Critical Analysis of "Can carbonated water support weight loss?"

Jan 27 / Drs. Bryan & Julie Walsh

Introduction

A newly published paper in the BMJ Nutrition, Prevention & Health claims that drinking carbonated water might help with glucose regulation and weight loss [1]. The author, Akira Takahashi, based on his experience and observations in hemodialysis for chronic kidney patients, suggests that the carbon dioxide (CO2) from carbonated water, gets absorbed into the bloodstream, where it enters into red blood cells (RBCs), causes an increase in intracellular pH in RBCs, resulting in enhanced glycolysis and thus glucose uptake and utilization. The author postulates this mechanism based on his observation that glucose levels typically decrease during a hemodialysis session. It is a nice story and the media has picked it up for the attractive soundbites it creates - "Sparkling water might help you lose weight!" - this postulate is fundamentally flawed and oversimplifies complex physiological processes.

While the author does acknowledges that the glucose disposal caused by drinking carbonated water is likely small and insufficient as a standalone weight-loss intervention, that is a gross understatement and this inference is likely going to cause more harm than good with the general public. 

Here we outline a number of problems with this paper, it's hypothesis, and ultimately, why it might create false hopes in people reading the news bites from the press release related to this paper.

Problem #1: Hemodialysis vs Sipping Fizzy Water

One of the most glaring issues in the paper is the author's attempt to draw parallels between CO2 absorption during hemodialysis and the consumption of carbonated water. This comparison is not only inappropriate but also misleading for a number of reasons including, but not limited to:
  1. Duration: Hemodialysis is a procedure where a patient's entire blood volume is continuously filtered through a machine for 4 straight hours, carefully balanced with precise concentrations of electrolytes and buffers. On the other hand, casually sipping carbonated water means taking a few swallows of fizzy liquid over a few minutes until your drink is finished. Unless someone plans to hook themselves up to a continuous carbonated water IV for four hours straight (which would be both impossible and stupid), these two situations aren't even remotely comparable in terms of duration, volume, or physiological impact.

  2. Exposure: In hemodialysis, blood comes into immediate, direct contact with carefully calibrated buffers and dialysis fluid – it's an instantaneous exchange at the cellular level. Drinking carbonated water, however, means carbonation from the fizzy drink has to pass through the digestive system first – stomach, intestines, and multiple layers of tissue - before anything can even reach the bloodstream. These are fundamentally different pathways with entirely different physiological impacts and timeframes.
In other words, putting something directly into the blood continuously for four hours versus transiently through the gastrointestinal tract over the course of a few minutes is a totally inappropriate comparison and one that shouldn’t even be made in the first place.

"Putting something directly into the blood continuously for four hours versus transiently through the gastrointestinal tract over the course of a few minutes is an inappropriate comparison and one that shouldn't be made."

Problem #2: Oversimplification of Complex Physiology

The paper's mechanistic reasoning suffers from several critical flaws:
  1. Neglect of Homeostatic Mechanisms: The body has pretty sophisticated systems to maintain pH balance and regulate blood glucose levels. These include respiratory compensation, buffer systems, and hormonal regulation, which are not adequately addressed in the paper.

  2. Overestimation of RBC Role: The author might be overemphasizing the role of RBCs in glucose metabolism, failing to acknowledge that RBCs account for only a small fraction of total glucose disposal in the body. He observes that glucose decreases during a hemodialysis treatment but is merely postulating why, how, and where it goes.

  3. Ignoring Concentration Gradients: As discussed above, the paper does not consider the significant differences in CO2 concentration and exposure time between hemodialysis and carbonated water consumption.

  4. Lack of Consideration for CO2 Fate: The author does not adequately discuss the physiology or fate of orally ingested CO2, including how much is ingested and what happens to it once it is consumed.
These mechanistic flaws undermine the paper's central hypothesis and highlight the need for a more rigorous and comprehensive analysis of the proposed pathway.

Problem #3: CO2 Exposure from Carbonated Water

The article also fails to provide a thorough quantitative analysis of CO2 exposure from carbonated water consumption. 
  1. Typical CO2 Content: Carbonated beverages generally contain 3-4 volumes of CO2 per liquid volume. This means that for every 1 liter of carbonated water, there are 3-4 liters of dissolved CO2 under pressure.

