I have recently started dieting (about 3 weeks now) by drinking slim fast shakes for breakfast and lunch and eating less for dinner. I snack on pretzels or honey nut cheerios during the day. I have also started riding a stationary bike for 20 min/day (about 9.5-10 miles). I'm not losing as much as I think I should for such a drastic change in my habits... why?

Not enough exercise for calorie reduction, mdjohn. I would suggest a more vigorous workout. Start walking my dear, cut out the honey nut cheerios and pretzels (you can snak on regular cheerios though). Drink plenty of water. SlimFast is good but still contains carbs that are not good for a natural weight loss. The only good thing is you mix it with 2% milk !

Walking is a complete cardio exercise. Weigh yourself only in the morning before eating, not after working out.

Tick

I'm pretty sure you are NOT cycling 10 miles in 20 minutes, at least not without soaking the whole room in sweat. 20 minutes is not enough exercise for weight loss, and your snacks are not the best.

Honey nut cheerios have more sugar than most of the stuff on the candy counter.

Try eating good meals, and no snacks between them. If you must snack, make it an apple or a small tub of cottage cheese.

Most people think that by soaking yourself in sweet will help you loss pounds. WRONG. That is a cardio workout. To do a fatburning work out power walk, long slow excersise trick the body in to lossing wieght. We are mamules and because of this if your heart rate goes to high. Its like runnig from danger. So your body instead keeps the fat cals and instead burns the ones in your stomach. These cals go ony to your heart and lungs. Your body doesn't know when it will eat again,so it stores them. Trying to a slow walk on a tread mill. For me my setings are 7 incline and 3.2 miles an hour. I do it for an hour a day. This of course is the ideal setings for a 20 -30 years of age man. Start slow . And talk to a trainer about a program. Also only eat good carbs not bad. Weat is good, whole grain is good for your heart but not if your trying to loss wieght.