Can we live without sugar?
The harmful effects of sugary foods are well known: overweight, diabetes, cavities, hypertension, accumulation of fat in the liver... To the point where many want to wean themselves off them. But can we live without sugar? The Rumor Detector takes stock.
If eating is generally a pleasure, it is also a condition of our survival. The body draws the energy it needs to function from the food we eat. And even while we sleep, it draws this energy from reserves constituted by certain organs.
This energy comes from three main groups of macronutrients: proteins, carbohydrates and fats.
The role of sugars
The body digests food by mixing it in the stomach with gastric juice, an acidic liquid that contains enzymes. During digestion, carbohydrates - sugars found in starchy foods such as bread, pasta, rice or cereals, as well as fruits, vegetables and sweets - are broken down into another type of sugar, called glucose. This sugar, once in the intestine, is released into the bloodstream to feed the cells.
Once in the bloodstream, glucose can be used for energy or stored in the liver and muscles as glycogen. The amount varies from person to person, but an average-sized man weighing 70 kg stores about 100 grams of glycogen in his liver.
When you stop eating carbohydrates for several hours, the liver glycogen is broken down into glucose and released into the bloodstream to keep blood sugar levels from getting too low. Glycogen stored in the muscles, although in larger quantities, remains in the muscles to meet their energy needs and cannot be released into the bloodstream to feed the cells.
Glucose, the main source of energy
Glucose is the main fuel for the brain, which needs a constant supply of energy to function and carry out vital functions, such as breathing or sending signals to the nervous system. It consumes 20% of our daily energy. When brain cells, or neurons, have trouble getting or using this fuel, they function less well and can die. And when neurons die en masse, there is a loss of brain capacity.
So sugar seems necessary. Except that glucose is not the only fuel the brain can use. In people whose diets don't include enough carbohydrates to provide the required 110 to 145 grams of daily glucose, or when the body lacks carbohydrate storage, the brain can use ketones to meet much of its energy needs.
Ketones, an alternative to glucose
After 24 to 48 hours without carbohydrates, the body's glycogen level is depleted. The liver then intensifies its production of water-soluble compounds called ketones, created by the breakdown of fatty acids. Ketones are made from ingested fat or body fat stores. They can then cross the blood-brain barrier to provide energy to the brain.
Researchers have shown that for some people on a strict ketogenic (keto) diet, ie low in carbohydrates (sugary foods, bread, pasta, rice, juice, etc.) and high in fat (red meat, fatty fish, cheese, butter, avocado, nuts and seeds, etc.), ketones can provide up to 50% of basic energy needs and up to 70% of the brain's energy needs. The body must be carbohydrate-deprived for at least three to four weeks before reaching this ketone state.
And for the rest of the brain's energy needs, the liver can produce all the glucose it needs, through a process called gluconeogenesis, says a 2019 literature review. The liver uses compounds from protein consumption and the breakdown of body or dietary fat, to recreate glucose.
In other words, the brain can meet all of its energy demands through liver-stored glucose, gluconeogenesis, and ketone production, regardless of whether carbohydrates are ingested.
The 2005 US Food and Nutrition Board manual (Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids) states that "the lower limit of dietary carbohydrate compatible with life is apparently zero, provided adequate amounts of protein and fat are consumed.
However, people who eat a lot of carbohydrates and decide to cut out sugar must go through a period of adaptation. Because the brain is not used to using ketones, glucose will remain its main source of fuel for some time, drawing on the body's reserves. Once the body adapts to a very low-carb or no-carb diet, the brain uses ketones to meet much of its energy needs, and the liver should produce as much glucose as needed to cover the rest, according to a study published in 2017 .
The brain needs glucose. However, that glucose doesn't have to come from carbohydrates alone; it can be produced by the liver. Eating sugar to fuel the brain and provide energy to the muscles is an option, not a requirement. Although eliminating all traces of sugar from our diet seems difficult since it is found everywhere, even in some vegetables!
Author: Agence Science Presse - Catherine Crépeau