carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and encompass sugars, starches, and fibers found predominantly in plant foods. Structurally, carbohydrates can range from simple monosaccharides such as glucose and fructose to complex polysaccharides such as starch and dietary fiber. In the context of human nutrition, carbohydrates are categorized based on their digestibility and chemical structure, including simple sugars (e.g., glucose and sucrose), oligosaccharides, and complex carbohydrates such as starches and fibers. The term 'carbohydrate' originates from their elemental composition, historically referred to as 'hydrates of carbon'. Chemically, they serve as a critical energy substrate, with glucose being the primary fuel for the brain and central nervous system under normal dietary conditions. Carbohydrate intake is an integral part of most traditional diets around the world, supplying both energy and essential nutrients. In addition to simple energy substrates, carbohydrate‑containing foods provide dietary fiber, which plays a pivotal role in digestive health and metabolic regulation. Unlike fats and proteins, carbohydrates can be stored in limited amounts as glycogen in the liver and muscles, serving as an available fuel reserve during periods of increased energy demand. Carbohydrates are abundant in grains, fruits, legumes, starchy vegetables, dairy, and many processed foods. Complex carbohydrates generally take longer to digest and have a lower glycemic impact compared with simple sugars, which are rapidly absorbed and can cause sharp increases in blood glucose levels. Dietary guidelines universally emphasize carbohydrate quality, recommending the majority of carbohydrate intake come from whole, minimally processed sources rich in fiber, vitamins, and minerals rather than refined and added sugars.

Carbohydrates perform several physiological functions that are central to human metabolism. The primary function of carbohydrates is to provide energy: glucose derived from carbohydrate digestion is the preferred fuel for the brain, red blood cells, and many other tissues. Through glycolysis and subsequent oxidative metabolism, glucose yields ATP, the energy currency of cells. In addition to fueling daily activities, carbohydrates spare protein from being used for energy, allowing amino acids to support tissue growth and repair rather than being oxidized for fuel. Dietary fiber, a non‑digestible form of carbohydrate, supports gastrointestinal health by increasing stool bulk, improving transit time, and serving as a substrate for beneficial gut microbiota. Fiber has been associated with lower risk of constipation and may modulate the gut microbiome toward a profile associated with metabolic health. Evidence suggests high‑fiber diets are linked with improved lipid profiles and reduced risk of coronary heart disease, possibly through bile acid binding and modulation of cholesterol metabolism. Complex carbohydrates and fiber are also associated with better glycemic control, as they slow gastric emptying and reduce postprandial glucose spikes. Systematic reviews show that diets emphasizing whole grains and fiber‑rich sources are associated with lower incidence of type 2 diabetes and improved metabolic markers compared with diets high in refined carbohydrates. Some populations consuming diets rich in plant‑based carbohydrates have historically demonstrated lower rates of cardiovascular disease and obesity, underscoring the importance of carbohydrate quality in disease prevention. While low‑carbohydrate dietary patterns are used therapeutically for weight loss and glycemic control in type 2 diabetes, comprehensive reviews indicate that carbohydrate intake must be considered in the context of overall diet quality and nutrient adequacy, with higher quality carbohydrate diets supporting long‑term health outcomes.

Determining optimal carbohydrate intake depends on age, sex, physiological status, activity level, and overall energy requirements. National dietary reference standards generally set a minimum carbohydrate intake of about 130 grams per day for adults and children, which reflects the approximate amount needed to fulfill essential glucose requirements for brain function and prevent ketosis. Many health authorities and dietary guidelines recommend that carbohydrates comprise 45–65% of total daily energy intake for adults to support energy needs and maintain metabolic health. While the 130 g/day figure represents a physiological minimum, actual carbohydrate needs will vary based on total calorie requirements. Highly active individuals such as athletes may require substantially higher carbohydrate intakes to support glycogen stores and endurance performance. Conversely, very low‑carbohydrate diets — often defined as less than 45% of total calories from carbohydrates — are used in specific clinical contexts such as diabetes management and weight loss, but should be supervised to ensure nutritional adequacy. During pregnancy and lactation, carbohydrate needs increase due to the energy cost of fetal growth and milk production, and recommendations often encourage higher carbohydrate intake within a balanced diet. In older adults, maintaining adequate carbohydrate intake is important to support energy needs and prevent unintentional weight loss and muscle catabolism. It is important to emphasize carbohydrate quality: prioritizing whole grains, fruits, vegetables, and legumes ensures adequate intake of fiber and micronutrients. Dietary planning should consider individual health conditions, metabolic goals, and personal preferences to establish carbohydrate intake that supports both energy needs and long‑term health outcomes.

Carbohydrate deficiency is uncommon in populations consuming mixed diets that supply sufficient total calories. When carbohydrate intake is extremely low, the body shifts metabolism toward increased fat oxidation and ketone production. This metabolic adaptation is normal in fasting and ketogenic diets, but may be associated with transient symptoms such as fatigue, headache, dizziness, irritability, and difficulty concentrating during early adaptation. Persistent very low carbohydrate intake may lead to chronic low energy availability, potentially affecting exercise capacity and hormonal balance. Some individuals may experience digestive changes due to low fiber intake, such as constipation or alterations in bowel habits. In clinical settings, prolonged carbohydrate restriction without adequate energy intake can contribute to ketoacidosis in susceptible individuals, particularly those with type 1 diabetes, highlighting the essential role of insulin in carbohydrate metabolism. Overall, specific clinical deficiency diseases attributed solely to inadequate carbohydrate intake are not recognized, and symptoms tend to reflect broader energy deficits rather than a unique carbohydrate deficiency. Populations at risk for low carbohydrate intake include those following very restrictive diets without professional guidance, individuals with eating disorders, and those with limited access to diverse foods. Healthcare providers monitor signs of energy deficiency, electrolyte imbalances, and metabolic adaptations when carbohydrate intake is substantially reduced, particularly in vulnerable populations.

