Carbohydrates are a type of macronutrient, and they provide the body with energy. Whether they are essential to the diet remains a highly debated topic. In this Honest Nutrition feature, we define the term “carbohydrates,” discuss whether they are essential in the diet, and explain what happens when people restrict them.
To address whether carbs are essential, it is essential to explore the role of carbohydrates — specifically glucose — in the body and diet.
Carbohydrates are one of the three main classes of macronutrients, alongside proteins and fats. They provide energy to the body, which breaks them down into glucose.
There are three types of carbohydrates: starches, simple sugars, and dietary fiber. However, people may use the term carbohydrates loosely, and incorrectly, to define just starchy foods.
Using this term to refer solely to starchy foods such as bread, rice, and potatoes may lead to an incomplete understanding of the role of carbohydrates in the diet and the body.
Furthermore, carbs are present in a wide variety of foods. Here are some dietary sources of carbs:
|Food group||Carbs per serving, in grams||Examples|
|grains||15||quinoa, sweet potato, and rice|
|legumes||17||pinto beans, black beans, and lentil peas|
|dairy||11||cow’s milk, yogurt, and cheese|
|fruits||15||cherries, raspberries, and stone fruit|
|vegetables||5||Brussels sprouts, bok choy, and cabbage|
These numbers show that nonstarchy vegetables provide about one-third the amount of carbohydrates compared with grains, legumes, and fruits. However, they are not completely devoid of carbs.
Given that food sources of carbohydrates also provide essential micronutrients — such as B vitamins for energy production and disease-fighting phytochemicals — these foods remain important in the diet.
However, the diversity of food sources of carbohydrates means that higher intakes of starchy foods specifically may not be necessary.
Carbohydrates in the body
The body breaks down carbohydrates into its major energy-producing form — glucose.
Glucose then undergoes a series of enzyme reactions that generate the energy-producing substrates pyruvate and lactate in a process called glycolysis.
Under normal metabolic conditions and oxygen availability, pyruvate is the main precursor to energy production in the mitochondria of the cell, and the body consistently produces small amounts of lactate via the lactate shuttle.
However, when an oxygen debt occurs — such as during intense physical activity — lactate, in the form of lactic acid, becomes the primary substrate that the body uses to provide energy to muscle cells.
In the mitochondria, the further processing of pyruvate produces acetyl-coenzyme A (acetyl-CoA), a two-carbon compound that combines with carbohydrate-derived oxaloacetate to form citrate and start the tricarboxylic acid (TCA) cycle.
Amino acids and fatty acids also produce some acetyl-CoA.
The TCA cycle, which people may also refer to as the Krebs cycle, generates carbon dioxide and adenosine triphosphate (ATP) — a signaling molecule and the “energy” that enables muscles to contract.
Each glucose molecule goes through two rounds of the TCA cycle, producing ATP and carbon dioxide. This gas enters the blood and leaves the lungs during exhalation.
What happens when you restrict carbohydrates?
A steady supply of glucose is necessary to maintain oxaloacetate levels and the regular functioning of the TCA cycle.
When dietary carbohydrates are restricted, the TCA cycle is diminished, and ketone production becomes predominant for energy needs.
The body then relies on ketogenic amino acids and fatty acids to produce acetyl-CoA. However, instead of entering the TCA cycle, acetyl-CoA substrates are metabolized to form the ketone bodies acetoacetate and beta-hydroxybutyrate.
The production of ketone bodies takes place in the liver, but these molecules enter the circulation and travel to the brain, heart, kidney, and skeletal muscle to supply energy.
This alternative metabolic pathway for energy production in the body has motivated many low carbohydrate diets as an approach to reduce the chronic disease risk associated with the excess intake of dietary carbohydrates, particularly that of refined carbohydrates and added sugars.
Two of the groups of people most likely to restrict carbohydrates are athletes and those trying to lose weight.
Keto adaptation, which refers to the metabolic adjustment to using fat as the predominant energy source when carbohydrates are restricted, can take as little as 5–6 days.
A 2018 study involving weightlifting athletes demonstrated that a low carbohydrate ketogenic diet reduced body fat without compromising lean muscle mass and powerlifting performance.
However, many studies in endurance athletes have revealed that this dietary approach lowers peak power and impairs exercise performance.
For this population, therefore, keto adaption may not be the optimal dietary choice. However, more long-term studies are necessary to confirm this.
Several scientific studies even advocate for high carbohydrate diets for endurance and elite athletes.
When appropriate, weight loss improves metabolic health and reduces obesity-related comorbidities.
Weight loss is often a major motivator for those following low carbohydrate, high fat (ketogenic) diets, and based on the carbohydrate-insulin theory, carbohydrate restriction does offer metabolic advantages.
However, although carbohydrate restriction is beneficial for some people — with a ketogenic diet promoting weight loss and a reduction in blood sugar markers — it can increase the risk of heart disease and the levels of LDL cholesterol, the so-called bad cholesterol.
Furthermore, many researchers have disregarded the carbohydrate-insulin theory as too simplistic given the lack of evidence to demonstrate that a high carbohydrate intake is fattening for most people.
In fact, the restriction of dietary fat leads to more significant body fat loss than carbohydrate restriction in people with obesity.
Instead, overall caloric intake — regardless of the percentages of carbohydrates, protein, and fat intake — appears to matter more for long-term diet adherence and weight loss.
Glucose tolerance and carbohydrate restriction
Glycemic index and glycemic load are predictive measures of how a carbohydrate food or combination of foods may affect insulin and blood sugar levels.
For instance, foods with a low glycemic index, which include complex carbohydrates such as quinoa, should have less effect on blood sugar levels than high glycemic index foods such as simple sugars.
However, this system is inconsistent and does not account for individual differences in tolerance to carbohydrates and glucose. These differences help determine exactly how much foods raise blood sugar in some people.
Individuals may react differently to carbohydrates in the diet, depending on their glucose tolerance, which doctors based on the results of fasting blood sugar, glycated hemoglobin, and oral glucose tolerance tests.
For those with impaired glucose tolerance or insulin resistance, which will include people with prediabetes and hormonal imbalances such as polycystic ovary syndrome (PCOS), carbohydrate restriction is beneficial for improved metabolic health.
It is important to monitor which carbohydrate foods spike blood sugar levels in these individuals because glucose tolerance may also vary from day to day and depending on the time of the day.
The bottom line
Carbohydrates are a macronutrient that provides the body with energy. They occur naturally in a variety of foods, including grains, legumes, fruits, vegetables, and dairy. Despite this, many people use the term carbohydrates to describe starchy foods and simple sugars.
In the body, carbohydrates generate pyruvate, acetyl-CoA, and oxaloacetate in the tricarboxylic acid cycle, playing a key role in energy production.
In the absence of carbohydrates, the body uses ketogenic amino acids and fatty acids to produce ketone bodies for energy production. This process is called keto-adaptation.
Carbohydrates are not the only source of energy production in the body, but they provide supportive essential nutrients for energy production, such as B vitamins.
A low carbohydrate diet may improve metabolic markers in people with glucose intolerance and insulin resistance, but it may not be appropriate for other groups of people, such as endurance athletes.