BMI z score in overweight children in the control group, and no change in normal-weight or overweight children in the intervention group. In the study by Foster et al,59 the incidence of overweight over 2 years was 14.9% in control schools, but 7.5% in intervention schools. Finally, Muckelbauer et al61 and Taylor et al63 reported an increase in the prevalence of overweight from 25.9% to 27.8% in the control group and no change (23.4%-23.5%) in the intervention group. Given the randomized nature of these studies, the consistent prevention of weight gain across studies indicates that exposure to advice to drink water (with or without other beverage change or health promotion advice) can reduce weight gain in the general pediatric population.
In the study where BMI increased in both the control and intervention groups, 4-fold increases in juice intake counterbalanced decreases in sugar-sweetened beverage intake.62 The null finding is consistent with results of another intervention trial (that did not promote drinking water), where changes in milk intake negated decreases in sweetened caloric beverage intake.65 Excess calories from milk and 100% juice are associated with weight gain in children.66,67
The intervention trials in children are informative about weight maintenance/gain, but not weight loss. They focus on representative samples of children, as opposed to selected samples of overweight children, motivated to lose weight. The studies in children do not involve conditions that promote the breakdown and oxidation of body fat (ie, increased time spent fasting or exercising at <70% V˙O2 max).
In Adults Following Hypocaloric Diets, Drinking Water Is Associated With Weight Loss
Two intervention studies report the effects of drinking water promotion on weight loss in adults. Both studies involve restricted samples of overweight individuals, motivated to diet and consuming hypocaloric diets.
Dennis et al19 randomized 48 overweight or obese men and women to a hypocaloric diet plus 500 mL water at each meal, or to a hypocaloric diet only (control). The hypocaloric diet included 1200 kcal/d (about 5000 kJ/d) for women and 1500 kcal/d (about 6200 kJ/d) for men and followed US department of Agriculture pyramid guidelines. Intake of caloric beverages was discouraged. Weight loss after 12 weeks was approximately 2 kg greater in the water group compared with the control group.
Secondary analyses of data from the Stanford A to Z Study weight loss intervention focus on 155 overweight premenopausal women who reported drinking less than 1 L/d water before beginning 1 of 4 popular weight-loss programs (Atkins, Zone, LEARN, and Ornish).68 In general, the 4 programs recommend drinking 1 L/d or more water and limited/no intake of caloric beverages. After 2 months of diet classes, study participants in all 4 diet groups reported higher water intake and lower caloric beverage intake. Intake of 1 L/d or more drinking water was associated with significantly greater weight loss in all 4 diet groups, independent of change in diet composition and physical activity.
In summary, accumulated data from studies of the highest, most rigorous quality indicate that drinking water prevents excess total energy intake and promotes fat oxidation compared with caloric beverages. Drinking water instead of caloric beverages could reduce total energy intake of US children and adults by at least 10%.69,70
Although data from long-term, randomized interventions are still needed to confirm and characterize the effects of drinking water on weight loss in overweight children and obesity prevention in the general adult population, there is a strong evidence base for recommending drinking water for weight management.
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