Safety Issues With the Slim-Fast Diet

The Slim-Fast meal plan combines meal-replacement shakes and bars with fresh foods you prepare yourself. Although there are some benefits to this plan, including teaching you proper portion control and an emphasis on consuming supplemental fruits and vegetables, it also has its drawbacks, including lack of variety that can lead to poor diet adherence. "U.S. News & World Report" notes that there are no serious safety concerns with the diet; however, active and pregnant women's nutrition needs may not be met on the Slim-Fast plan. Consult your doctor before beginning a new diet.

Calories Count

The Slim-Fast plan prescribes a 1,200-calorie daily goal for women and a 1,600-calorie daily goal for men. Although 1,600 calories is enough for most men to lose weight safely, 1,200 calories may not be enough for some women, including those who weigh over 165 pounds and those who are very active. According to Dr. Kristie Leong, the absolute minimum number of calories you should consume while dieting is 1,200. When you eat too few calories, you risk a slowed metabolism and health problems from not getting the nutrients your body needs. The National Heart, Lung, and Blood Institute says that women who weigh more than 165 pounds and active women may need up to 1,600 calories a day for safe weight loss.

Nutrient Deficiencies

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If you belong to the group of people whose calorie needs aren't met by the Slim-Fast plan, you may experience nutrient deficiencies that could result in health problems 1. Leong cites iron and protein deficiency as among the biggest risks. Low iron intake can lead to anemia, especially in women who need more iron than men to support menstruation. Lack of protein negatively affects the immune system, resulting in increased risk of colds and flu. "U.S. News & World Report" says that all dieters may not get enough potassium -- an electrolyte mineral important for regulating blood pressure -- on the Slim-Fast plan.

Rapid Weight Loss

A safe rate of weight loss is 1 to 2 pounds per week, according to Nutrition.gov. To meet but not exceed this goal, your diet plan should contain 500 to 1,000 calories less per day than your regular diet. Eating fewer than this number of calories, especially if combined with exercise, can lead to weight loss that is too rapid. The dangers of rapid weight loss include gallstones, thyroid problems, sexual dysfunction, osteoporosis and changes in blood pressure.

Slim-Fast During Pregnancy

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Pregnancy is not a safe time to attempt to lose weight, says OB-GYN James Smith 3. It's important that the fetus get all the calories and nutrients it needs for proper development. Smith says that although the Slim-Fast shakes and bars can be used as occasional snacks, a pregnant women needs a more balanced diet than the Slim-Fast plan provides. He reports that there may also be dangers of getting too much of some nutrients, such as vitamin A, from the meal-replacement bars and shakes in addition to the prenatal vitamins most women take.