Resistant Starch: Weight Loss Miracle?

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Ok, there’s a simple rule of thumb—if you see the words “weight loss” and “miracle” in the same sentence, you should get your guard up.  A recent book promoting a new weight loss plan bases it’s premise on increasing your intake of resistant starch to fill your belly without absorbing calories and shrinking fat cells in the process.The funny thing is, the basic science behind this plan actually isn’t entirely off.  I can’t say as much for the claims about actual weight loss and shrinking fat cells, but the basic idea is sound.  Starch is comprised of glucose molecules linked together as amylose and amylopectin.  Amylose is linear and therefore packs together tightly; amylopectin is a branched structure and therefore does not pack together as tightly.  The more tightly packed the molecules are, the harder they are to digest.  Therefore, foods containing higher concentrations of amylose tend to travel through the small intestine without being digested.  When they reach the colon, these starch molecules get fermented by beneficial bacteria to create small chain fatty acids such as butyrate, which have been shown to improve cellular health in the colon and decrease colon cancer.  In this way, resistant starch functions more like fiber.

Resistant starch is naturally found in legumes (beans, lentils), brown rice, barley, buckwheat and whole wheat.  It can also be formed in small amounts when some foods are cooked and then cooled.  This process occurs, for instance, in sushi rice and cooked-then-cooled potato salads and pasta salads.  Resistant starch is also found in some foods we wouldn’t normally eat, like green bananas and plantains, and chemically-modified corn starch (high-amylose maize).  Some companies are now marketing banana starches and high-amylose maize starch as powders to add to other foods to raise their resistant starch content.  (It can be found in some ice creams, even!)

Some research also shows that resistant starch can have an effect on glucose and insulin levels, modifying dramatic rises after a meal in the same way that fiber has a modifying effect.  Companies selling these powders use this information to market their products as beneficial for those with metabolic syndrome and type II diabetes.  Some research in rats also shows that when compared to eating easily digested starch, resistant starch diets led to smaller sized fat cells.  In humans, long term diets of resistant starch led to lower triglyceride levels and total cholesterol levels.  I think it’s this information that the new weight loss book uses to claim beneficial effects of increasing your resistant starch content of your diet.

There’s only one catch here:  there is currently no commonly-agreed upon way to measure the amount of resistant starch in a food.  Also, since glucose and insulin levels are also effected by fiber content of food and many other variables, any research looking at resistant starch’s effect on these levels would have to take this into account, but most published studies have not done so.  In one study which carefully accounted for fat and fiber content of the foods being eaten, there was no difference in glucose levels and insulin levels between resistant starch diets and “regular” diets.  However, at moderate levels of resistant starch, there was a measurable difference in oxidation of fats, implying that less fat would be stored following this meal.  Of note, though, is that this only occurred in a diet with 5% resistant starch; when increased to 10%, the benefit was lost.  To reach a diet of 10% resistant starch, it would be necessary to use modified food powders; so basically, this study points out that the products would be a waste of time!

Bottom line:  starches aren’t all bad for you.  And it’s the same old story—eat more beans and legumes, eat more whole grains, and don’t worry so much that sushi uses white rice!

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