WHAT IS A LOW CARBOHYDRATE DIET?

As regular GI News readers already know, the three most common kinds of carbohydrate in foods and drinks are sugars (e.g., fructose, galactose, glucose, lactose, maltose, sucrose, etc…), starches (e.g., amylopectin and amylose) and dietary fibres (e.g., β-glucans, cellulose, hemicelluloses and pectins).

Available and unavailable carbohydrate

Sugars and starches are also commonly known scientifically as available carbohydrates because they can be digested by human digestive enzymes, absorbed and metabolised. On the other hand, dietary fibres are by definition carbohydrates that are resistant to human digestive enzymes and are therefore also commonly known scientifically as unavailable carbohydrates.

Dietary fibre is generally agreed to be beneficial to the health of most humans. Therefore, when talking about low carbohydrate diets, most proponents are actually talking about low sugar and starch diets, or diets that are low in available carbohydrates, but not necessarily unavailable carbohydrates (although many versions of “low carb diets” inadvertently lead to inadequate fibre intakes).

How much available carbohydrate do we need?

Available carbohydrates are largely used as an energy source by our bodies, being the preferred fuel for our brains, nervous systems, and red blood cells. The average adult brain and nervous system requires approximately 140 g of glucose per day and red blood cells require 40 g. Gluconeogenesis (using lactic acid, certain amino acids and glycerol) can provide about 130 g of glucose per day. Therefore, the absolute minimum dietary requirement is approximately 50 g of glucose per day.
Carbohydrates have many other roles, however, including structural, genetic (RNA and DNA), and in the function of certain proteins, and we must not forget that they add taste, texture and colour to our foods and drinks. Carbohydrates are found mostly in plant foods, milk and yoghurt.

Low carbohydrate diet definition

While low carbohydrate diets are certainly not new, having been around in clinical form for over a century, a scientific definition has until recently been surprisingly lacking. In 2015, a group of low carbohydrate diet proponents proposed a 4-tier system based on grams of available carbohydrate and percentage of energy from available carbohydrate, and it appears to have become the de facto standard definition since:

For a typical adult following a very low carbohydrate diet, 10% of energy, or 50 g of available carbohydrate, is equal to 2-slices of bread plus 1-piece of fruit for the entire day. For many children with lower energy requirements, it’s even less. This is one reason why most people find it difficult to follow a very low carbohydrate diet – it simply does not fit in with their personal food preferences (e.g., pleasure of eating) or social requirements (e.g., community, culture, religion, etc…). Also, perhaps unsurprisingly, it’s difficult to obtain all of the essential vitamins, minerals and dietary fibres from a very low carbohydrate diet, so dietary supplements are therefore typically required. Fibre is thought to improve bowel health overall and decrease the risk of gastrointestinal tract cancers (e.g., colo-rectal cancer), amongst other things, so long-term consumption of a low/er fibre diet will likely have detrimental effects on health. This is one reason why few nutrition counsellors recommend very low carbohydrate diets as discussed in this month’s Diabetes Care.

On the other hand, through careful selection of vegetables, fruits, and dairy foods, it is reasonably easy to construct a variety of healthy eating patterns that can be labelled as low carbohydrate using this definition. This is one reason why it is currently quite fashionable (again) to recommend low carbohydrate diets to people with type 2 diabetes as discussed in this month’s Diabetes Care.

Moderate carbohydrate diets are becoming increasingly the norm for people living in developed parts of the world like Australia, Canada, Europe and the United States of America. Spoilt for choice, we can consume a wide range of foods and drinks and consequently obtain considerable amounts of energy from alcohol, fats and protein. The much-lauded Mediterranean diet is a moderate carbohydrate diet.

High carbohydrate diets are still consumed in many poorer parts of the world where starchy foods like grains (e.g., maize, oats, rice, wheat, etc…) and starchy vegetables/tubers (e.g., potatoes, sweet potatoes, yams, etc…) are the staples. In developed nations, vegetarian and vegan diets are also typically high carbohydrate and are currently very fashionable. It is possible to get all of the essential nutrients from high carbohydrate diets. However, special care needs to be taken with vegan diets at certain life stages (e.g., childhood, pregnancy, etc…) with key nutrients like iron and vitamin B12, and for some people, dietary supplements may be required. Additionally, low GI varieties of starchy and sugary foods and drinks are recommended to ensure that the dietary glycemic load is not too high (ideally less than < 95 g per 2000 Calorie / 8,400 kJ per day).

How much carbohydrate are people eating?

In 1995, the National Nutrition Survey found that the average Australian adult consumed 251 g of carbohydrate per day, or 45.3% of energy and 25 g of dietary fibre. In 2011-12, the Australian Health Survey determined that the average adult consumed 226 g of carbohydrate per day, or 43.5% of energy, and 22.9 g of dietary fibre.

In the USA in 2017-2020 adults consumed 227 g of carbohydrate per day, or 43.1% of energy and 16.6 g of dietary fibre. 

Therefore, at the moment, the average Australian and American adult consumes a moderate carbohydrate diet, both lower in dietary fibre than recommended.

What about people with diabetes?

Dietary surveys of what people with diabetes actually eat are rare. A survey of 164 people with diabetes living in the Blue Mountains of Australia in the mid 1990’s found that they consumed 214 g of carbohydrate per day (41% of energy) and 29 g of dietary fibre, suggesting that they ate less available carbohydrate but more fibre than the average Australian adult at that time and consumed a moderate carbohydrate diet overall.

Summary

Essentially everyone agrees that not all carbohydrates should be avoided, but the amount of available carbohydrate advisable to be consumed for good health is much debated. A relatively new system for categorising very low, low, moderate and high carbohydrate diets was proposed in 2015 and has become popular amongst scientists. While it is possible to consume a low carbohydrate diet, most people in the developed world consume a moderate carbohydrate diet, with vegetarians and vegans being notable exceptions, enjoying a high carbohydrate diet. Very low carbohydrate diets typically require vitamin, mineral and fibre supplementation. Very high carbohydrate diets are usually nutritionally adequate, but vegan diets may also pose health risks at certain life stages and may also require dietary supplements. 

Read more:

1. Mann and Truswell. Essentials of Human Nutrition. Oxford University Press, 2012.
2. Feinman and colleagues. Dietary carbohydrate restriction as the first approach in diabetes management: critical review and evidence base. Nutrition. 2015
3. Barclay and colleagues. Macronutrient intake, glycaemic index and glycaemic load of older Australian subjects with and without diabetes: baseline data from the Blue Mountains Eye study. Br J Nutr 2006.

Dr Alan Barclay, PhD, is a consultant dietitian and chef with a particular interest in carbohydrates and diabetes. He is author of Reversing Diabetes (Murdoch Books), and co-author of 40 scientific publications, The Good Carbs Cookbook (Murdoch Books), Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment Publishing).