Carbohydrates are the body's most important source of energy, but for the body to function properly, it needs a balanced diet that also includes protein and fat. Let's look at what carbohydrates are, and why relying exclusively on plant foods can lead to a deficiency in important nutrients.
What carbohydrates are
Carbohydrates (glycides) are organic compounds containing a carbonyl group and several hydroxyl groups. The name for this class of compounds comes from the words "hydrates of carbon," and was proposed by Carl Schmidt in 1844. The first carbohydrates known to science were described by the general formula Cx(H2O)y, formally making them compounds of carbon and water.
"Sugars" is another name for low-molecular-weight carbohydrates: monosaccharides, disaccharides, and oligosaccharides. Carbohydrates are an integral component of the cells and tissues of every living organism, making up, by mass, the bulk of the organic matter on Earth. For all living organisms, carbohydrates ultimately originate from the process of photosynthesis carried out by plants.
The classification of carbohydrates
Every carbohydrate is made up of individual "units" — saccharides. Based on their capacity for hydrolysis into monomers, carbohydrates fall into two groups: simple and complex.
Simple carbohydrates (fast)
- Monosaccharides — contain a single unit (glucose, fructose, galactose)
- Disaccharides — contain two units (sucrose, maltose, lactose)
- Oligosaccharides — three to ten units
Monosaccharides raise blood sugar quickly and have a high glycemic index, which is why they're called fast carbohydrates. They dissolve easily in water, and are synthesized in green plants.
Complex carbohydrates (slow)
Polysaccharides — contain more than ten units. Foods rich in complex carbohydrates raise glucose levels gradually, and have a low glycemic index, which is why they're called slow carbohydrates. They include:
- Starch — the main storage polysaccharide in plants
- Glycogen — the main storage polysaccharide in animals and humans
- Cellulose (fiber) — a structural polysaccharide in plants
The functions of carbohydrates in the body
In living organisms, carbohydrates serve many important functions:
An energy function
Carbohydrates are the primary source of energy: oxidizing 1 gram of carbohydrate releases 4.1 kcal of energy and 0.4 g of water. This makes them the most readily available, fastest source of energy for the body's cells.
Structural and support functions
Carbohydrates take part in building various support structures. Cellulose is the main structural component of plant cell walls; chitin serves a similar function in fungi, and provides rigidity to the exoskeleton of arthropods.
A storage function
Carbohydrates serve as stored nutrients: glycogen in animals, starch and inulin in plants.
An osmotic function
Carbohydrates take part in regulating the body's osmotic pressure. Blood contains 100–110 mg/L of glucose, whose concentration affects the blood's osmotic pressure.
Sources of carbohydrates in the diet
The main dietary sources of carbohydrates are:
- Bread and baked goods
- Potatoes
- Pasta
- Grains and cereals
- Sweets and confectionery
- Fruits and berries
- Vegetables
Pure sugar is a pure carbohydrate. Honey contains 65% fructose and 25–30% glucose. Fiber and pectins, poorly digested by the human body, also belong to the carbohydrate group.
Why carbohydrates aren't enough: the role of protein
Proteins (also called polypeptides) are high-molecular-weight organic compounds made of alpha-amino acids linked in a chain by peptide bonds. In living organisms, a protein's amino-acid composition is set by the genetic code, and synthesis mostly draws on 20 standard amino acids.
The functions of protein in the body
Proteins serve a wider range of functions in living cells than other biopolymers, such as polysaccharides and DNA:
A catalytic function
Enzymes are proteins with specific catalytic properties. By 2013, more than 5,000 enzymes had been described. They catalyze the breakdown of complex molecules (catabolism) and their synthesis (anabolism), including DNA replication and repair and RNA transcription.
A structural function
Proteins form the basis of many bodily structures. Hair and nails are made of the protein keratin, collagen and elastin are key components of connective tissue, and tubulin forms microtubules.
A transport function
Many proteins take part in transporting substances. Hemoglobin carries gases in the blood; albumins transport fats.
Defensive and immune functions
Proteins play a key role in the immune response, protecting the body from infection and other harmful effects.
A receptor function
Proteins take part in cellular signaling systems. The protein rhodopsin, for example, is essential to the visual receptors, generating a nerve impulse in response to photons of light.
Protein in the diet
Protein is an important part of the diet for animals and humans alike. Its main sources: meat, poultry, fish, milk, nuts, legumes, grains, vegetables, fruit, berries, and mushrooms.
During digestion, enzymes break the protein consumed down into amino acids, which are then used to build the body's own proteins, or broken down further to generate energy.
