A protein-sparing modified fast (PSMF) is a very low calorie diet with some protein, fluids, and vitamin and mineral supplementation. The diet is to last about eight months. While people often lose weight they frequently regain it afterwards.
PSMFs were first designed in the 1970s. Health concerns include dehydration being a health concern, hence fluids, vitamin and mineral (potassium, magnesium, sodium, and calcium) supplementation is mandatory and doctor's supervision is recommended. PSMF diets consist of foods that are naturally rich in good-quality protein and particularly extremely low in fats (chicken breast, extra lean beef, tuna, egg white, ham, cottage cheese) Several programs derived from the original PSMF have been created such as the Dukan Diet, incorporates resistance training to minimize or prevent LBM breakdown, and minimal to no aerobic exercise (typically restricted to walking a few times a week).
Methodology
Complete fasting produces the largest possible calorie deficit of any diet. Long-term fasting, however, introduces multiple health risks which can lead to death. Even if major health problems are avoided, a complete fast results in the loss of muscle, organs, and other lean body tissue, while most dieters are only attempting to lose excess fat. This muscle-wasting effect is a natural consequence of a diet deficient in both calories and in protein.
A PSMF attempts to spare the dieter the health risks of a complete fast by introducing the minimum amount of protein necessary to prevent muscle-wasting effects, while still eliminating fats and carbohydrates. Typically, depending on activity level, .8-1.2 g per pound of lean body mass (not total body weight) is taken. Protein beyond this minimum amount is also eliminated, as the body would use it for energy. Further lean body mass (muscle, organs, etc.) are spared through resistance training and limiting aerobic activity.
For example, for a 225-pound male with 30% BF, and thus, 157.5 pounds of lean body mass and 67.5 pounds of fat, and weight trains three times a week, he would consume 1.0 g per lb of LBM * 157.5 lb of LBM = 157.5 g of protein which is 630 calories/day. With a fish oil supplemention (40 calories), and some minimal fat which is inherent in almost all foods, the total caloric intake could be around 800 calories/day. Protein powders are generally not recommended on this diet since whole foods lead to better satiety and this is an important factor on a very low calorie diet, though casein powder could be considered due to its ability to slow down digestion. Low calorie vegetables are required on the diet for the bulk, and fiber and carbohydrate-rich vegetables like corn, carrots, potatoes, etc. must be avoided on this diet. Any calorie-free fluids are permitted, including water, coffee, tea, and diet drinks.
The amount of protein consumed varies by individual. It is generally calculated based on lean body weight and adjusted based on activity level, but generally results in a net daily intake of well under 1000 calories. For an average male with a TDEE (total daily energy expenditure) of 2500 calories/day, and a caloric intake of 1000 calories/day, that would mean a caloric deficit of 1500 calories/day. Over 7 days, that would be 10,500 calories lost, and 3 pounds of weight (and up to 3 pounds of fat since 1 pound of fat is 3500 calories of energy) lost.
In addition to lean protein, people on a PSMF must typically consume supplements to avoid the nutritional deficiencies inherent in such an extreme and low-calorie diet. These include essential fatty acid supplements that have EHA and DHA in them (typically obtained from fish oil capsules), vitamins, and minerals typically potassium, magnesium, sodium, and calcium. A general multivitamin supplement, designed to augment a healthy diet, is inadequate - for example, someone on a PSMF needs significant amounts of minerals such as sodium and potassium. Potassium intake, for instance, has been recommended in general for both men and women to be at 4,700 mg a day Lyle McDonald, in particular, recommends supplementing 3-5 g of sodium, 1 g of potassium (typically as potassium citrate), 500 mg of magnesium (magnesium citrate), and 600 mg of supplemental calcium.
It is highly recommended to limit how long one does a PSMF diet for hormonal and energetic reasons. In general, people who are more obese can stay on the PSMF diet longer than those who are leaner since their bodies' fat reserves can supply more energy. As one's body fat percentages decrease, it becomes more important to limit the diet to no more than a few weeks, whereas a very obese person could do it for much longer given proper micronutrient supplementation. Example) A 300-pound person at 40% BF has 120 pounds of fat, and each pound of fat can supply about 31 calories energy to the body/pound of body fat. Thus, this person could theoretically produce up to 31 calories/lb of fat * 120 pounds of fat = 3,720 calories from his fat reserves in a day. A 160-pound person at 10% body fat, however, with 16 pounds of fat could only produce 31 calories/lb of fat * 16 pounds of fat = 496 calories from his fat reserves.
A long-term PSMF diet undertaken carelessly and without the care of a physician may lead to serious health risks.