Monday, September 24, 2007

True or False: A Carb is a Carb is a Carb?

True or False: A Calorie Is A Calorie Is A Calorie?

True. A calorie is a unit of measure for energy. Specifically, a calorie is the amount of energy needed to raise one gram of water 1 degree Celsius (at atmospheric pressure). What we call a calorie is technically a kilocalorie, the amount of energy needed to raise 1 kilogram of water 1 degree Celsius.

However, "a calorie is a calorie is a calorie" loses its absoluteness when you consider the vehicle used to carry that calorie into the body, and the state of the body accepting the calorie.
"It's not just the calories [contributing to the rise in obesity]. That's the point. It's the effect of the calories on the hormones, and the effect of the hormones on how your body decides to use the calories you're eating -- is it going to burn them as fuel or store them as fat -- and that effect on hunger."
- PBS Frontline, Interview with Gary Taubes, 2004
So ...
True or False: A Carb is a Carb is a Carb?

True. If you saw a carbohydrate strolling down a fashion runway you would be hard-pressed to call it fat. Technically, a carbohydrate is an organic compound, an aldehyde or ketone with lots of -OH groups attached. It is very distinct structurally from a fat or protein.

However, carbohydrates come in many forms, which cause them to elicit different effects upon entering the body.

Sherri passed on this study which appeared in this month's journal Obesity. It describes a case where different forms of carbohydrate (but the same amount of carbohydrate, and the same amount of calories) fed to mice resulted in different effects.

Hepatic Steatosis and Increased Adiposity in Mice Consuming Rapidly vs. Slowly Absorbed Carbohydrate
Here's a summary:
Quick-Burning Carbs May Cause Fatty Liver

Study Basics
  • Male mice were fed either high glycemic index (GI) carbohydrates or low GI carbohydrates.
  • High GI carb was provided by high amylopectin cornstarch. Low GI carb was provided by high amylose cornstarch.
  • Diets were isocaloric and controlled for macronutrient and micronutrient content. (Diets had the same calories, and the same amounts of carbohydrate, fat, and protein.)
  • Study lasted 25 weeks
  • No difference between the groups in body weight.
  • Mice fed high GI diet had higher total body fat.
  • Mice fed high GI diet had fattier livers (containing almost twice the amount of triglycerides).
  • Mice fed high GI diet had higher blood levels of insulin (a fat-storing hormone) and triglycerides.
The two carbohydrates in the study differed structurally. This caused a difference in their rate of absorption, and consequently, their impact on postprandial (after-eating) blood glucose, and insulin response.

The high Glycemic Index cornstarch's carbohydrate looked like this:

The low Glycemic Index cornstarch's carbohydrate looked like this:

How the above structures of starch translate into different effects in the body:
  • Amylose (bottom figure) is one long, continuous chain of glucose units. Enzymes nip off the end glucose, so you can only free one glucose unit at a time = slower digestion.
  • Amylopectin has lots of end glucose units, lots of places for enzymes to nip off terminal glucoses simultaneously = faster digestion.
  • Thus, our bodies experience a faster and higher rise in blood glucose after eating starches with more amylopectin. (The Glycemic Index ranking of a food takes this into account.)
  • Glycogen, the storage form of starch in our bodies is even more branched than amylopectin, plant's storage form of starch. When we mobilize our glycogen, boy can we dump sugar into our blood!
The implication of this study is that eating carbohydrates with a high GI may deposit more fat (triglycerides) in our liver, bloodstream, and possibly other organs than eating the same amount of carbohydrates with a lower GI.

Foods with a high GI tend to be highly refined, including any product made from flour, e.g. bread, breakfast cereals, or pasta. White and 100% whole wheat breads have similar GIs because the action of grinding the grain frees the starch for digestion.

See my post on Glycemic Index/Glycemic Load for how to eat a lower GI diet.

If you've had an abnormal liver test, had higher-than-normal levels of liver enzymes, the message this study brings suggests an alternative, drug-free remedy.
Photos from Mario's Biology Place. Mário Moniz de Sá is an instructor at Langara College, Canada.

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