Carbohydrates
Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen, typically in a 2:1 ratio of hydrogen to oxygen. They are classified as simple (monosaccharides and disaccharides) or complex (polysaccharides) based on their structure and digestion speed. Common simple carbohydrates include glucose and sucrose, while complex carbohydrates include starches and fiber. The primary function of carbohydrates in the body is to serve as a source of energy, which is primarily converted into glucose. Excess glucose can be stored as glycogen for later use.
Dietary guidelines suggest that carbohydrates should make up 45 to 65 percent of total calorie intake, focusing on healthier sources like whole grains, fruits, and vegetables. There is concern over the increased consumption of refined sugars, which have been linked to health issues such as obesity and diabetes. Various diets, including low-carb options, have emerged as strategies for weight loss and health management, though some experts advocate for the inclusion of complex carbohydrates for better long-term health outcomes. Understanding carbohydrates and their impact on health is essential, especially in light of rising health challenges associated with diet.
Carbohydrates
DEFINITION: Organic compounds consisting of carbon, hydrogen, and oxygen, in which hydrogen and oxygen are in a 2:1 ratio.
ALSO KNOWN AS: Sugars, saccharides
ANATOMY OR SYSTEM AFFECTED: Blood, brain, endocrine system, gastrointestinal system, intestines, kidneys, muscles, pancreas
Digestion and Absorption
Dietary carbohydrates are macronutrients. Carbohydrates may be simple or complex, depending on their chemical structure and how quickly they are broken down and absorbed. Carbohydrates include monosaccharides, such as glucose, fructose, and galactose; disaccharides (two monosaccharides linked together), such as sucrose, maltose, and lactose; and polysaccharides (many monosaccharides linked together in polymers), such as starches and fiber. Simple carbohydrates include monosaccharides and disaccharides, while complex carbohydrates include polysaccharides.
Carbohydrates are broken down into glucose by the enzyme amylase. Starches are first broken down in the mouth by salivary alpha-amylase and then in the small intestine by alpha-amylases of both salivary and pancreatic origin. The resulting simpler sugars are further digested by enzymes linked to the inner lining of the small intestine: maltase, sucrase, and trehalase, which yield absorbable monosaccharides. These sugars cross the cells lining the small intestine via specialized molecular transport mechanisms and then diffuse into the intestinal capillaries and reach the bloodstream.
Metabolism
In the body, the main role of carbohydrates is energy production and storage. Carbohydrates can also be joined to proteins (glycoproteins, for cell-cell interactions) or fatty acids (glycolipids, which provide energy and can be markers for cellular recognition).
The body converts most digestible carbohydrates into glucose, which is a universal energy source for cells. Excess glucose is stored as glycogen (glycogenesis), which can then be broken down (glycogenolysis) when energy is needed. Glucose is maintained at a constant level in the blood by the interplay of insulin, glucagon, and other hormones.
Carbohydrate-related diseases are often genetic in nature, linked to inborn errors in enzymes or cellular transporters. Examples are galactosemia, glycogen storage diseases, and lactose intolerance. Diabetes mellitus is a metabolic disorder characterized by excessive blood glucose. Type 1 diabetes is caused by insulin deficiency; type 2 can be the result of insulin resistance, impaired insulin secretion, and increased glucose production.
According to the 2020–2025 Dietary Guidelines for Americans, carbohydrates, preferably starches and natural sugars, should represent 45 to 65 percent of total calorie intake. In 2022, the renowned Mayo Clinic confirmed those percentages remained accurate advice. Refined simple sugars provide calories but very little nutrition, and their intake should, therefore, be limited. Too much dietary sugar can lead to health problems such as tooth decay, malnutrition, and weight gain. Fiber-rich fruits and vegetables, whole grains, and legumes are healthy and nutritious sources of carbohydrates. Similarly, in 2023 guidance, the World Health Organization recommended that carbohydrates make up 40 to 70 percent of total calorie intake, with this carbohydrate intake coming primarily from vegetables, whole grains, pulses, and fruits.
Perspective and Prospects
Food availability in developed countries has reached unprecedented levels, and the per capita consumption of carbohydrates, particularly in the form of refined sugars, increased dramatically in the late twentieth and early twenty-first centuries. Since the 1990s, the incidence of obesity has been climbing steadily, and so has the incidence of diabetes and related health problems. Research in nutrition and carbohydrate metabolism has been geared toward addressing the problem, which has reached epidemic proportions. Progress continued to be made in dietary manipulations and drug development.
Diets low in carbohydrates have gained popularity as a way to lose weight or to manage health problems such as diabetes, cardiovascular disease, and high blood pressure. Low-carb diets emphasize proteins such as meat, poultry, fish, and eggs and nonstarchy vegetables such as asparagus, brussels sprouts, cauliflower, celery, cucumbers, salad greens, peppers, and tomatoes. Low-carb diets exclude most grains, legumes, fruits, breads, sweets, pastas, and processed foods. Some experts have reported that there is insufficient evidence for the heart-healthy benefits of low-carb diets in the long term and have recommended that individuals favor complex carbohydrates, such as legumes and whole grains, over simple sugars and processed foods.
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