How long after eating should I wait to exercise, or does it not matter? I am mainly talking about cardio, not weightlifting. I am trying to lose weight, not gain it, and in large amounts.
It takes 30 to 40 minutes for food to digest in your stomach and 3 to 4 hours for it to absorb in your intestines. I agree that there is no strict formula for this, since it also depends on what you eat and how you intend to expend that energy.
From my experience if I eat a large meal which is heavy in carbohydrates I see my best performance with a high intensity exercise 6 to 8 hours later. I have also consumed protein shakes within 15 minutes of a weight training circuit and have seen strengthened results. I recommend you experiment and find your own ideal wait length.
Unfortunately, there is no strict formula for this and the answer strongly depends on the intensity of exercise.
If you are uncomfortable when exercising, it has a direct effect on my performance. If I eat a full meal, I have to wait 2 to 3 hours before high intensity exercise.
That said, before a low intensity exercise like a long run (>10 miles) I always try to get some calories in me. Banana's, bagel, powerbar, and tons of fluids. More than 15 miles, I try to add calories during.
The simple fact of the matter is that after eating your body diverts blood flow to your stomach and intestines to digest and soak up the goodies in your food. If you try to exercise too soon then your body won't be able to cope with the increased requirement of blood to the muscles as well. Not to mention that the increased internal pressure on the abdomen may cause you to chunder (puke).
How long you should wait is really a subjective question just as with weight loss - where the individual's biovariables determine the rate at which exercise will have an effect. But about an hour is a typical duration to wait, preferably 2.
In your question you state, that you want to lose weight, but you aren't doing any weight training.
If you really want to lose huge amounts of weight (fat I presume?) you should absolutely include weight training into your workouts. (First weight training, then cardio training) A combined training is much more effective in burning fat.
If you are worried about gaining muscle, you can do a few things to prevent it. (Although I can't image why one should want this, since additional muscle tissue means additional calorie consumption and thus faster weight loss)
- Have a negative calorie balance. (Should not be a problem in your case)
- Avoid eating protein.
- Avoid eating after your workouts. (Looks like you are already planning to do this)
- Increase the repetitions and decrease the weights.
Not a direct answer to your question, but if you're trying to lose weight, cardio will work for the first 6-8 weeks, and after that, it may actually have the reverse effect. I've seen it time and time again with my clients (I'm a personal trainer) who've been doing cardio for a long time, but inevitably weight collects around the belly.
So what do you do? You really should do weightlifting to lose weight. You don't have to worry about gaining weight as a result of weightlifting if you're in a caloric deficit. To gain weight, you need a caloric surplus, and since your calories are going to be low enough to lose weight, weightlifting will only have the effect of toning and speeding up the metabolism.
Exercises like squats (which can be seen here: http://www.youtube.com/watch?v=Kp4zZmVc2nQ), lat pulldowns, pushups, overhead presses, and other will help you with your goal.