Food & Nutrition

The ultimate guide to carbohydrates

Confused about carbs? Here's everything you need to know about carbohydrates.
Published 22 May 2019 | Updated 21 November 2023

Carbs may be the world’s most confusing macronutrient. On the one hand, most health experts say carbohydrates help form the foundation of a healthy diet. On the other hand, some advocates of low-carb diets suggest we’d be slimmer, stronger, more focused and more energised if we just quit eating carbs.

In reality, no carb-containing food is inherently good or bad. Carbohydrates come in many forms, with various implications for health. Once you have a little know-how on the science of carbohydrates you’ll be empowered to make informed food choices that support your wellness goals.

What are carbohydrates?

Carbs are essentially sugar molecules—groupings of carbon, hydrogen and oxygen—that occur either alone or linked together in different combinations. Most of the carbs we eat are digested and broken down into glucose to provide energy for our muscles and brain; in fact, carbohydrates are the body’s preferred source of fuel, says Silvia Klinger, registered dietitian and nutritionist. Some carb types support digestion, gut health and other functions. Taken together, carbohydrates are classified as macronutrients, meaning the body needs relatively high amounts in the diet for energy.

Definition of carbohydrate types

Some of us hear the word ‘carbs’ and immediately think ‘bread’. In fact, the world of carbs is diverse, Klinger says. Carbohydrates fall into one of three main groups, with many foods containing more than one type of carbohydrate.

  • Sugars. It’s not just the granulated white stuff you spoon into your coffee. This carb group also includes sugars that occur naturally in whole foods, such as the fructose in fruits and lactose in dairy, including yoghurt. Also in the sugar category are sweeteners found in commercially made foods and drinks. On ingredient labels, sugar goes by a number of names, including syrup, dextrose and molasses.
  • Starches. Food sources include grains such as rice, oats and barley, as well as grain-based products such as bread, cereal and pasta. Potatoes, beans, peas and corn are major sources of starch, too.
  • Fibre. Found only in plant-based foods like fruits, veggies and grains, fibre adds bulk to your diet. It has a number of subtypes—for example, there’s pectin in citrus fruits, inulin in onions and cellulose in kale—which are classified as either soluble or insoluble depending on how they interact with water. Different types of dietary fibre support health in different ways.

Simple and complex carbohydrates

Nutrition experts further classify carbs as simple or complex depending on the number of sugar molecules the carbs contain. Sugars fall into the simple category because they’re made up of just one or two molecules. Starches and fibres, meanwhile, comprise at least three sugar molecules and are therefore classified as complex carbs. Here’s a little more about what that means for the body at mealtimes.

  • Simple carbohydrates: sugars. Thanks to their basic molecular structure, sugars require little digestive breakdown. This means glucose enters the bloodstream soon after a person eats. “Simple carbs can be beneficial, especially for people who are very active physically and need quick bursts of energy during exercise,” says Mindy Hermann, registered dietitian and nutritionist.

    The presence of fibre in a food can slow the release of simple sugars to result in a more sustained energy release and greater satiety, Hermann notes. That’s one reason she recommends fibre-rich whole foods such as apples over sweetened foods like biscuits, which tend to lack fibre. “Unlike fruit, processed foods with added sugar really aren’t that filling,” Hermann says. “Some people find that [foods with added sugar] can lead to overeating.”

  • Complex carbohydrates: starches and fibre. As noted above, ​complex carbs are larger chains of sugar molecules linked by chemical bonds. For starches, digestive enzymes in the gut need time to break the chains into single units, so starches tend to provide a steadier and more gradual source of glucose (energy) than simple sugars do, says Susan Mitchell, PhD, registered dietitian and nutritionist.

    Dietary fibre, meanwhile, works a little differently from starches and sugars. Our bodies lack the enzymes needed to break down the chains in fibre into simple sugars, Mitchell explains. As a result, fibre passes through the gut without being used as energy. Fibre supports health in other ways depending on its form. Soluble fibre, found in foods like oat bran and legumes, combines with water and forms a gel for a stabilising effect on blood sugar, for example. Insoluble fibre, found in foods like whole grains and veggies, helps move food through the digestive tract, supporting regular bowel movements and bowel health.

Function of carbohydrates

Carbohydrates play a complex and integral role in human health. Here’s a closer look at how carbs affect the body.

Carbs and energy

After you eat a carb-containing meal or snack, it travels to the small intestine for digestion and absorption into the bloodstream. Any complex carbs are broken down into single units of glucose, which then enter the bloodstream. Once glucose makes its way into cells, it’s synthesised into a molecule called ATP (adenosine triphosphate), a cell’s main carrier of chemical energy. Although cells can also make ATP from the building blocks of protein- and fat-based foods, glucose from carbs is the body’s preferred and most widely used source of energy.