Carbohydrates are widely distributed in plant‑based foods and dairy products. Whole grains such as brown rice, oats, quinoa, and whole wheat products are rich in complex carbohydrates and provide fiber, vitamins, and minerals alongside energy. Starchy vegetables such as potatoes, sweet potatoes, corn, and peas are concentrated sources of digestible carbohydrates and also contribute fiber and phytochemicals. Legumes including lentils, chickpeas, black beans, and kidney beans are carbohydrate‑dense while also offering plant protein and significant amounts of soluble and insoluble fiber. Fruits such as bananas, apples, oranges, and berries contain natural sugars coupled with dietary fiber, antioxidants, and micronutrients, which slow digestion and support glycemic control. Dairy products like milk and yogurt provide lactose, a naturally occurring carbohydrate, along with protein and calcium. Foods rich in simple sugars include table sugar, honey, syrups, and many processed snacks; these contribute energy with minimal essential nutrients and should be limited. The quality of carbohydrate sources impacts health outcomes: diets emphasizing whole, minimally processed sources are associated with better metabolic profiles and reduced chronic disease risk compared with diets high in refined sugars and processed grains. Choosing foods with higher fiber content and lower glycemic impact supports sustained energy and digestive health. Balancing carbohydrate sources across meals can help stabilize blood glucose and prevent energy spikes and crashes.

Carbohydrates are digested primarily in the gastrointestinal tract. Enzymes such as amylases break down complex starches into maltose and dextrins, which are further cleaved into glucose by brush border enzymes. Simple sugars such as glucose and fructose are readily absorbed via transporters in the small intestine; glucose absorption is facilitated by sodium‑dependent glucose transporters, and fructose by GLUT5 transporters. The rate of absorption influences postprandial glucose and insulin responses: foods high in fiber and complex carbohydrates generally slow digestion and absorption, leading to lower glycemic peaks compared with refined sugars. Dietary fiber, particularly soluble fiber, is not digested by human enzymes but is fermented by gut microbiota in the colon, producing short‑chain fatty acids that confer benefits for colon health and glucose metabolism. Factors influencing bioavailability include the physical form of the food, processing, cooking method, and the matrix of nutrients present. For example, intact whole grains and legumes release carbohydrates more slowly than refined flours. Antinutrients such as phytates may slightly modify carbohydrate digestion but are generally not a concern in balanced diets. Combining carbohydrate foods with fats and proteins also slows absorption and moderates glycemic responses.

Carbohydrate supplements are commonly used in sports and clinical settings to deliver rapid energy. Sports drinks, gels, and maltodextrin powders provide easily digestible carbohydrates to sustain performance during prolonged exercise. In clinical nutrition, carbohydrate supplementation may be necessary for individuals with poor appetite, undernutrition, or increased energy demands. However, for the general population consuming a varied diet, carbohydrate supplements are unnecessary because dietary intake typically meets energy needs. Excessive supplementation of simple carbohydrates can contribute to weight gain and metabolic dysregulation if not balanced with energy expenditure. Choosing whole food sources over isolated carbohydrate supplements offers additional nutrients and fiber that support long‑term health outcomes. When carbohydrate supplements are used, timing around exercise and portion control are important to avoid undue spikes in blood glucose.

Carbohydrates have no established tolerable upper intake level because the body can metabolize large amounts within normal dietary practice. Toxicity is not defined for carbohydrates in the way it is for vitamins or minerals. However, excessive intake of refined sugars and processed carbohydrate foods is associated with adverse health outcomes including obesity, insulin resistance, dyslipidemia, and increased risk of type 2 diabetes and cardiovascular disease. Diets with high proportions of added sugars contribute to excess calorie intake and poor nutrient density. Public health guidelines recommend limiting added sugars to reduce these risks. Chronic overconsumption of sugar‑sweetened beverages and highly refined carbohydrates is linked with metabolic syndrome and weight gain. Therefore, while carbohydrates as a nutrient are not toxic per se, the quality and context of carbohydrate intake are central to long‑term health.

Carbohydrate intake can influence the pharmacokinetics and effects of certain medications. For instance, high carbohydrate meals can affect the absorption of oral diabetes medications and insulin dosing requirements. Rapidly absorbed carbohydrates can necessitate adjustments in insulin administration in individuals with type 1 or type 2 diabetes. Carbohydrate counting is commonly used to fine‑tune insulin doses and certain oral hypoglycemic agents to achieve glycemic targets. Medications like alpha‑glucosidase inhibitors (e.g., acarbose) directly affect carbohydrate digestion and slow the breakdown of complex carbohydrates to modulate postprandial glucose. Additionally, carbohydrate‑rich meals may alter the absorption of some antibiotics when taken concurrently due to changes in gastric emptying rates. Patients using glucocorticoids may experience higher blood glucose levels after carbohydrate meals, requiring monitoring and potential medication adjustment. Healthcare providers should consider meal composition when prescribing drugs that interact metabolically with carbohydrate intake.

Common Forms: sports gels, maltodextrin powders, energy drinks

Typical Doses: Varies by use; 30–60 g/hour during endurance exercise for athletes

When to Take: Around prolonged exercise sessions when rapid energy is needed

Best Form: simple glucose or maltodextrin for rapid energy

⚠️ Interactions: affects insulin dosing in diabetes, alpha‑glucosidase inhibitors slow carbohydrate digestion