A ketogenic diet emphasizes high fat and very low carbohydrate intake, forcing the body to shift its primary fuel source from glucose to fat. This metabolic shift, known as ketosis, produces compounds that affect the body’s acid-base balance. Concerns about a keto diet causing low carbon dioxide (CO2) in the blood often arise when blood tests show low total CO2 or bicarbonate levels. Low CO2, medically termed hypocapnia, is generally a sign that the body is actively compensating for an underlying metabolic change. This physiological mechanism maintains stability in the blood’s acidity.
Understanding Ketogenesis and Acid Production
The ketogenic diet severely restricts carbohydrate intake, typically to under 50 grams per day, which depletes the body’s stored glucose (glycogen) reserves. With glucose unavailable, the liver begins to break down fat into fatty acids, a process called beta-oxidation, to create an alternative fuel source. This accelerated fat breakdown results in an excess of acetyl-CoA, a molecule that cannot be fully processed by the liver’s energy cycle.
The liver reroutes this excess acetyl-CoA to produce ketone bodies, which are released into the bloodstream as fuel for the brain and other tissues. The three main ketone bodies are acetoacetate, beta-hydroxybutyrate, and acetone. Acetoacetate and beta-hydroxybutyrate are organic acids that release hydrogen ions (H+) when dissolved in the blood, effectively lowering the blood’s pH. This influx of acidic compounds presents a metabolic challenge that the body must address immediately.
How the Body Regulates Blood pH
The body must maintain the blood pH within a very narrow range, typically between 7.35 and 7.45, for physiological processes to function correctly. This tight regulation is primarily achieved through a sophisticated chemical defense system known as the bicarbonate buffer system. This system involves a reversible reaction between carbon dioxide (CO2) and bicarbonate (HCO3-) in the blood.
In this buffering process, any excess acid (like the H+ released by ketone bodies) is immediately “soaked up” by the bicarbonate ions, which are the body’s alkaline reserve. The chemical reaction converts the acid and bicarbonate into carbonic acid (H2CO3), which then rapidly dissociates into CO2 and water. This reaction effectively neutralizes the acidic challenge, but it results in a net consumption of bicarbonate and an increase in dissolved CO2.
The partial pressure of CO2 (PaCO2) is the respiratory component of the buffer system, tightly controlled by the lungs. The kidneys regulate the metabolic component by controlling the excretion and reabsorption of bicarbonate. When the body faces a metabolic acid load, such as from ketone production, the goal is to stabilize the blood pH by adjusting these two components. Low CO2 in a blood test is often the result of this regulatory mechanism in action, not an isolated problem.
The Compensatory Role of Respiration in Keto Diets
The mild metabolic acid load created by the increased production of acidic ketone bodies triggers a predictable and immediate physiological response called respiratory compensation. Specialized chemoreceptors in the brain and arteries detect the slight decrease in blood pH and the corresponding drop in bicarbonate levels. This signal prompts the respiratory center to increase the rate and depth of breathing, a process known as hyperventilation.
This hyperventilation acts to “blow off” the excess CO2 that was generated when bicarbonate buffered the ketone acids. By rapidly expelling more CO2 through the lungs, the body effectively shifts the entire buffer equilibrium toward the side of less acid. This mechanism successfully raises the blood pH back towards the normal range, counteracting the metabolic acidosis.
When an individual in nutritional ketosis undergoes a blood gas test, the results often show a lower-than-normal CO2 level (hypocapnia) alongside a mildly reduced bicarbonate level. This low CO2 is not a primary respiratory problem but the direct evidence of the lungs successfully compensating for the mild metabolic acid challenge from the ketones, ensuring the overall blood pH remains stable and safe.
Differentiating Nutritional Ketosis from Ketoacidosis
It is important to distinguish nutritional ketosis from the dangerous medical condition known as ketoacidosis. Nutritional ketosis, the goal of a well-formulated ketogenic diet, is a controlled metabolic state where ketone body levels are typically in a mild range, usually between 0.5 and 3.0 mmol/L. In this state, the body’s powerful buffering systems, including the respiratory compensation that lowers CO2, maintain the blood pH within the normal, safe range.
Ketoacidosis, most commonly Diabetic Ketoacidosis (DKA), is a pathological condition resulting from an absolute or severe lack of insulin, usually in individuals with type 1 diabetes. In DKA, ketone production is uncontrolled and reaches dangerously high levels, often exceeding 15 to 25 mmol/L. This overwhelming acid load completely exceeds the body’s buffering capacity, leading to severe metabolic acidosis where the blood pH drops below 7.30 and can be life-threatening.
While the body attempts the same compensatory hyperventilation in DKA, it is insufficient to normalize the pH. Anyone on a keto diet experiencing extreme fatigue, persistent nausea, vomiting, or signs of severe dehydration should seek immediate medical attention, as these symptoms can signal the development of a pathological state. For healthy individuals, the low CO2 observed is merely a sign that the body’s robust acid-base control mechanisms are functioning as intended.