The idea that “carbs are bad” has left many people confused and misinformed about the vital role they play in a part of a balanced and well-rounded nutrition plan. They have become quite a polarising topic, with many people promoting the adoption of low carb diets such as Atkins and the Ketogenic diet, so I’ve done the hard work for you and reviewed the literature on the topic to help you make informed choices around nutrition and fuelling.
Firstly, let’s define what is actually meant by “carbs”…
Carbohydrates (CHO) are one of the three macronutrients, along with protein and fat, that make up all food and are essential for the body to function. Furthermore, CHO can be broadly classified as one of three types:
- - Sugars are simple, short-chain compounds (e.g. glucose, fructose). These are responsible for a quick, short-term spike in blood glucose
- Starch is a long chain of sugar compounds found in food that comes from pants and includes bread, pasta, grains, and potatoes. These provide a slower and steadier release of enrgy over time
- Dietary fibre is also a long chain of sugar compounds found in the cell walls of foods that come from plants but cannot be digested by the gut. Good sources of fibre include wholegrain bread, wholewheat pasta, beans, and lentils
A few other terms you’ll come across when navigating the plethora of carbohydrate information and recommendations are:
- Glycaemic index: used to compare food based on standardised amount of CHO available
- Glycaemic load: GI multiplied by total amount of CHO in a typical serving and allows a comparison of foods to predict glycaemic response
Australian dietary guidelines recommend that CHO make up between 45 to 65% of your total daily caloric intake, but what is more important is the CHO type when it comes to risk of chronic disease1. Your daily CHO intake should be sourced predominantly from low energy-density and/or low glycaemic index sources2. For example, brown rice, potatoes, and wholemeal bread are not as calorie-laden as hot chips or pastries which also contain high amounts of fat and other nutrients such as sodium. Higher total glycaemic load is an independent risk factor for type II diabetes, cardiovascular morbidity and mortality incl. stroke, and some cancers1. Free sugars such as those found in honey, syrup, and added to food and drinks should not make up more than 5% of the total calories you consume during the day (this roughly equates to no more than 30g/day). Higher intakes of added sugars also raise triglycerides, total cholesterol, blood pressure, and other cardiovascular disease risk factors3. In a meta-analysis of trials in adults, reduced intake of free sugars, with no other restrictions on diet, was associated with modest decrease in body weight4. It is important to note that sugars occur naturally in fruit, vegetables in milk but these do not count as free sugars and do not need to be limited within the diet!
When putting together a meal or snack, a combination of protein, CHO and small amounts of fat are the most conducive to satiety and weight loss5. Fibre-containing carb options such as pasta, brown rice, couscous and quinoa are filling and satisfying when combined with a source of protein. Carbs keep you fuller for longer; and per gram contain far fewer calories compared to fats (16 vs 36) so you can eat a larger volume for the same number of calories.
CHO foods also contain a lot of other important vitamins and minerals, especially fibre, so if avoiding an entire food group, you are missing out on a lot of other vital nutrients. Starchy CHO provide B vitamins essential for the functioning of the nervous system, to support healthy skin, metabolism and release of energy from food, and functioning of healthy blood cells. Wholegrain carbohydrates contain fibre as well as other essential nutrients such as magnesium and B vitamins. There is strong evidence that eating wholegrains is linked with lower body weight, a slimmer waist, and reduced risk of weight gain.
CHO as a Fuel Source
Carbohydrate is an important energy source during exercise over a range of intensities due to its ability to be used by aerobic and anaerobic pathways6. Maintaining an appropriate level of CHO intake to fuel demands of exercise aids performance of prolonged endurance exercise and intermittent high intensity exercise. During moderate-high intensity exercise the body is unable to use fat as a fuel quickly enough and requires carbohydrate to produce energy. CHO is used as the preferential fuel source at higher intensities because it provides a high volume of energy per volume of oxygen delivered to the mitochondria, thus improving efficiency. Carbohydrates also regulate the anabolic (muscle building) response to resistance training. Sufficient glycogen availability before, during and after weight training promotes favourable exercise-induced adaptations7. When insufficient CHO is consumed, hypertrophy is inhibited and muscle breakdown increases.