  2. Fate of Ingested CO2: The journey of CO2 from a carbonated beverage to potential metabolic effects is complex and largely ignored by the authors. Here's a breakdown of what might happen to ingested CO2.

    When carbonated water is consumed, the temperature of the body along with the lower atmospheric pressure of the stomach, increases the CO2 release from the water. As many people have experienced, this leads to gastric distension that leads to belching, sometimes a considerable amount. Since the stomach does not participate much in gas exchange, the amount of CO2 absorbed into the bloodstream from carbonated water is limited, not to mention highly variable.
It seems that when carbonated water is consumed, a very small amount of orally ingested CO2 reaches the bloodstream to significantly impact systemic physiology. Additionally, this also completely ignores the body’s buffering systems, which would likely be engaged to offset any significant physiological changes from CO2 ingestion. Hemodialysis, on the other hand, can be manipulated to counter any physiological buffering that takes place during the session.

Real-world CO2 Absorption Limitations

The paper overlooks several crucial factors that limit CO2 absorption from carbonated beverages:
  1. Rapid Degassing: Upon opening a carbonated beverage, CO2 immediately begins to escape. Therefore, how much is in the container prior to opening is very different than when it is opened, poured out, and sits at atmospheric pressure for a period of time. The longer it sits, the less carbon dioxide remains.

  2. Gastric Barrier: The stomach lining is not optimized for gas absorption. Most gas exchange occurs in the lungs, which are specifically adapted for this purpose.

  3. Dilution and Neutralization: Stomach acid and other gastric contents dilute and partially neutralize the carbonic acid formed by dissolved CO2, potentially further reducing absorption.

  4. Transit Time: The relatively quick passage of liquids through the stomach limits the time available for significant CO2 absorption.
These factors collectively contribute to the minimal systemic absorption of CO2 from carbonated beverages, a crucial point that the authors fail to address adequately.

Hemodialysis versus Carbonated Water: Apples and Oranges?

The paper's comparison of CO2 absorption during hemodialysis to that from carbonated water consumption is fundamentally flawed, as this table demonstrates:
 

Hemodialysis Conditions Carbonated Water Consumption
Duration 4-hour continuous exposure Brief, intermittent exposure
Contact Direct blood-dialysate interface Indirect, through gastrointestinal absorption
CO2 Carefully controlled and monitored Variable and uncontrolled

These stark differences highlight why the comparison is wildly inappropriate:
  1. Duration Differences: The sustained exposure during hemodialysis allows for significant cumulative effects, whereas the brief exposure from drinking carbonated water is unlikely to produce lasting physiological changes.

  2. Exposure Method: The direct blood-dialysate contact in hemodialysis facilitates efficient gas exchange. In contrast, CO2 from carbonated water must overcome several physiological barriers before entering the bloodstream.

  3. Concentration Gradients: Hemodialysis maintains a consistent CO2 gradient to drive diffusion. With carbonated water, the CO2 gradient rapidly diminishes as the beverage degasses and is diluted in the stomach.

  4. Physiological Barriers: The gastrointestinal tract presents multiple barriers to CO2 absorption, including the stomach lining, pH changes, and competing gases. These barriers are absent in the controlled environment of hemodialysis.
By drawing this inappropriate comparison, the authors of the paper significantly overestimate the potential physiological impact of CO2 from carbonated water.

Problem #4: Physiological Limitations

Questioning RBC Glucose Metabolism

The paper's hypothesis about carbonated water's effects on glucose disposal centers on red blood cell (RBC) metabolism is based on the findings of a 2004 study on hemodialysis-induced hypoglycemia. While the glucose disposal observed during hemodialysis could indeed stem from increased RBC glycolysis through the proposed CO2-mediated mechanism, it's worth considering this in a broader metabolic context. RBCs typically account for 10-15% of total glucose disposal, while skeletal muscle, brain, and other tissues are responsible for the majority of glucose disposal. This brings up the first question about the proposed mechanism of glucose disposal by the paper and whether it's primarily driven by RBC metabolism as proposed, or if other tissues and mechanisms might contribute significantly.

However, the more important question isn't about the mechanism of glucose disposal during hemodialysis, but rather whether drinking carbonated water could meaningfully replicate this effect. Even if RBCs are responsible for the glucose disposal seen during hemodialysis, the physiological conditions during dialysis (continuous exposure, controlled CO2 quantities, direct blood contact) are vastly different from those following carbonated beverage consumption. The transient, uncontrolled , and variable nature of CO2 exposure from carbonated water, combined with rapid respiratory compensation and the limited glucose uptake capacity of RBCs, suggests that any effect on systemic glucose levels would likely be minimal compared to the controlled environment of hemodialysis.

