Welcome back to "Rock Your Body," The State News' fitness and nutrition column. Each week, we hit up our local experts with some questions and pass on their wisdom to you.

But before we get to the goods, we thought we'd tell you a little bit about who'll be giving you advice.

Tom Ostrander: Tom is the owner of Powerhouse Gym, 435 E. Grand River Ave. He is one of Michigan's top amateur bodybuilding competitors. Tom is a personal trainer with experience ranging from professional athletes to stay-at-home moms.

Jim Pivarnik: Jim is a professor of kinesiology and epidemiology. Trained as an exercise physiologist, his specialty is in physical activity and exercise on chronic disease risk. He is also director of MSU's Center for Physical Activity and Health.

This week, we asked:

SN: How does sugar affect your body? Are substitutes good for you?

Tom: When sugar enters the body it causes a jump, or spike, in blood sugar levels. The body responds by releasing insulin from the pancreas to stabilize the blood sugar. While sugar will bring you up and give you that "wired" feeling, insulin will do the opposite; it will make you feel tired and lethargic. That is why you must constantly ingest sugar to keep that "high" going or it will be short lived. Excessive insulin released in the body is stored as body fat. All carbohydrates are converted into some form of sugar, thus low-carbohydrate diets are a fad right now (no carbs, no insulin released).

Sugar, however, does have its place. Right after intense exercise, such as weight training, sugar is highly effective at restoring lost glycogen. At that time I recommend a drink containing glucose polymers such as Gatorade. Gatorade might be one of the best products ever created for restoring glycogen.

As far as sugar substitutes, I would not say they are good for you, but I have failed to see anything bad about them in my own experiences. I prefer Splenda because it is actually made from sugar.

Jim: There are many forms of sugar, both simple and complex, but all are classified in the nutrient category of carbohydrates. The storage form of carbohydrate in the body is glycogen, with the muscles and liver being the main storage depots.

An average person can store up to 2-2.5 pounds of glycogen at a time. Unfortunately, there is little we can do to increase this amount, so an excess intake of sugar and other carbohydrate (excess of what is used regularly) will eventually be converted to fat.

Not all sugars and other carbohydrates (such as starch) are metabolized equally. Carbohydrates that cause high increases in blood sugar when ingested are of greatest concern. Foods that contain these carbs are high on the glycemic index. The glycemic index ranks carbs based on their immediate affect on blood sugar.

Examples of foods high on the index is table sugar. Sugar from fruit is low on the index.

There are many forms of sugar substitutes on the market today, and most are approved by U.S. governmental agencies. These allow individuals with a sweet tooth to enjoy the taste of their favorite food with significantly fewer calories.

SN: How important is stretching? What are some good stretches?

Jim: Although it seems intuitive that stretching a muscle before using it will also help prevent injuries, surprisingly, not all research supports this notion. Most athletes, however, will tell you they perform better after stretching.

It is recommended that one perform static, rather than ballistic (bouncy) stretches. Static stretching is performed by lengthening the muscle and holding light tension on it for approximately 20-30 seconds.

Most overall body stiffness occurs in major joints, so they should be targeted in any stretching program. Stretches for the groin, back, legs and shoulders are recommended for any flexibility enhancing program.

Tom: Stretching before and after exercise is a good idea that is often overlooked. Stretching before exercise warms up the muscles and prepares them to be contracted. When stretching is done after exercise it helps begin the cool down and recuperation process.

There are no special or magic stretches that will increase performance.

Stretching a muscle will also help elongate the muscle and keep it from becoming brittle. It is good to stretch for 10-15 minutes before and after exercise. With all stretches, make sure to ease into and out of the stretch without bouncing.