Minimal consumption of carbohydrate (< 50 grams/day) for an extended period of time results in a phenomenon called ‘keto-adaptation’. Keto-adaptation denotes an altered metabolism in which fat becomes the predominant energy source, consequently, shifting the body from a state of fat storage to fat oxidation (1). The biochemical modifications essential to transition from a “glucocentric” (reliance on glucose for fuel) to an “adipocentric” (reliance on fatty acids and ketone bodies for fuel) metabolism require carbohydrate restriction for several weeks to months (2). Two common approaches to become keto-adapted include sustained consumption of a very low-carbohydrate/high-fat ketogenic diet (KD) or an extended period of fasting.
Low Insulin / High Glucagon
Surges of glucose enter the bloodstream after consumption and digestion of dietary carbohydrates. To prevent glucose from reaching high/toxic levels in the circulation, the pancreas releases insulin, a hormone that signals tissues to store excess glucose in the form of glycogen or leads to storage of fat in adipose tissue (body fat). On the other hand, restriction of carbohydrate intake results in limited exogenous glucose availability (i.e., dietary glucose) and as a result, blood glucose levels must be maintained endogenously (inside your own body). For example, unlike a mixed diet (moderate to high carbohydrate), a study demonstrated that individuals who adapted to a KD for 12 weeks showed no increase in blood glucose or insulin following a meal (3). Therefore, consuming a carbohydrate-restricted diet such as the KD would not raise blood glucose levels directly.
During dietary carbohydrate restriction, the pancreas releases another hormone known as glucagon in response to low blood glucose levels (2). Carbohydrate-restriction thus leads to a low insulin/high glucagon ratio which regulates fuel metabolism by promoting four metabolic responses.
Breakdown of liver glycogen to glucose
Synthesis of glucose in the liver
Fat breakdown from adipose tissue
Synthesis of ketone bodies in the liver
Glycogenolysis and gluconeogenesis maintain blood glucose levels endogenously, while lipolysis and ketogenesis provide alternative sources of fuel in the form of fatty acids and ketone bodies (4,5).
Glycogen Alterations and Early Water Loss
Within the first 1-2 days of carbohydrate-restriction, glycogen reserves (stored carbohydrate) in the liver and muscle are utilized and depleted to supply glucose demands (6). The human liver contains ~70-100 grams of glycogen and muscle contains ~400 grams (6). Each gram of glycogen carries 3 grams of water, thus glycogen and water depletion in the liver and muscle go hand in hand (6). Rapid weight loss generally occurs in the first two days of carbohydrate-restriction due to glycogen and water loss. Hence, if liver and muscle glycogen stores deplete (500 grams) along with the connected water (500 grams glycogen x 3 grams water), you can estimate that 1-2 kgs can be easily lost within the first couple of days (6). Following glycogen depletion, gluconeogenesis increases to provide glucose to red blood cells and the central nervous system while fatty acid oxidation amplifies to provide energy for all other tissues (7).
Research studies demonstrate modifications in glycogen content and utilization resulting from keto-adaptation. For instance, lean men that consumed a KD for 4-weeks demonstrated a reduction in muscle glycogen and blood glucose utilization by three- to four-fold during steady state exercise (cycled ~65% of VO2max until exhaustion) (8). Fat oxidation predominantly fueled the submaximal exercise after 4-weeks of keto-adaptation (8). The author of this study noted that the greater glycerol release rate from increased triglyceride breakdown helps power gluconeogenesis throughout exhaustive exercise in the keto-adapted state (8). A longer 20-month study compared peak fat oxidation and muscle glycogen content between elite ultra-endurance athletes either consuming a KD or a high-carbohydrate diet (9). The ketogenic athletes demonstrated a two-fold higher rate of peak fat oxidation and a greater capacity to utilize fats for energy during high-intensity exercise (9). Interestingly, the ketogenic and high-carbohydrate athletes demonstrated similar glycogen content before exercise, similar glycogen utilization during exercise, and similar glycogen repletion after exercise in response to performing a maximal graded exercise test followed by a 180-minute submaximal treadmill run (9). The popular consensus regarding a KD in terms of muscle glycogen is that it reduces muscle glycogen content and therefore may damper performance which may be true for shorter-term studies, however, this 20-month study demonstrates that a longer keto-adaptation period may be necessary to sufficiently increase energy supply via fatty acid oxidation and KD pathways to restore and spare muscle glycogen content (1).