Homeostatic pH Regulatory Mechanisms

A key consideration when evaluating the potential effects of carbonated water on metabolism is the body's sophisticated homeostatic mechanisms. While the paper's proposed mechanism focuses on CO2-mediated changes in RBC metabolism, it's worth examining this in the context of normal physiological regulation. The body maintains tight control over pH and CO2 levels through multiple integrated systems, including respiratory compensation, buffer systems, renal, and cellular ion transport mechanisms. Most of these systems are designed to respond fairly quickly to even minor alterations in acid-base balance or CO2 levels.

The important question becomes whether the CO2 from carbonated water could overwhelm or significantly influence these regulatory systems. During normal consumption of carbonated beverages, ingested CO2 encounters a number of buffering systems, including the carbonic anhydrase pathway, the buffering capacity of hemoglobin, and compensation by the respiratory system to eliminate excess CO2. These homeostatic mechanisms suggest that any physiological effects from carbonated water's CO2 content would likely be both transient and minimal, particularly when compared to the controlled conditions of hemodialysis where such mechanisms may be altered or bypassed.

Problem #5 - Methodological Issues

While some of these have been mentioned previously, this paper has several methodological considerations that warrant discussion all at once:

1. Overextension of Dialysis Data 

The extrapolation from hemodialysis studies to carbonated water consumption raises important questions about comparative validity. Hemodialysis patients have compromised renal function and altered metabolic states, potentially making them distinct models from healthy individuals consuming carbonated beverages. The controlled, prolonged exposure to CO2 in dialysis differs substantially from the brief, variable exposure experienced when drinking carbonated water. Additionally, dialysis involves numerous physiological changes beyond CO2 exposure that may influence the observed effects.

2. Inappropriate Comparisons 

The paper draws parallels between notably different physiological scenarios. The direct blood-dialysate interface in hemodialysis operates under fundamentally different conditions than the complex process of gastrointestinal absorption. While dialysis occurs under carefully monitored conditions, as discussed previously, beverage consumption represents a more variable and uncontrolled exposure. There's also a meaningful distinction between chronic exposure in regular dialysis treatments and the acute, intermittent exposure typical of carbonated water consumption.

3. Lack of Real-World Validation 

The paper's hypotheses would benefit from additional empirical evidence from real-world scenarios. The absence of clinical trials specifically examining the effects of carbonated water consumption on weight loss or glucose metabolism in human subjects leaves many questions unanswered. Additionally, considering the lack of existing literature on carbonated beverage consumption and weight loss as well as why regular consumers of carbonated beverages don't show significant metabolic effects weaken the author’s proposed mechanism.

4. Oversimplification of Complex Biochemistry and Physiology 

The paper presents an intriguing but a rather myopic view of how carbonated water might influence metabolism through RBC glycolysis. While the proposed mechanism is biochemically possible, it exists within a complex network of physiological systems that maintain homeostasis. The body's pH buffering systems, enzyme saturation kinetics for RBC glycolysis, and multiple glucose regulatory pathways all influence how CO2 fluctuations might affect cellular metabolism. A more comprehensive analysis considering these interacting systems, along with quantification of the potential magnitude of these effects under normal physiological conditions, would help evaluate the practical significance of the proposed mechanism. Unfortunately, that quantification was not presented.

While the author presents an interesting hypothesis based on his own clinical observation, the paper's methodological limitations overshadow its potential contributions. The oversimplified analysis may unfortunately serve more to generate misleading headlines than to advance our understanding of carbonated water's metabolic effects.

Valid Benefits of Carbonated Water

While the paper's main claims regarding enhanced glucose metabolism and weight loss are unsupported, it's important to acknowledge that carbonated water may indeed offer some benefits through other mechanisms. These effects, however, are primarily mechanical rather than metabolic:

1. Increased Satiety due to Gastric Distention

Carbonated water may contribute to increased feelings of fullness through gastric distention. The CO2 bubbles in carbonated water can cause the stomach to expand, potentially triggering stretch receptors that signal satiety to the brain. Some studies have shown that carbonated beverages can increase feelings of fullness compared to non-carbonated alternatives [2]. However, the effect is likely short-lived and may not significantly impact overall calorie intake over time.