Amino acids animals cannot synthesize themselves are called essential. Animals obtain them from the protein in their food. Using protein as an energy source matters especially during starvation, when the body's own proteins, particularly in the muscles, become a source of energy.
The role of fat in the diet
Fats (triglycerides, triacylglycerides) are organic compounds — esters, the products of esterifying carboxylic acids with the trihydric alcohol glycerol. In living organisms, they serve, above all, structural and energy functions.
The functions of fat
An energy function
Fat has an energy value of about 9.3 kcal per gram, equivalent to 39 kJ/g. This is more than double the energy value of carbohydrates, making fat the most energy-dense nutrient.
A structural function
Fats are the main component of cell membranes, providing their structural integrity and function.
A storage function
The body's primary energy reserve is stored in fat cells. Fat is one of the main energy sources for mammals.
An insulating function
Thanks to its extremely low thermal conductivity, fat deposited in the subcutaneous layer serves as insulation, protecting the body from heat loss.
Types of fat
Natural fats most often contain the following fatty acids:
- Saturated fatty acids — stearic and palmitic acids (mainly in animal fats)
- Unsaturated fatty acids — oleic, linoleic, and linolenic acids (mainly in vegetable oils)
The higher a fat's content of unsaturated acids, the lower its melting point. Vegetable oils are generally liquid at room temperature, while animal fats are usually solid.
Fat digestion
Emulsifying fat in the intestine (a necessary condition for its absorption) requires bile-acid salts. Saturated fats are broken down in the body by 25–30%, while unsaturated fats are broken down completely.
Balanced nutrition: the synergy of macronutrients
For the body to function normally, it needs a balanced diet, including all three main macronutrients: carbohydrates, protein, and fat. Each group serves unique functions that cannot be fully replaced by the others.
Why carbohydrates alone aren't enough
Although carbohydrates are the body's primary energy source, a diet consisting exclusively of "greens" — vegetables, fruit, and berries — can lead to:
- A deficiency of essential amino acids — the body cannot synthesize every protein it needs
- A shortage of fat-soluble vitamins — vitamins A, D, E, and K require fat for absorption
- Hormonal imbalance — many hormones are synthesized from fats
- Loss of muscle mass — without enough protein, muscle tissue is broken down
- Impaired enzyme function — many enzymes are themselves proteins
Optimal macronutrient ratios
According to nutritionists' recommendations, the optimal balance of macronutrients in a healthy adult's diet is:
- Carbohydrates — 45–65% of total calories
- Protein — 10–35% of total calories
- Fat — 20–35% of total calories
These proportions can vary depending on age, sex, activity level, and individual characteristics.
How macronutrients are metabolized
Carbohydrate metabolism
Carbohydrate metabolism in humans and higher animals consists of several processes:
- Hydrolysis of dietary polysaccharides and disaccharides into monosaccharides in the gastrointestinal tract
- Glycogenesis (synthesis) and glycogenolysis (breakdown) of glycogen in tissues
- Glycolysis — the breakdown of glucose in the body
- Gluconeogenesis — the synthesis of carbohydrates from non-carbohydrate sources
Protein metabolism
Most microorganisms and plants can synthesize all 20 standard amino acids, but animals obtain them from dietary protein. Proteins are broken down during digestion through hydrolysis by protease enzymes.
Fat metabolism
Fats are digested with the help of bile acids and lipases. They can be used as an energy source, as building material for membranes, or stored in fatty tissue.
The consequences of an unbalanced diet
A diet based exclusively on carbohydrates from plant foods can lead to serious problems:
Protein-energy malnutrition
A protein deficiency leads to muscle atrophy, weakened immunity, impaired wound healing, and disrupted enzyme synthesis.
A deficiency of essential fatty acids
A lack of fat can cause problems with the nervous system, skin issues, and impaired absorption of fat-soluble vitamins.
Hormonal disorders
Many hormones are synthesized from cholesterol and other lipids, so a fat deficiency can disrupt hormonal balance.
Conclusion
Carbohydrates, without question, are the most important source of energy for the human body, especially for the brain and muscles to function. Vegetables, fruit, and berries provide the body not just with carbohydrates, but with vitamins, minerals, and fiber as well. For the body to function fully, however, a balanced diet is essential.
Protein supports tissue growth and repair, enzyme and hormone synthesis, and immune function. Fat serves as a concentrated energy source, a component of cell membranes, and a precursor to many biologically active substances.
A diet of "greens" alone, however rich and varied, cannot provide the body with every nutrient it needs. Maintaining health and optimal function requires a diet including all three groups of macronutrients in the right proportions, supplemented by vitamins and minerals.
Understanding the role each macronutrient plays helps in building a balanced diet, one that gives the body everything it needs for a healthy, active life.