Fat Mobilization and Utilization
With limited levels of dietary glucose entering the system for fuel, fatty acids become the primary source of energy during carbohydrate-restriction (2). Mobilization of fatty acids from adipose tissue escalates in the face of a chronic low insulin/high glucagon ratio to supply energy in addition to any dietary fat consumed (2,4). With a few exceptions such as erythrocytes and brain cells, most cells can use fatty acids for energy (10). Fatty acids undergo oxidation in the mitochondria of cells to form a molecule called acetyl coenzyme A (acetyl-CoA). Acetyl-CoA combines with the compound oxaloacetate to then enter an aerobic metabolic pathway, called the citric acid cycle, for conversion into energy (ATP).
Fatty acid oxidation (utilizing fatty acids for energy) increases with keto-adaptation and may be observed via the respiratory quotient (RQ) during exercise. The RQ ratio equals the amount of carbon dioxide eliminated from the body divided by the amount of oxygen consumed by the body (RQ = CO2 eliminated/O2 consumed). An RQ = 1.0 would indicate carbohydrates as the primary fuel source whereas an RQ = 0.7 would indicate fats as the primary fuel source. A 4-week KD study demonstrated a significant drop in RQ during sub-maximal exercise from 0.83 to 0.72 in endurance-trained men, indicating greater fatty acid oxidation after adapting to the ketogenic diet (8).
Starvation studies demonstrate the carbohydrate-restricted adaptation to utilizing primarily fatty acids for energy. A 21-day starvation study examined fuel utilization patterns in 5 obese humans 11). Subjects lost fat and lean mass; skeletal muscle accounted for 7% of the loss while fat stores accounted for the other 93% (11). The brain obtained energy indirectly from fat stores (directly from ketone bodies synthesized from partially oxidized fatty acids) and from glucose synthesized by gluconeogenesis (1). This study suggests that a normal weight human could derive adequate energy from fat stores to survive starvation for several months (11).
Ketone Body Metabolism
As previously mentioned, gluconeogenic processes increase to maintain blood glucose levels during carbohydrate restriction. Oxaloacetate is a compound with two roles during carbohydrate-restriction: 1) it acts as a glucose precursor used in gluconeogenesis and 2) it combines with acetyl-CoA after partial fatty acid oxidation to enter the citric acid cycle for energy production (12,13). Increased gluconeogenesis during carbohydrate restriction reduces oxaloacetate’s availability to combine with acetyl-CoA, thus redirecting excess acetyl-CoA to the liver mitochondria for the initiation of ketogenesis (12,13). Ketone body precursors include fatty acids and ketogenic amino acids (i.e., leucine, isoleucine, lysine, phenylalanine, tyrosine, and tryptophan). β-hydroxybutryrate (β-HB), acetoacetate (AcAc), and acetone are ketone bodies synthesized but not utilized by liver cells (13).
Utilization of ketone bodies by brain cells signifies adaptation to extended fasting or a KD. The brain typically derives 100% of its energy from glucose (140-150 grams of glucose/day), however, ketone bodies provide > 50% of the brain’s energy during carbohydrate restriction (14). Consequently, ketone bodies aid in the maintenance of blood glucose levels, allowing for a reduced rate of gluconeogenesis over time, and sparing muscle protein (14).
Energy substrate adaptations in skeletal muscle during carbohydrate restriction were demonstrated in the forearm muscles of 8 obese men after fasting 1 night, 3 days, and 24 days (15). After a 1-night fast, AcAc and β-HB utilization by the muscle was low. After a 3-day fast, circulating levels and muscle utilization of AcAc and β-HB were significantly elevated. After 24 days of fasting, AcAc and β-HB levels were further elevated in arterial blood but the utilization by the skeletal muscle was not elevated past day 3. Instead, the forearm muscle primarily utilized free fatty acids for energy, with a decrease in utilization of both glucose and ketone bodies (15). Keto-adaptation, therefore, leads to tissue-specific fuel preferences such that skeletal muscle consumes fatty acids whereas brain cells utilize ketone bodies and glucose.
