Beta-hydroxybutyric acid, often simply called BHB, is one of the three primary ketone bodies produced by the human body. It is a chemical compound the body creates when it shifts its energy production from its usual source, carbohydrates, to an alternative fuel. BHB can be utilized by various tissues.
The Body’s Natural Production of BHB
The natural production of beta-hydroxybutyric acid, a process known as ketogenesis, primarily occurs within the mitochondria of liver cells. This metabolic shift is initiated when the body’s primary carbohydrate stores, such as liver and muscle glycogen, become depleted. When glucose availability is low, insulin levels decrease, which in turn allows for the release of fatty acids from fat stores.
These liberated fatty acids travel to the liver, where they undergo a series of transformations. Through a process called beta-oxidation, fatty acids are broken down into acetyl-coenzyme A (acetyl-CoA). When acetyl-CoA accumulates in excess because the citric acid cycle cannot process it all, it is diverted into the ketogenesis pathway. This pathway then synthesizes ketone bodies, with acetoacetate being the first one produced, which is subsequently converted into BHB by the enzyme BHB-dehydrogenase.
The primary triggers for this endogenous BHB production include prolonged fasting, where carbohydrate intake is minimal or absent. Similarly, following a ketogenic diet, characterized by very low carbohydrate intake and high fat consumption, significantly stimulates ketogenesis. Extended periods of strenuous physical activity can also lead to increased BHB levels as glucose stores are utilized and fatty acid oxidation rises.
BHB’s Role as an Alternative Fuel Source
Beta-hydroxybutyric acid (BHB) is released into the bloodstream and serves as a readily available energy substrate for various tissues. Unlike fatty acids, BHB is water-soluble, allowing it to circulate freely without requiring specialized transport proteins. This enables BHB to reach organs that cannot directly utilize fatty acids for energy.
The brain, in particular, relies heavily on glucose for its energy needs, but BHB provides an effective alternative when glucose is scarce. BHB can efficiently cross the blood-brain barrier, a protective membrane that restricts the passage of many molecules, including fatty acids. Once inside the brain, BHB is converted back into acetyl-CoA, which then enters the citric acid cycle to generate adenosine triphosphate (ATP), the cell’s main energy currency. During periods of low glucose, BHB can supply up to 70% of the brain’s energy requirements.
Beyond the brain, the heart and skeletal muscles also readily utilize BHB for fuel. The heart can derive a significant portion of its energy from ketone bodies, especially during conditions of metabolic stress or heart failure. Skeletal muscles also take up BHB from the blood, converting it into acetyl-CoA to power muscle contraction, particularly during prolonged exercise when muscle glycogen stores are depleted.
Exogenous BHB Supplements
Beyond the body’s natural production, beta-hydroxybutyric acid can also be introduced externally through supplements, known as exogenous BHB. These supplements typically come in the form of BHB salts or ketone esters. BHB salts usually combine BHB with minerals like sodium, calcium, or magnesium, while ketone esters are a more direct form of BHB linked to an alcohol. The main purpose of these supplements is to elevate blood ketone levels without requiring strict adherence to a very low-carbohydrate diet or prolonged fasting.
Individuals often use exogenous BHB supplements to help transition into a state of nutritional ketosis, which can be challenging to achieve through diet alone. By increasing circulating ketone bodies, these supplements may help mitigate common side effects associated with the initial phase of a ketogenic diet, sometimes referred to as “keto flu” symptoms. They are also used by athletes who seek performance benefits, as BHB can provide an alternative fuel source for muscles during endurance activities.
Some people also use these supplements to support cognitive function, particularly under conditions of metabolic stress like fasting or exercise. While exogenous ketones can acutely raise blood BHB concentrations, the extent and duration of these effects can vary. Research continues to explore the full range of applications and efficacy of these external BHB sources.
Clinical Significance of BHB Levels
Measuring beta-hydroxybutyrate (BHB) levels in the blood provides insights into an individual’s metabolic state, with particular clinical significance in certain medical conditions. While moderate elevations of BHB are characteristic of nutritional ketosis, a state where the body is efficiently burning fat for fuel, extremely high levels can indicate a serious medical emergency.
The most prominent and life-threatening condition associated with dangerously elevated BHB is diabetic ketoacidosis (DKA). DKA primarily affects individuals with type 1 diabetes, though it can occur in type 2 diabetes under severe stress. In DKA, a severe lack of insulin prevents glucose from entering cells, leading to uncontrolled fat breakdown and excessive ketone body production, including BHB. This rapid accumulation of acidic ketones, coupled with high blood glucose levels, overwhelms the body’s buffering systems, causing the blood to become dangerously acidic.
BHB levels in DKA are much higher than those seen in nutritional ketosis. Symptoms of DKA include rapid breathing, dry mouth, nausea, vomiting, abdominal pain, and a fruity breath odor due to acetone. Monitoring BHB levels is therefore a common diagnostic tool in emergency settings to confirm DKA and guide appropriate treatment, which typically involves insulin administration and fluid replacement.