Do Potatoes Inhibit Weight Loss?

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Two of the most current popular diet books, Enter the Zone by Barry Sears, and Sugar Busters by H.L. Steward et al, claim that high-carbohydrate diets make you fat. They explain this by saying that a high-carbohydrate diet leads to high insulin levels which in turn prevent your body from burning fat stores. They go on to claim that the foods that have a high glycemic index (GI), such as potatoes, corn, carrots and parsnips, are the most harmful because they cause a lot more insulin to be released. These authors claim that diets higher in protein and fat and lower in carbohydrate are the key to keeping insulin levels low so more body fat can be burned off.
Why are they wrong?
The GI is a laboratory measure of how high your blood sugar rises in response to an equal amount of carbohydrate from various food sources. While it is true that a high insulin level does inhibit fat burning, the GI does not accurately predict how much insulin the body produces. It also tells us little about how filling or satiating a food is.

Carrots only have 195 calories per pound, boiled potatoes about 400 calories per pound, while white bread contains around 1250 calories per pound. You would have to eat a lot more carrots or potatoes to equal the carbohydrate calories in white bread. At the same carbohydrate level, carrots and potatoes take up much more room in your stomach than bread. Greater stomach expansion causes it to empty faster which is one reason that carrots and potatoes have a GI similar to white bread. Since carbohydrate absorption does not begin until food leaves the stomach, foods with a low calorie density (e.g. carrots and potatoes) are more likely to have an exaggerated GI compared to foods with more concentrated calories like bread.

Dietary fat delays stomach emptying, so any carbohydrate consumed with a lot of fat usually results in a lower GI. However, because dietary fat increases insulin output, carbohydrate consumed with fat generally has a much greater insulin response than one would predict from the rise in blood sugar alone. For example, ice cream or cheesecake, with their high fat content have a lower GI than potatoes or rice, but these fatty foods cause a high insulin output similar to rice or potatoes. Dietary protein can also magnify the insulin response when consumed with carbohydrate. In fact, foods with little or no carbohydrate, like steak and eggs, still cause a fairly substantial insulin response even though they have little effect on blood sugar.
What causes excessive insulin release?
Ultimately, the number one factor that influences insulin output is total calories consumed. If you eat a high calorie meal, regardless of its CHO, protein or fat content, your body will experience a large insulin release and this will inhibit the release of body fat stores; in fact it may even add to them. In simple terms, it is excessive calories from protein, fat or carbohydrate that ultimately cause high insulin levels and fat storage.
GI is a poor predictor of satiety.

Researchers studying satiety wanted to see how full or satiated people felt after consuming an equal calorie level from a variety of food sources. They asked people to rate how satiated they felt after a meal and also observed how much food they ate 2 hours later. A high-satiety food left subjects more satisfied after eating a set amount of calories and also caused those people to eat less 2 hours later. It turns out that the highest satiety food tested was the potato which is also one of the highest GI foods. And as we’ve seen, it is how many calories that people eat before they are full that most influences insulin output and determines whether body fat stores expand or become depleted. A diet made up of higher satiety foods, like potatoes, will likely lead to less hunger, a lower calorie intake and weight loss. Low satiety foods, like white bread, croissants and candy bars can often lead to excess caloric intake and weight gain.

The key to weight control in the long run appears to be the consumption of low calorie dense foods such as baked potatoes, brown rice, barley, fresh fruits, carrots, corn and other vegetables. This is because these foods provide more satiety per calorie than foods with concentrated calories so people feel satisfied with fewer calories. Not surprisingly, the best advise for optimum health and weight control is to increase your consumption of vegetables, fruits, whole grains and beans with modest amounts of nonfat dairy products and lean meat, fish and poultry. Glycemic index does have some clinical value but this is primarily for improving blood sugar control in people with diabetes.

Next month we will examine the research that critics of high-carbohydrate diets often cite when claiming that high-carbohydrate diets cause metabolic problems. We will examine how research design has lead some to the incorrect conclusion that all high-carbohydrate foods and diets contribute to a complex of metabolic problems, termed “Syndrome X.”

Dr. Jay Kenney, PhD, RD, FACN

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