Not all glucose ends up being used for ATP production. The liver and muscles also stash some glucose away in the form of glycogen to protect against future shortages. Any glucose not needed after that is stored as fat.

Carbs, blood sugar and insulin

Glucose can’t do its job without a major assist from insulin. In healthy people, the pancreas secretes insulin (a hormone) in response to rising blood levels of glucose after a carb-containing meal or snack. Insulin then ‘unlocks’ the body’s cells so glucose can enter. In some people, however, the body’s cells don’t respond well to insulin, a condition known as insulin resistance. This can lead to chronically high blood sugar levels—and health impacts such as pre-diabetes and type 2 diabetes. Many people are able to improve their insulin response with lifestyle measures such as maintaining a healthy weight, eating a balanced diet, regular physical activity etc.

Carbs and digestion

Fibre is probably most famous for preventing constipation. In taking on water, soluble fibre helps form soft, bulky stools. Insoluble fibre adds further bulk and, with a broom-like effect, keeps waste material moving along to support regular bowel motions.

That’s not all fibre does during the digestive process. When some types of fibre (such as pectin) reach the colon, they help feed the beneficial bacteria in your gut—aka probiotics—through a process of fermentation. As those bacteria feast on the fibre, some produce short-chain fatty acids believed to play a role in immunity, inflammation control and more. Other gut bacteria may use their fibre fill-up to influence hormones that regulate appetite.

Many adults in Australia fall short of getting the recommended 25g to 30g of fibre per day. You can boost your fibre intake by eating whole grains and whole grain foods, as well as legumes, fruits, vegetables, nuts and seeds.

Carbs and weight control

“It can be hard not to get swept up in the carb confusion,” Klinger says. “The truth is, carbs can still be a part of your weight-loss plan.” After all, even diets billed as low-carb contain some carbs. (Keep reading for more on low-carb diets.)

For weight management, Hermann recommends choosing foods that contain complex carbs because they tend to be digested and absorbed more slowly than foods that contain little fibre or just simple sugars (white bread and sweets being classic examples). “Much of the time, simple carbs provide a lot of calories, but not a lot of nutrition,” Hermann says.

Carbs, brain function and mood

Despite accounting for just 2% of body weight, the brain uses about 20% of the body’s glucose, making it a major consumer. That energy goes toward maintaining neurons and generating neurotransmitters. The brain depends on glucose (carbs) as its major source of energy, which likely explains why lower-than-normal blood levels of glucose, a condition known as hypoglycemia, can make people feel confused, tired or irritable. Hypoglycemia warrants prompt medical attention; contact your doctor if you experience symptoms.

‘Good’ vs. ‘bad’ carbohydrate foods

At WeightWatchers®, we steer clear of value judgments concerning individual foods. After all, no food is eaten in isolation! A healthy pattern of eating includes selections from a variety of food groups while allowing you the flexibility to plan for any food you love.

That said, if you’re looking to boost your overall nutrient intake while getting your carbs, consider putting these selections on your plate.

Healthier carbohydrate-containing foods

  • Vegetables.​ From Brussels sprouts to beetroot, all veggies contain some measure of carbs. Eating a variety will serve you well. In addition to being good sources of many vitamins and minerals, “veggies provide an A-to-Z mix of naturally occurring plant compounds called phytonutrients, which help protect us from heart disease, cancer and other diseases,” Mitchell says.
  • Whole fruits. The fibre in any given fruit slows the release of its simple sugars, making fruit a nutritious option for those of us with a sweet tooth. And though some people may worry that fruits’ naturally occurring simple sugars will add kilos on the scale, research following U.S. men and women for 24 years found that the more fruit and vegetables people ate, the less weight they gained over time.
  • Legumes, peas and lentils boast a unique combination of starch, fibre and protein, which can help to satisfy appetite. A meta-analysis of 11 clinical trials found that people who regularly ate legumes (pulses) had a reduced risk of developing cardiovascular disease, possibly due to beneficial effects on blood pressure, cholesterol, blood sugar and more.
  • Whole grains. In addition to delivering complex carbs, including fibre, whole grains are rich in magnesium antioxidants and vitamin E . Research suggests that a diet high in whole grains can help lower the risk of developing type 2 diabetes and cardiovascular disease.
  • Tubers. ​​Sceptics may scoff that potatoes and sweet potatoes are starchy, but again, that’s not an inherently negative quality. Remember, starch is a complex carb! Depending on the variety, a tuber vegetable might be a good source of potassium, vitamin C, vitamin B6 and more.

Low-carbohydrate diets

From keto to paleo, low-carb diets are very popular at the moment. “Low-carb diets often result in short-term weight loss but are tough to maintain because they’re so restrictive”, Hermann says. If dieters return to their former eating habits, any weight that was lost tends to come back, she notes.

