Understanding Carbohydrate Metabolism

In another article, I discussed the importance of metabolic health, the intricate balance of chemical reactions that keep your body functioning. Carbohydrates play a central role in this, as they are one of the primary sources of energy for metabolic processes. Understanding how carbohydrates are metabolised and their impact on your health—and adjusting your diet accordingly—is crucial to maintaining a well-functioning metabolism and avoiding many of the health issues linked to poor metabolic health

Sugars and Monosaccharides

Carbohydrates come in various forms, with the simplest being monosaccharides, often called simple sugars. Three commonly occurring monosaccharides in the diet are glucose, fructose, and galactose. These six-carbon molecules are the building blocks of all other carbohydrates and serve as the main fuel—particularly glucose—burned by the body to produce energy for metabolic processes.

Disaccharides

Monosaccharides can combine to form more complex carbohydrates. The simplest of these are disaccharides, which consist of two monosaccharide molecules linked together. Examples include:
•    Sucrose (table sugar): A glucose molecule linked to a fructose molecule.
•    Lactose (milk sugar): A glucose molecule linked to a galactose molecule.

Oligosaccharides

Chains of 2–10 monosaccharides form oligosaccharides, which are found in foods like fruits, grains, vegetables, pulses, and even human breast milk. These are an important part of soluble fibre, also known as prebiotics. Most dietary oligosaccharides cannot be broken down by digestive enzymes in the gut. As a result, they pass through the small intestine without being absorbed and travel to the large intestine, where they serve as food for the gut microbiota (gut microbiome).

This promotes the growth of beneficial bacteria, inhibits harmful bacteria, and results in the production of short-chain fatty acids (SCFAs) such as butyrate, propionate, and acetate. SCFAs have numerous positive effects on metabolic health, including:

• Supporting the gut lining (preventing “leaky gut”).
• Boosting the immune system.
• Improving insulin sensitivity and cardiac health.
• Potentially reducing brain inflammation and helping to prevent neurodegenerative conditions like Alzheimer’s disease.

SCFAs also stimulate the release of appetite-regulating hormones such glucagon-like peptide-1 (GLP-1), which promote satiety and may help prevent overeating.

This hormone mechanism is targeted by new weight-loss medications like Ozempic, which are increasingly seen by many in the medical establishment as potential solutions to the obesity epidemic. However, these medications are expensive, in short supply, and critically, have unknown long-term safety profiles for obesity treatment.

Polysaccharides

Polysaccharides are longer chains of monosaccharides that include starch and cellulose:

• Starch: The most common carbohydrate consumed, consisting of thousands of glucose molecules. Starch is typically broken down into glucose in the small intestine.
  • Different types of starch vary in digestion rates:
    Rapidly digested starch: Quickly broken down and absorbed.
    Slowly digested starch: Broken down more gradually.
    Resistant starch: Passes into the large intestine, where it nourishes the microbiota or is excreted.
• Cellulose: Unlike starch, cellulose cannot be digested by humans. It reaches the large intestine intact, contributing to insoluble fibre (roughage), which supports gut health and regularity.

The Glycaemic Index and Glycaemic Load

The type and amount of carbohydrates in your diet play a critical role in determining metabolic health. Unfortunately, modern Western diets often include large quantities of:

• Simple monosaccharides (e.g., those in fizzy drinks, sweets, cakes, and fruit juices).
• Rapidly digested starches (e.g., white bread, peeled potatoes, white rice, and maize).

These foods cause rapid spikes in blood glucose levels, giving them a high glycaemic index. They also result in large amounts of glucose being absorbed into the blood, contributing to a high glycaemic load.

When blood glucose rises rapidly, the pancreas secretes insulin—a hormone that facilitates the uptake of glucose into cells, particularly muscle and liver cells, to reduce blood sugar levels. However, there is a delay in the feedback loop between blood glucose levels and insulin secretion. This delay often causes blood glucose to spike excessively, followed by a rebound dip when insulin remains active even after blood glucose levels return to normal or fall below normal. This dip can cause lethargy and hunger, prompting a cycle of high-sugar snacking that over time leads to insulin resistance and the development of Type 2 diabetes.

Fat Storage

Excess dietary carbohydrates must be stored somewhere. Small amounts are temporarily and healthily stored in the liver and muscles as glycogen, a short-term energy reserve. However, when glucose is consumed in excess, particularly from high-glycaemic carbohydrates, it leads to:

Body fat accumulation, especially around the abdomen, thighs, and arms.
Fat deposits in major organs, such as the liver (causing Non-Alcoholic Fatty Liver Disease, or NAFLD) and in arteries (increasing the risk of cardiovascular disease, heart attacks, and strokes).

The Problem with Fructose

Fructose, commonly added to fizzy drinks, confectioneries, breakfast cereals, and processed foods, poses a unique metabolic threat. It accounts for about 10% of the average American diet (and nearly as much in Europe) and is heavily associated with processed foods containing High Fructose Corn Syrup (HFCS).

While small amounts of fructose can be metabolised in the intestine, larger amounts are metabolised exclusively in the liver using a pathway that bypasses the usual regulatory steps of glucose metabolism. This pathway enables the liver to continue converting fructose into fat, even when energy levels are already high. Historically, this adaptation was beneficial, allowing energy to be stored as fat for use during food shortages. However, in modern diets with year-round energy surpluses, this leads to excessive fat synthesis, contributing to:

• Obesity and visceral fat accumulation.
• Insulin resistance and type 2 diabetes.
• Non-Alcoholic Fatty Liver Disease (NAFLD).
• Cardiovascular disease.

The Bigger Picture

High-glycaemic-index and high-glycaemic-load diets are harmful not only due to excessive fat storage but also because they contribute to chronic inflammation, reduced insulin sensitivity, and an increased risk of Type 2 diabetes.

While this article focuses on the basics of carbohydrate metabolism, future articles will explore the broader impacts of these diets, their link to chronic disease and how polyphenols can help improve your metabolism.

27th Jan. 2025 - Dr. Steve Collins