Dieting can be deadly, say researchers, and doctors should think twice before subjecting their patients to strict diets that could have long-term consequences such as raising the risk of heart attacks, cancer, and diabetes.

The study, published in the journal Psychosomatic Medicine, found that dieters have higher levels of the harmful stress hormone cortisol, which has been linked to an increased risk of many deadly diseases. In addition, cortisol can actually make dieters gain weight and may explain why so many diets fail.

The researchers also discovered that instead of dieters feeling better about themselves, many had higher levels of psychological stress.

"Regardless of their success or failure (in losing weight), if future studies show that dieting increases stress and cortisol, doctors may need to rethink recommending it to their patients to improve health," the researchers were quoted in the Daily Mail.

"Chronic stress, in addition to promoting weight gain, has been linked with coronary heart disease, high blood pressure, diabetes, and cancer," they said. "Dieting might potentially add to this stress burden and its consequences would best not be ignored."

The study, which was conducted by California University in San Francisco and Minnesota University, examined 121 women who undertook a standard three-week diet. The subjects consumed 1,200 calories a day, about half of the recommended 2,000.

The saliva of each woman was tested both before and after dieting and studied for cortisol levels. The samples taken after dieting showed a significant increase.

The researchers speculated that one reason for the rise in cortisol could be that it is used to increase energy levels in the body, and since the body's calories were restricted, the body felt stressed and automatically released cortisol.

Such a severe restriction of calories is too severe for the body to to deal with, Catherine Collins, chief dietitian at St. George's Hospital in London, told the Daily Mail. She suggests a better goal would be limiting calories to between 1,500 and 1,800 a day combined with an exercise program.

"Very low calorie diets do cause problems, and it's not that unexpected that cortisol levels went up," she said. "We need cortisol for 'fight or flight' situations.

"Chronic exposure over a long time can affect our cholesterol levels, increase blood pressure, and even raise the risk of depression."

Instead of following restrictive diets, you can cut hundreds of calories and barely miss them with these tweaks from About.com.

• At breakfast, have lean turkey sausage instead of pork.
• Substitute skim for whole in your latte.
• Replace pan pizza with thin crust.
• Use mustard instead of mayonnaise.
• Replace fries with a salad.
• Choose sherbet instead of ice cream.
• Drink diet soda instead of regular.