Glucose is the brain’s preferential source of energy. When muscle glycogen and blood glucose stores are low, this leads to impaired skill and concentration, increased perception of effort6, depression and mood swings. For example, After overweight women followed a "low-carbohydrate" diet for a week (completely eliminated carbohydrates from their diets) they did worse on tests of working memory and visuospatial memory than their counterparts who followed a "low-calorie" diet, based on American Dietetic Association guidelines, in a study from Tufts University. Researchers also suspect that CHO promote the production of serotonin, a feel-good brain chemical. In a study from the Archives of Internal Medicine, people who followed a very low carbohydrate diet (only 20-40g daily) for a year experienced more depression, anxiety and anger than those assigned to a low-fat, high-carb diet.
Low Carb Diets
Low carbohydrate diets have really grown in popularity as a strategy for weight loss over the past 50 years due to the belief that by lowering insulin we can prevent entering an anabolic fat-storing state and thereby induce weight loss8. Results from a large number of sources refute the carbohydrate-insulin hypothesis9– when highly rigorous controlled-feeding studies have been performed, neither lowered insulin levels, increased resting metabolism, or increased fat loss have been observed on low CHO diets.
Low carb diets appear to be superior for promoting rapid weight loss over the initial 6-12 months10, however there is little evidence for their long-term efficacy. This initial weight loss is partly due to water loss. These diets are not sustainable, making it hard to eat normally and leading to feelings of deprivation and hunger8. A meta-analysis of randomised controlled trials revealed that while individuals assigned to low-CHO diets without energy restriction lost more weight than individuals consuming low-fat diets after the first 6 months, within a year there was no difference in weight loss. They also noted unfavourable changes in LDL cholesterol10. When you limit carbohydrates, you deprive your body of a main source of fuel — and many essential nutrients that you need to stay healthy. Hall et al (2016) demonstrated this in a controlled-feeding study where 17 overweight or obese men consumed a low-CHO ketogenic diet and lost weight through a calorie deficit of 300kcal/day but was not accompanied by increased fat loss and actually experienced more muscle breakdown (loss of fat-free mass)11. Very low carb diets such as Atkins or the aptly named Ketogenic (Keto) diet restrict daily CHO intake to only 20-50g (less than 10% total daily energy intake) and induce glycogen depletion which increases oxidation of muscle proteins7 and ketone production. The body can use ketones, but it is a far from optimal fuel source, especially for the brain. If you’re using ketones as fuel the body is under enormous pressure and if they build up over time can cause further problems such as kidney damage8.
Carbs are not all bad and do not need to be eliminated to lose weight. Eating too much of anything will likely lead to weight gain, since obesity risk is primarily determined by total caloric intake9. But any diet that produces a calorie deficit (whether from reducing fat, protein, or CHO), will have be effective for weight loss8 – “a calorie is a calorie”. Strong data indicates that energy balance is not materially changed during isocaloric substitution of dietary fats for carbohydrates. A meta-analysis of 32 controlled feeding studies with isocaloric substitution of CHO for fat found that both energy expenditure and fat loss were actually improved with lower fat diets12. Weight loss on low carbohydrate diets is also generally associated with eating less energy-dense or processed food and higher protein intakes can help with appetite control and maintenance of muscle mass. However, in the medium and long term these diets do not seem to result in faster weight loss, or better maintenance of weight loss, than traditional energy restricted diets.
When you talk about carbohydrate foods, many people automatically refer to cases of high calorie diets laden with large amounts of sugar and foods mentioned above, which are linked with obesity and other diet related diseases such as heart disease. For those with a healthy weight and active lifestyle, there is no reason to completely remove them from the diet.
Rather than restrict carbs, choose healthier ones more often.