People often confuse a low carbohydrate diet with a low GI diet. The term “low-carbohydrate diet” (Very low carbohydrate ketogenic diet) is sometimes applied to diets that restrict carbohydrates to less than 10% of daily caloric intake (<50 g/day on a 2000 Calorie (8,400kJ) /day diet), but can also refer to diets that simply restrict or limit carbohydrates to less than recommended proportions (< 26% of total energy coming from carbohydrates or 130g/day from a 2000 Calorie (8,400kJ) /day diet). A moderate carbohydrate diet provides 130-230g/day or 26-45% of energy from carbohydrates and a high carbohydrate diet is where carbohydrates provide > 230g/d or >45% of energy from a 2000 Calorie (8400kJ) diet.
In the short term, low-carbohydrate diets may cause you to lose weight because they restrict kilojoules (energy). The body begins to use body stores of glucose (glycogen) from the liver and muscles to replace the carbohydrate it is not getting from foods and drinks. Around 3-4 g of water is stored with each gram of glycogen, so the rapid initial weight loss that occurs on a low-carbohydrate diet is mostly due to water and glycogen loss, not body fat. Systematic reviews and meta-analyses of the the scientific evidencei have found no differences in weight loss between low carbohydrate diets and conventional weight loss diets in the short to medium term (3-6 months), or longer term (1-2 years). There are no studies investigating the health effects of consuming low carb diets for more than 2 years so the long term health effects of a low carbohydrate diet are unknown.
Why should we worry about the amount of carbohydrate we eat?
Australia’s Nutrient Reference Values suggest people have from 45% to 65% of their total energy intake from carbohydrate. This is purposefully a wide range, to account for factors such as a person’s level of physical activity, personal food preferences and cultural background, amongst many others.
Carbohydrates are important as they are the preferred fuel for the brain, central nervous system, red blood cells, kidneys and exercising muscles. They are a vital source of energy found in all plants, and foods that come from plants such as fruit, vegetables, cereals and grains. The carbohydrate lactose is also found in mammalian milks. In fact, human breast milk contains the most carbohydrate of all mammalian milks, highlighting its importance to rapidly developing infants. Good sources of carbohydrates include breads, breakfast cereals, pasta, rice and other grains, legumes (beans, peas, lentils), starchy vegetables (corn, potatoes, yams), fruit, milk and yoghurt.
When digested, the starches and sugars in carbohydrate containing foods are broken down into millions of (primarily) glucose molecules which are released into the bloodstream or processed in the liver. When blood glucose levels rise your body releases a hormone called insulin, which allows glucose to enter cells where it can be used to provide fuel for our brains, muscles and other vital organs.
How much carbohydrate do we eat?
Most Australians don’t eat too much carbohydrate. The latest Australian Health survey (2011-12) shows that Australians are eating a moderate carbohydrate diet (43.5% of energy from carbohydrates). We do however, eat too much of the wrong type of carbohydrate. Australians need to improve the quality of the carbohydrate containing foods that they eat
Low GI “diets” are more about the quality of the carbohydrate rather than the quantity. The GI is a measure of how quickly or slowly carbohydrate foods are broken down and absorbed into the blood stream and converted to glucose. Low GI foods (≤55) are characterised by a slower and lower rise in blood glucose levels following ingestion. High GI foods (≥70) are characterised by a faster and higher rise and fall in blood glucose levels.
This is where the Glycemic Load, or GLii comes into play as it is an important factor for blood glucose management because it takes both the type and amount of carbohydrate into account. It is recommended that you keep to a GL intake of less than 100 units per day.
The GL helps explain why a range of healthy diets with quite different carbohydrate contents can be consumed to successfully manage blood glucose levels, from Mediterranean styleiii to vegetarianiv. Low GI and GL diets are also associated with the highest dietary quality in people with diabetesv.
In the end the choice of how much carbohydrate is up to the individual and their needs but the quality of the carbohydrate (GI) remains important for long term health and wellbeing.
Note: To view references please click on the subscript within the relevant paragraph.