Posted: 05 Jan 2010 05:41 AM PST
Humans have limited capacity to store carbohydrates. Beyond the glucose and glycogen in our blood and tissues, we have relatively little carbohydrate to draw from in time of energy need. That's why long-distance runners and triathletes have to carry sugar sources to keep blood sugar from plummeting.
Fat, of course, is different. We have virtually unlimited capacity to store energy as fat.
Because we have limited carbohydrate storage capacity, what can the body do with the excessive quantities of carbohydrates that Americans ingest? What becomes of a bagel for breakfast, wheat crackers for snacks, a whole wheat sandwich for lunch, pretzels, and whole wheat pasta that many people eat every day, not to mention the chips, soft drinks, and juices?
Excess carbohydrates are diverted to an interesting metabolic pathway called de novo lipogenesis (DNL). This refers to the liver's ability to make triglycerides from excessive carbohydrates in the diet. Triglycerides are packaged for release into the blood as VLDL. VLDL, in turn, interacts with other lipoproteins, creating small LDL particles, reduced HDL and smaller, less protective HDL. High VLDL will be measured on a standard cholesterol panel as higher triglycerides.
A University of California (Berkeley, San Francisco) group has done much of the work describing DNL.
A diet weighed towards carbohydrates, especially if 50% or greater calories are carbohydrate, is sufficient to provoke plenty of DNL, even in slender people. DNL is a big part of the reason why low-fat (and, thereby, high-carbohydrate) diets result in higher triglycerides. DNL really gets turned on many-fold if the triglycerides are "simple," rather than "complex."
Overweight people, however, can demonstrate five-fold greater DNL even with lesser quantities of carbohydrate intake (e.g., 40% fat, 46% carbohydrate, 14% protein):
From Schwarz et al 2003. Mean (± SEM) fractional de novo lipogenesis in lean normoinsulinemic (NI), obese NI, and obese hyperinsulinemic (HI) subjects after 5 d of consuming a high-fat, low-carbohydrate diet and in different lean NI and obese HI subjects after 5 d of consuming a low-fat, high-carbohydrate diet. Values with different superscript letters are significantly different.
Excessive carbohydrates, a la standard low-fat diets, are good for nobody. The concept of de novo lipogenesis fills in a theoretical hole that now explains why people who eat carbohydrates have higher triglycerides, VLDL, and, eventually, insulin resistance and diabetes.