Metabolic responses to the acute ingestion of two commercially available carbonated beverages: A pilot study

Over the past few decades, rates of obesity in the United States and many industrialized countries have risen at alarming rates [1]. This is disconcerting given the vast measures that have been taken to counter this effect. For example, the government's Food Guide Pyramid has been overhauled [2]; companies have introduced and/or reformulated their food products to promote lower carbohydrate or lower fat alternatives; schools have begun to ban junk foods from their vending machines; and several professional organizations have published guidelines regarding physical activity levels to promote weight loss [3–6]. Despite these efforts, Americans continue to gain weight [1].

Some experts have linked the increase in obesity to the consumption of sugar-containing soft drinks [7, 8]. While it is well-known caffeine has a dose-dependent effect on metabolic rate [9, 10], the effect of repeated (daily) use of caffeinated beverages (soft drinks, coffee, etc) on obesity has not been well characterized. Recently, several companies have released soft drink alternatives that claim to have thermogenic (calorie burning) effects above and beyond those of normal caffeinated beverages. One such product, Celsius™ is marketed as a replacement to high calorie, high sugar soft drinks, coffees, and energy drinks. Celsius™ contains 200 mg of caffeine per 12 oz, along with a proprietary blend of vitamins and phytochemicals. Many of these ingredients, particularly caffeine, guarana extract, and green tea extract can be found in capsule form as dietary supplements and have been previously shown to cause acute increases in metabolic rate [11–15]. For example, Rudelle et al. recently reported that consumption of a beverage containing mainly green tea catechins (700 mg), caffeine (100 mg), and calcium (211 mg), when consumed three times per day for three days, increased 24-hour energy expenditure (EE) by 4.6% in healthy male and female subjects [16]. Although the increase was modest, the researchers concluded that "such a beverage may provide benefits for weight control", at least when consumed as part of a prudent diet and regular physical activity.

This "proof-of-concept" study sought to determine the metabolic effects of a similar type of beverage. We surmised that determining how much of an increase in metabolic rate Celsius™ ingestion provided relative to its caloric content (in this case, ~5 Calories per 12 oz) was an obligatory step in assessing its effects on human physiology and its potential as a weight loss aid. Thus, the purpose of this study was to measure acute changes in metabolic rate after the consumption of 12 oz Celsius™, and on a separate day, 12 oz of a popular caffeine-containing soft drink (Diet Coke ®).

As a first step in a series of studies designed to assess the effects of Celsius™ when consumed as part of a daily diet, the results of this pilot study indicate that acute ingestion of Celsius™ has thermogenic properties in healthy men and women. Since RER values were not different between trials, these data also indicate that both beverages had similar effects on substrate oxidation. Although no adverse events were reported during either trial (assessed via questionnaires), it is important to note that Celsius™ contains 200 mg of caffeine per 12 oz, an amount which has been shown to transiently increase blood pressure in some normotensive subjects [18, 19]. Indeed, Berube-Parent et al. reported increases of 7 and 5 mm Hg for systolic and diastolic blood pressures, respectively, in subjects who consumed 600 mg of caffeine per day, given as 3 × 200 mg throughout the day [20]. Thus, future studies should measure blood pressure responses to ascertain the potential effects of Celsius™ on systemic hemodynamics, particularly in subjects with preexisting hypertension.

Rudelle et al. recently reported a 4.6% increase in 24-hour energy expenditure after subjects consumed a supplement beverage containing mainly caffeine (100 mg), green tea (700 mg [standardized to 94 mg EGCG]), and calcium (211 mg) three times per day for three days [16]. In agreement with our study, the energy contributions of carbohydrate, fat, and protein were not altered by the beverage, but the total daily energy expenditure was significantly increased by the beverage. Blood pressure, heart rate, and urinary catecholamines were also unaltered. These data also concur with those of Dulloo et al., who reported a 4% increase in 24-hour energy expenditure after the consumption of a similar mixture of caffeine and green tea (standardized for EGCG content) [12]. Although we can not exclude the possibility that other ingredients within the Celsius™ formula have supportive physiological roles, the literature to date appear to buttress the position that caffeine and green tea are the principal active components in Celsius™. For example, taurine is a conditionally essential amino acid involved in bile acid conjugation, osmoregulation, detoxification, cell membrane stabilization, and modulation of cellular calcium flux and neuronal excitability [26]. Although clinically useful in the treatment of cardiovascular disease, hypercholesterolemia, seizure disorders, ocular disorders, diabetes, hepatic disorders, cystic fibrosis, and alcoholism, we are unaware of any studies demonstrating a direct effect of taurine on RMR. Similarly, glucuronolactone is a glucose metabolite produced in the liver that is purported to improve concentration and memory [27]. Finally, ginger extract has been reported to have anti-nausea effects and may also have anti-inflammatory properties [28]. As with taurine, the effects of glucuronolactone and ginger extract on RMR are unknown. Future studies should determine the effects of these popular ingredients on energy metabolism so that their contribution (if any) to the observed results can be delineated.

These preliminary findings indicate that acute consumption of Celsius™ stimulates thermogenesis to a greater extent than Diet Coke^®, at least over three hours post-ingestion in healthy men and women. These findings are not surprising considering that Celsius™ contains 4.4 times more caffeine than Diet Coke^® per 12 oz. It is tempting to speculate that daily consumption of Celsius™ might improve body composition over several weeks of continuous use, particularly when some epidemiologists have estimated that a caloric deficit of as little as 100 kcal/d may be sufficient to prevent weight gain in the majority of Americans [29]. However, this assumes that the effects we observed on metabolic rate are sustained and not attenuated during chronic use. It is our understanding that these types of studies are currently being planned by other research groups, therefore the long-term safety and efficacy of Celsius™ remains to be demonstrated.