The paper's mechanistic reasoning suffers from several critical flaws:
- 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.
- 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.
- 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.
- 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.
The article also fails to provide a thorough quantitative analysis of CO2 exposure from carbonated water consumption.
- 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.
- 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.
The paper overlooks several crucial factors that limit CO2 absorption from carbonated beverages:
- 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.
- 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.
- Dilution and Neutralization: Stomach acid and other gastric contents dilute and partially neutralize the carbonic acid formed by dissolved CO2, potentially further reducing absorption.
- 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.
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:
- 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.
- 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.
- 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.
- 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.
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.
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.
While some of these have been mentioned previously, this paper has several methodological considerations that warrant discussion all at once:
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.
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.
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.
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.
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:
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.
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.
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].
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 |