What Are Starch and Fructose, and How Does Excess Sugar Become Fat?

What is starch?

Starch is a polymer compound of pure glucose and our main dietary carbohydrate. Carbohydrates primarily provide energy, but also form complex compounds such as glycoproteins and glycolipids and have functional roles in receptors and their structure.

How does sugar become fat?

Consuming more energy than daily expenditure has undesirable consequences. Biochemically, dietary starch is broken down to glucose by salivary and pancreatic amylase, absorbed from the upper small intestine (jejunum), and carried in blood first to the liver. Of absorbed glucose, 60% is metabolized there. After meals the liver uses glucose for energy through a pathway called glycolysis . It is regulated at three steps, the most important being the phosphofructokinase 1 step.

The liver does not use glucose only for energy. It uses 50% for energy, 10% for glycogen synthesis and the remaining 40% for fatty acid synthesis. Glycogen is stored carbohydrate maintaining blood glucose during fasting; produced fat is energy stored particularly in adipose tissue for fasting.

Where does bad cholesterol LDL come from?

Carbohydrate-derived fatty acids are packaged as VLDL (very-low-density lipoprotein) and released into blood to deliver endogenous, body-synthesized fatty acids to cells. During metabolism VLDL becomes IDL (intermediate-density lipoprotein), then LDL. This is the point I wish to emphasize, because excess LDL can oxidize in circulation and cause atherosclerosis, cardiovascular diseases . Ultimately, LDL is produced through metabolism of dietary carbohydrates.

Endoplasmic reticulum stress and diabetes

When blood glucose exceeds 86 mg/dL, pancreatic beta cells secrete insulin, an anabolic (building, growth-promoting) hormone central to metabolism. It directs glucose particularly into muscle and fat for immediate use. Excess sugar from high carbohydrate/starch intake increases insulin demand. Insulin is produced on ribosomes in the endoplasmic reticulum, which is stimulated to make more. Improperly folded, incompletely functional insulin then begins to be secreted, termed endoplasmic reticulum stress. This is a mechanism responsible particularly for later-onset type 2 diabetes.

What is fructose? How does it become fat faster?

Fructose is another carbohydrate unit. Its metabolism joins glucose's pathway below glycolysis's key control enzyme phosphofructokinase 1. Consequently, fructose metabolism proceeds much faster than glucose metabolism. Excess fructose intake therefore increases fatty acid synthesis, stored as triglycerides . In people eating fructose-rich diets, amounts of the following substance, known to harm the heart and kidneys, also increase: uric acid . Its levels increase, and one consequence of excessive uric acid is GOUT.

Carbohydrate-rich eating produces excess electron-carrying compounds (NADH and FADH2) through fuel oxidation. These are used in the inner mitochondrial membrane to obtain the body's main energy unit, ATP . If overproduced, they exceed electron transport chain capacity. Electrons in NADH and FADH2 then convert molecular oxygen into harmful superoxide radicals, triggering oxidative stress. These agents disrupt cell membranes and participate in aging, cataracts, atherosclerosis, diabetes and cancer.

We have partly described mechanisms and consequences of excess carbohydrate consumption. What should we do?

If energy-rich eating is not matched by expenditure, these consequences follow. Substances not synthesized in the body but required from outside are called essential compounds . Examples include essential amino acidsand essential fatty acids. However, there is no essential carbohydrate. This is because a metabolic pathway called gluconeogenesis synthesizes glucose from noncarbohydrate compounds. Glucose can become fructose, mannose and galactose. The body therefore experiences no deficiency with very low carbohydrate intake, unlike protein and fat.

For healthier living, moderation and consuming only as much as we expend are beneficial.

The figure shows carbohydrate building blocks. The smallest are monosaccharides (glucose, fructose and galactose), which combine into disaccharides (maltose, lactose and sucrose) and polysaccharides (starch, glycogen, cellulose, chitin).

- Note: Almost everyone now thinks themselves a sugar and fat expert, so we had to use more detail and medical terminology than intended.