By Marie Spano, ms,rd,cscs,cssd
In the quest for a ripped physique, many people steer clear of carbohydrates. Athletes and fitness models have jumped aboard the high-protein, low-carbohydrate bandwagon to shed weight. Should you, too, cut the carbs and watch the pounds drop off? Low-carb diets work, and carbohydrate isn’t an essential nutrient. But a more realistic approach may be to switch from no carbs to slow carbs.
WHY YOU NEED CARBS
A plethora of diet books focus on carbohydrates for two main reasons. First, carbohydrates raise blood sugar, which in turn raises insulin, a storage hormone that can help your body lower your blood sugar by shuttling it through a process that stores carbohydrate in the body in the form of glycogen in muscle and liver and fat in fat tissue.
Secondly, dieting is easier when all you have to do is say yes or no to eating a particular food. No need to assess hunger, count calories, monitor portion sizes or read food labels. Simply cut out the foods you are likely to overeat and you’ll cut calories and lose weight. And cutting out carbs makes sense. After all, you are more likely to go back for seconds when your meal includes a creamy Italian risotto as opposed to grilled chicken and a side of seasoned vegetables.
THE INFO ON INSULIN
The biochemistry behind carbohydrate metabolism involves the hormone insulin, which is released from cells within your pancreas. And, the type and amount of carbohydrate consumed determines the amount of insulin released. So, let’s say you sit down at your desk in the middle of the day to a high-carbohydrate meal and then peck away on your computer keyboard (a task that requires very little energy in the form of calories burned). Your blood glucose, the most important stimulus for insulin secretion, will rise. Because your body doesn’t need the immediate influx of calories for physical activity (or wound healing), the insulin produced from your pancreas, in response to the increase in blood sugar, will quickly help your body utilize the glucose in your blood. If your muscles don’t need the fuel and you’ve already consumed enough calories, your body will take the glucose through a series of steps to manufacture glycogen for storage in muscle and liver tissue, or fat in fat tissue.
But, that is not all insulin does— it also inhibits the activity of hormone-sensitive lipase in adipose tissue. Hormone-sensitive lipase is responsible for taking fat out of your fat tissue for use as energy. It’s like a double-edged sword— insulin decreases fat tissue mobilization and use and increases fat storage.
By now, you may want to cut out every gram of carbohydrate in your diet. However, insulin isn’t only responsible for transporting and metabolizing carbohydrates, creating fat tissue and inhibiting fat tissue mobilization. Insulin also helps amino acids make their way into cells and promotes protein synthesis. And, if you think carbs are your enemy, consider this: study after study shows that the amount of carbohydrate stored in your muscle as glycogen has a direct relationship to performance. If you cut your dietary carbohydrate intake down too low, your glycogen stores will also drop.
Think of your stored carbohydrate (glycogen) as your fuel tank. If your tank is one-quarter full, you won’t make it very far. But, if you start your exercise session on a full tank (which only lasts about two hours or less if you are exercising intensely), you can exercise more intensely (and therefore use more carbohydrate just like your car does if you slam your foot on the gas pedal) and for a longer period of time. If you walk around with low glycogen stores, your training will likely suffer at some point and you won’t obtain the body-changing or performance results you are looking for from your workouts.
THE SLOW CARB SOLUTION
Because cutting out all carbohydrates may make your dining experiences less than enjoyable and impair your training, it makes more sense to keep carbohydrates in your diet – but change the type of carbohydrate (and possibly the amount as well) you are consuming. Among the different types of carbohydrates, you may want to stick primarily with fiber and low-glycemic carbohydrates.
Though fiber won’t fuel your body for an interval workout or weight training session, since it provides no energy, it does provide bulk to your diet by absorbing 10 to 15 times its weight in water. That bulk will help decrease your risk for constipation, hemorrhoids, diverticulosis and possibly, colon cancer. Plus, the binding ability of fiber may decrease your body’s absorption of certain toxic compounds. Foods containing complex carbohydrates include grains, potatoes, beans and a variety of vegetables— all foods that naturally contain a variety of vitamins and minerals as well.
Now, why should you choose foods that are low on the glycemic index? The glycemic index is a measure of how fast a food raises blood glucose levels in comparison to a test food (white bread or pure glucose). Foods that are low on the glycemic index are digested and absorbed slowly and don’t increase blood sugar and insulin to the same extent that foods high on the glycemic index do. Keep your insulin down and you’ll also blunt the process of fat synthesis.
Studies examining how the consumption of foods low on the glycemic index affect body composition, specifically fat oxidation, are mixed. However, animal studies consistently show that low glycemic index diets can help reduce body fat. And though human studies have not consistently shown positive results, there is a plausible explanation for this. Though scientists can strictly control and even measure what lab rats consume, the diet of free-living humans, on the other hand, is difficult to control and measure.
Despite the complexity of studying humans, there are some studies that show the glycemic index may be a valuable tool for making dietary choices. A crossover study examining blood glucose concentrations after a low versus high glycemic index breakfast in eight women revealed that blood glucose concentrations were lower at 30, 45, 135 and 150 minutes after the low glycemic index breakfast. In addition, the insulin response was lower, while free fatty acid concentrations tended to be higher after consumption of the low glycemic index breakfast, indicating that fat oxidation was higher in this group. The low glycemic foods were also higher in fiber, which slows the release of sugar into the blood, and therefore may have contributed to the results noted in this study.
In another crossover study, seven adult women aged 25-46 with a BMI of 23.3-30.9 kg/m2 consumed four low glycemic index (59 ± 2) or high glycemic index (92 ± 3) meals per day that provided enough total calories for weight maintenance over a period of 20 days. The participants also performed 20 minutes of physical activity three times per week. Consumption of the low glycemic index meals resulted in lower blood glucose and insulinemia, and significant weight loss over the 20-day period.
Though not all studies indicate that a low glycemic index diet is beneficial for weight loss, most of the well-conducted studies, especially those using animals, suggest that a low glycemic index diet leads to a lower insulin response, which can increase fat oxidation, resulting in a reduction of body fat.
Despite popular belief, carbohydrates are not inherently fattening. However, the type and quantity of carbohydrates (in addition to your overall calorie intake) can help you pack on the pounds. Therefore, your best bet is to choose carbohydrates that are low on the glycemic index, and monitor your portion sizes.
References:
Champe PC, Harvey RA. Biochemistry 2nd Edition, J.B. Lippincott Company. Philadelphia, PA 1994.
Lopes da Silva MV, de Cassia Goncalves Alfenas R. Effect of the glycemic index on lipid oxidation and body composition. Nutr Hosp, 2001; 26(1): 48-55.