Enhanced Ketogenic Pathways
The cellular capacity to utilize ketone bodies for fuel expands as circulating ketone concentrations consistently remain elevated (16). Chronic ketone elevations up regulate expression of monocarboxylate transporter-1 (MCT-1), a protein that transports ketone bodies from circulation into tissue cells to be metabolized into energy (16). Generally, sufficient levels of ketogenic and ketolytic enzymes exist at the initiation of carbohydrate-restriction, therefore, the primary adaptation required to enhance utilization of ketone bodies for fuel, especially in the brain, is the cellular enhancement of MCT-1 expression (16,17).
Previous studies suggest greater survival time during periods of starvation or fuel-related pathological conditions (i.e., epilepsy) with keto-adaptation due to an increase in substrate-transporting capacity driven by chronically elevated levels of ketone bodies (18). For instance, six-weeks of a ketogenic diet, compared to a standard diet, increased MCT-1 and glucose transporter (GLUT) expression in rat brain cells (18). Elevated circulating levels of ketone bodies signal the brain to increase its capacity to uptake any available form of circulating energy substrate, including ketone bodies and glucose (18). Optimizing the transport system via increased expression in MCT-1 enhances the cellular utilization of β-HB and AcAc thus creating an efficient metabolic pathway over time (18).
Electrolytes are charged substances required for normal bodily functions (i.e., regulates the nervous system and muscular activity) and include sodium, potassium, calcium, bicarbonate and magnesium. Alterations in select electrolytes seem to occur primarily in the first week of carbohydrate-restriction. In a 28 day study, obese individuals consumed either a low-carbohydrate or high-carbohydrate diet (19). Compared to the high-carbohydrate group, the low-carbohydrate group demonstrated greater urinary sodium excretion in the first week but less total sodium excretion over the 28-day period (19). Potassium excretion was also greater in the low-carbohydrate group for the first 2 weeks of the diet, but after adapting to the diet for 28 days, potassium excretion did not differ from the high-carbohydrate diet (19). Fluid levels between the two groups were also similar regarding both intake and loss of fluid (19). This study demonstrates that electrolyte levels may lower in the first 1-2 weeks of carbohydrate restriction but balance out after adapting for a few weeks (19).
Similarly, a study in obese adolescents demonstrated normal electrolyte levels after consuming a KD for 8 weeks (20). Lean men also demonstrated unvaried potassium levels during a 4 week KD (8). A large 6-month less-controlled study demonstrated a decrease in serum sodium, chloride, bicarbonate, and uric acid levels in obese individuals with an increase in urinary calcium and uric acid excretion from baseline to 24 weeks (21). A 6-week low carbohydrate, high protein diet also showed an increase in calcium excretion (22). Therefore, it seems that sodium, potassium, and calcium may need to be supplemented with the diet.
Fasting vs KD Adaptations
Metabolic similarities of extended fasting and a KD stem from an absence of exogenous carbohydrate which shifts primary fuel substrate from glucose to fatty acids and ketone bodies (1). Gluconeogenesis (conversion of protein and glycerol to glucose) in the liver maintains blood glucose levels in both conditions (1). Through an extended fasting period, while not keto-adapted, the body adapts physiologically to using fatty acids (derived from adipose tissue) and ketone bodies for fuel but lean body mass breaks down to supply protein substrate (gluconeogenic amino acids) for gluconeogenesis (10). The KD, however, provides exogenous sources of dietary fat for energy requirements and dietary protein to power gluconeogenesis and elevated ketone levels to spare lean muscle mass (1). Therefore, the primary differences between the adaptations occurring between fasting and the KD are: 1) adipose tissue stores may be used at a greater rate for energy needs with fasting vs KD due to a lack of exogenous fat sources, and 2) lean body mass is maintained with a KD vs fasting due to elevated ketone levels and possible fulfillment of gluconeogenic substrate requirements (1,10,23).
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