Biochemistry of Sugars and Carbohydrates

Chapter summary

A sugar is a type of carbohydrate. “Carbohydrate” and “saccharide” mean the same thing.
The focus for obesity should be carbohydrates. The focus for fructose should be health.

Four groups: monosaccharides, disaccharides, oligosaccharides and polysaccharides.
Monosaccharides: glucose, fructose, galactose.
Disaccharides: sucrose (glucose + fructose), lactose (glucose + galactose) and maltose (glucose + glucose).
Complex: starch. Starch and cellulose have a similar structure, but humans can only digest starch.
Glucose drives cell energy (ATP). Fat and protein are also converted to a glucose-like compound. Too little is fatal and too much is toxic.

It is about 60% sweeter than glucose or sucrose.
In fruit it signals nutrient-rich food. In soft drinks it rewards sweet taste with calories and little else, which raises the “bliss point” that makes food hard to stop eating.
Soft drink intake, with high-fructose corn syrup (HFCS) or sucrose, has risen with obesity. This is correlation, not proof of cause.
Excess fructose may induce fatty liver, insulin resistance, abnormal blood fats and high blood pressure.
HFCS in soft drinks is about 55% fructose and 45% glucose. Sucrose is 50/50. HFCS has free glucose and fructose, whereas sucrose must be split. In the US, HFCS has displaced sucrose over 40 years while glucose has stayed steady.
Handling: Fructose is absorbed through the Glut-5 transporter. Much goes to the liver via the portal vein, and 20 to 30% escapes into the general circulation. Kidney cells can also metabolise it.

All carbohydrates are broken down to simple sugars before they are absorbed. Pancreatic amylase starts on starch.
The simple versus complex split has little value in judging nutritional quality. Some simple carbohydrates (fructose) raise blood glucose slowly, and some processed starches raise it quickly. Speed of digestion is what matters.

Low-GI view: A whole genre of low-GI and “eat mostly plants” books focuses on the glycaemic index. Supporters say choosing slow carbohydrates helps health.
The author’s view: Ultimately all carbohydrates become glucose, so reduce carbohydrates for obesity, and remove added fructose for health.
Fructose critics: Some, such as Bray (2007), question how bad fructose is compared with other sugars.
Fructose in fruit: Small amounts in whole fruit and vegetables are not the concern.

Bray, G. (2007) How bad is fructose? Am J Clin Nutr 86(4):895-896.
Kretowicz, M. et al. (2011) The impact of fructose on renal function and blood pressure. Int J Nephrology.
Cordain, L. et al. (2005) Origins and evolution of the Western diet: health implications for the 21st century. Am J Clin Nutr 81:341-54.
Fructose and Carbohydrate. Wikipedia.