You can't go wrong with the Mayo Clinic which provides a free online calculator to determine activity level, age, weight and other vital statistics. The calculator reduced my calories by about 450 per day (1650 to 1200) and offered a four page plan with mix and match categories and portion sizes based on the food pyramid.
With all the diet assistance you can find on the web, it amazes me how many people opt for the sketchy diet plans. But recently I was discussing weight loss with colleagues and they wanted a diet that allowed them to eat anything and didn't require exercise. They also wanted immediate results which led them to one of the more popular fad diets today, one that involves either injecting or ingesting a hormone and reducing calories to 900 a day. These otherwise reasonable people would rather pay hundreds of dollars to fly-by-night companies for a quick fix than take the Mayo Clinic's free advice to help them lose weight the natural, healthy way.
It makes sense to me that people of different activity levels respond differently to dieting. Carbohydrates are great for people with active lifestyles. But I can't see any sense in a sedentary person restricting their diet to meat and cheese without an exercise plan to clear their arteries of bacon fat and cholesterol. There's no organic Drano that'll blast through all that gunk - despite what you've read on the internet. It is simply a matter of balance: if you take in more calories than you burn, you will gain weight. Unfortunately it's a lot easier to gain weight than lose it.
So here are some tips to tip the scale in your favor: Park the car 20 yards farther from your office building. Take the stairs. Play outside with your kids or grandkids. Go for a walk. Join Curves. Substitute whole grains for white bread. Eat more veggies. Eat more fruits. Reduce calories. Reduce portions. Reduce fat. Reduce sugar. Prepare your own lunch. Drink a glass of water when you feel hungry. Eat small portions frequently. Try to get 60% of your daily calories before 2:00 pm.
And one bad food day does not give you license to go back to your old habits. Keep a food diary. Familiarize yourself with portion sizes and find out where your weaknesses are. And if you're going to eat a whole box of Thin Mints in two days (1600 calories), adjust your diet accordingly!