2. Potential Replacement for High-Calorie Beverages

One of the most tangible benefits of carbonated water is its potential to replace higher-calorie beverages in the diet. By substituting carbonated water for sugary sodas or other high-calorie drinks, individuals can significantly reduce their caloric intake. Beyond calorie reduction, carbonated water can contribute to daily fluid intake, potentially improving hydration status for some individuals. The effervescence of carbonated water may make it more appealing than plain water for some people, encouraging increased fluid consumption.

3. Potential Oral Health Benefits

Some studies suggest that carbonated water may have neutral or even positive effects on oral health. While carbonated water is slightly acidic, it's generally not acidic enough to pose significant risks to dental enamel, especially compared to sugary or citrus-flavored beverages [3]. The carbonation may also stimulate salivary flow, which can help neutralize acids and protect against dental caries [4].

4. Digestive Effects

Carbonated water may have some effects on the digestive system, though the evidence is mixed. Some studies suggest that carbonated water can improve swallowing ability in both young and older adults [5]. Anecdotal evidence and some limited studies indicate that carbonated water might help relieve indigestion and constipation, though more research is needed [6].

While these benefits are more grounded in evidence than the paper's primary claims, it's important to note that they are generally modest in magnitude and may not apply universally to all individuals.








































Study Participants Duration Key Findings Limitations
Liu et al. (2022)[13] 66 older adults 4 months Improved muscle endurance, no significant change in walking distance Small sample size, short duration
Andreux et al. (2019)[14] 60 elderly individuals 4 weeks Improved mitochondrial gene expression in muscle Short duration, limited functional outcomes
Singh et al. (2022)[15] 88 middle-aged adults 4 months Improved muscle strength and exercise performance Industry-funded, limited long-term data

"While carbonated water may offer some benefits, such as increased satiety through gastric distention and as a replacement for high-calorie beverages, these effects are primarily mechanical rather than metabolic."

Conclusion

The paper "Can carbonated water support weight loss?" published in BMJ Nutrition, Prevention & Health presents a hypothesis that is not supported by sound scientific evidence. The author has overreached in his mechanistic explanations, failed to consider the limited practical significance of their proposed pathway, and made inappropriate comparisons to hemodialysis.

Key points of critique include:
  1. Flawed Core Mechanism: The proposed pathway of carbonated water ingestion and subsequent CO2 absorption leading to enhanced RBC glycolysis and subsequent glucose reduction is based on oversimplified physiology and neglects crucial homeostatic mechanisms.

  2. Inappropriate Hemodialysis Comparison: The author draw unsupported parallels between the controlled, prolonged CO2 exposure in hemodialysis and the brief, variable exposure from carbonated water consumption.

  3. Quantitative Analysis Shortcomings: The paper fails to adequately quantify CO2 exposure from carbonated beverages and ignores the fact that most ingested CO2 is likely never absorbed into the bloodstream.

  4. Physiological Limitations: The role of RBCs in glucose metabolism is overstated, and the body's homeostatic mechanisms are not sufficiently considered with CO2 ingestion.

  5. Unsupported Clinical Implications: Claims regarding weight loss and glucose regulation are not backed by clinical evidence and are based on questionable assumptions about the magnitude of the proposed effects.

  6. Methodological Flaws: The paper suffers from overextension of dialysis data, inappropriate comparisons, lack of real-world validation, and oversimplification of complex physiological processes.
While carbonated water may offer some benefits, such as increased satiety through gastric distention and as a replacement for high-calorie beverages, these effects are primarily mechanical rather than metabolic.

Hopefully, this article underscores the importance of maintaining scientific rigor in nutritional research, especially in a day and age when the media is ready to pounce on any reasonably plausible soundbite to get them clicks and likes. Future studies on the effects of carbonated water should focus on well-designed clinical trials that directly measure relevant outcomes in appropriate populations. Additionally, researchers should be cautious about extrapolating data from medical procedures like hemodialysis to make claims about everyday dietary choices.

While carbonated water can be a part of a healthy diet, its effects on weight loss and metabolism are likely to be minimal and indirect. Consumers and healthcare professionals should focus on established principles of nutrition and weight management rather than relying on speculative mechanisms with little scientific support.

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