How a low-carbohydrate diet works

Some advocates of low-carb diets believe that limiting carbs accelerates weight loss by setting off metabolic changes that optimise fat burn and keep appetite in check. While research suggests that medically supervised carb restriction might be helpful to some people living with certain health conditions, there’s little scientific evidence that low-carb diets impart special weight-loss powers. A 2019 research review concluded that low-carb diets are no more effective than other dietary approaches to weight loss, building on previous research—like this small study of adults in 2009—that uncovered no low-carb weight-loss advantage beyond the first few months. Other studies have raised questions about the long-term safety of strictly limiting carbs.

Low-carb diets vary in their degree of carb restriction. The popular keto diet generally caps carbs at 20g to 50g a day, whereas a more moderate low-carb diet might allow for up to 130g a day (roughly the amount you’d get in 3 cups of cooked rice).

Hermann says the most sensible approach to a low-carb weight-loss diet would encourage people to limit foods and drinks that contain added sugars, which are often high in calories and low in nutrients, while still encouraging consumption of nutritious whole foods.

Low-carb diet foods

Diets that limit carbs generally require an increase in fat and protein intake. Just like with carbs, however, all fats are not created equal. For example, bacon contains zero carbs but is high in saturated fat, which is linked to coronary heart disease. On the other hand, unsaturated fats found in nuts, seeds and vegetable oils are considered heart-protective.

A healthier low-carb diet takes these factors into account, emphasising lean sources of protein such as chicken, fish, legumes, eggs and low-fat dairy—plus plenty of plant-based foods.

Examples of low-carbohydrate diets

  • Standard low-carb diet. A basic low-carbohydrate plan encourages dieters to get around 25% of energy intake from carbohydrates (up to 130g a day), with the rest coming from fat and protein.
  • Ketogenic diet. Considered a very low-carb eating plan, the keto diet restricts carbohydrates to 5% to 10% of total energy intake and protein is limited to about 20%. For better or worse, the remainder of daily energy intake is devoted to high-fat foods like butter and nuts.
  • Paleo diet. Also known as the Stone Age diet or caveman diet, the paleo approach is designed to mimic the meals of prehistoric human ancestors. It restricts grains, legumes, dairy, certain vegetable oils, added sugar and highly processed products. Instead, dieters are encouraged to eat lean meats, fish, fruits, vegetables, nuts and seeds.
  • Atkins diet. The Atkins diet uses a value known as net carbs—grams of total carbs minus grams of fibre. The Atkins diet generally allows 20g to 100g of net carbs a day. Because fibre doesn’t count toward carbs on the Atkins program, its various plans steer dieters toward high-fibre, lower-sugar produce. Fat and protein fill out the rest of a typical Atkins plan.

Potential side effects and risks of low-carb diets

Low-carbohydrate diets vary in their intensity and affect people in different ways. It’s best to consult with your doctor before switching to a low-carb eating plan (or making any radical change to your diet). Below are some potential pros and cons to consider.

Potential side effects of a low-carb diet:

  • Vitamin and mineral deficiencies
  • Digestive difficulties, particularly constipation
  • Headaches
  • Halitosis (bad breath)
  • Muscle cramps
  • Difficulty with adherence and concentration

Potential benefits of a low-carb diet:

  • Short-term weight loss
  • Improved blood sugar control in people with type 2 diabetes
  • Reduced blood pressure in people with hypertension
  • Improvement in levels of blood lipids

What should your daily carb intake be?

The latest Australian Dietary Guidelines recommend that carbs should make up between 45% and 65% of total energy. The optimal proportion of carbs for you depends on your lifestyle and personal health picture, Mitchell says. For example, if you are living with type 2 diabetes, your optimal carb intake might be on the lower end of that bracket to help keep blood glucose level within a healthy range, she explains. Lifestyle factors such as physical activity can increase the need for carbs. Your doctor can advise you on the carb intake that makes sense for you.

The upshot: Should you eat carbohydrates?

Given that they are the preferred energy source for the brain and muscles and support other important functions, carbohydrates are key in a healthy pattern of eating, a large body of research has determined. Carbs come in several forms and are abundant in many foods.

Health experts generally recommend limiting intake of carbs from added sugars and refined grains (white varieties of bread, rice etc). Instead, consider centering your diet on whole foods that deliver vitamins, minerals and phytonutrients along with carbs. Examples of nutritious, carb-rich foods include fruits, veggies, whole grains and legumes.

Despite the popularity of low-carb diets, there’s little evidence to suggest that people have more long-term weight-loss success when they severely cut back on this macronutrient group. In fact, whole foods that contain carbohydrates often promote satiety while being relatively low in calories. “Carbs can and should be part of a diet for weight management,” Hermann says.

A healthy approach to eating—whether or not you’re looking to lose weight—is a plan that’s sustainable in the long term and one that includes a variety of whole foods and allows for personal choice in what to eat.