Elizabeth Lyster: Everybody "knows" how to lose weight: diet and exercise, right? Well, what we eat and our activity levels do make a big difference in whether we achieve and maintain a healthy weight, size, and condition.

However, human metabolism changes over our lifetimes. When we're young (teenage and adult in our 20s), relatively minor changes in diet and exercise can produce weight loss. When I was in high school, when my jeans felt too tight, I just stopped eating cookies for a couple of weeks, and that did the trick! As we get older, our metabolism slows down. This slow-down in metabolism is like a hill that gets steeper as it goes down. That is, your metabolism is slower in your 30s than in your 20s, and it gets even slower in your 40s, your 50s, and so on. There are three major mistakes that women make when trying to lose weight. Because of slowing metabolism, these mistakes are especially costly for women over 40.

Here are my Top Three Women's Weight-Loss Mistakes, not necessarily in order of importance:

1. NOT ENOUGH FOOD! How counterintuitive! These days, people are learning quickly about this mistake. All the "trainers-to-the-stars" are writing books promoting the importance of healthy snacking and eating five to six times per day.

Think of your metabolism like a furnace -- if you don't keep fuel in the furnace, the fire will die down and go out.

Here is the heart of why diets don't work -- when you don't eat enough, your metabolism goes into "save" mode. Human bodies are designed this way, to withstand and survive starvation conditions. Feeling hungry may mean you are in this kind of counter-productive metabolic state.

So, it's not your fault -- eating less in this way will not lead to weight loss!

2. TOO MUCH EXERCISE: Exercise is an essential part of maintaining long-term health and weight control. However, there is in fact such a thing as doing too much exercise, or exercising too intensely. Exercise is hormonally depleting, especially for women over 40.

If exercise is experienced as stressful, this can stress the adrenal glands. Adrenal glands make cortisol in response to stress. When there is too much cortisol release, the body goes into the "save" mode I mentioned earlier, and will retain weight, especially around the midsection. This is one of the most tricky subjects for me to discuss with my patients who are over 40, are healthy eaters, exercise intensely, and are frustrated by lack of weight loss or even weight gain despite all their efforts.

You have to exercise smarter, not harder. For example, exercise programs that include weight-lifting will build muscle mass, which in turn raises the overall metabolic rate. This makes a person burn more calories, even at rest! In other words, it's not just about burning calories by exercising harder. If your best efforts at weight loss are not working well, you may be over-exercising.

3. NOT ENOUGH SLEEP! When you understand the work done by the adrenal glands, it becomes clear why not getting enough sleep, or good enough sleep quality, can cause weight retention or weight gain.

We talked in the last section about excess cortisol release caused by stress. Cortisol is released when we are tired and don't rest -- it's what keeps tired women from falling asleep at the wheel while they are ignoring their fatigue and running around doing all the things they have to do every day. Perimenopause and menopause-related symptoms can also interfere with good sleep quality. Depression (which is not a "natural" part of menopause) can also disrupt sleep. Stressful life events, such as the loss of a loved one, also cause cortisol release (both by interfering with sleep and from the stress itself).

Chronic lack of sleep leads to cortisol release, which also leads to craving junk food -- all of which of course leads to weight gain. Because this kind of weight gain is mostly around the abdominal area, we feel "bloated" and uncomfortable.

Poor eating habits related to fatigue and stress make this a vicious cycle that prevents reaching and maintaining a healthy weight.

So, what everybody "knows" about how to lose weight leaves out some crucial information. These top three mistakes that people make in trying to lose weight are especially costly for women over 40. Yes, you have to eat well and keep fit, but you have to keep a balance, and you have to get enough rest!

Here's to your health!

"Doctor Liz" is a gynecologist whose mission is to help women balance hormones, lose weight, and regain health and balance in their lives. She did her undergraduate studies at Cornell University, medical school at UC Irvine, and earned her Master of Public Health degree at UCLA in Health Education. Doctor Liz is in private practice in Laguna Beach, California and is mom to two active boys, ages 8 and 10.