Every few years, the BBC has an excellent documentary about weight loss. The most recent one to make it to public television here featured Dr. Michael Mosley, so when I saw he wrote a diet book, I was hoping it would be of the same quality. Instead, in his book "The Fast Diet," he expounds upon the latest diet craze in Britain, the "5:2 Diet," where one severely restricts calories two days each week and eats whatever one wants the other five.
The background rationale
Caloric restriction has been shown to extend lifespan and decrease the incidence of disease in all animals studied. The problem with this is that no animal voluntarily starves itself, with the exception of some humans (happy National Eating Disorders Awareness Week). Being continuously hungry, tired and cold is a poor trade-off for longevity. Because of this, studies have been done on intermittent fasting and it was found that those who ate anything they wanted, but only every-other day, lost weight - and diabetics who did so had blood glucose levels in the normal range. This, too, is too rigorous for long-term successful dieting, so the idea of eating 25% of normal calories twice a week was introduced... without any scientific justification. The reason alternate day fasting works is that a person who eats 2000-2500 calories daily cannot eat 4000-5000 comfortably; one simply averages fewer calories over a two day period - the calorie deficit of the two low calorie days in the 5:2 diet is easily overcome on the other five days. It won't work for long, as one learns the bad habit of gorging on the days one is not starving.
Some basic endocrinology
Your body is continuously balancing cell growth and cell death. When you take in enough carbohydrates or protein, your body secretes insulin to take up these nutrients and transport them where they are needed. Once fuel requirements are met, the remainder is used for growth, mediated by somatotropin and somatomedins, such as insulin-like growth factor 1. After all cell replacement and growth needs are met, the remainder is used to grow fat cells, which store energy for possible future use as energy.
When one continuously has just barely enough fuel to meet one's needs, the body becomes more efficient, partly by up-regulating receptors for somatomedins. Fat stores get depleted, body temperature lowers, heart rate slows... and whenever one has an excess of calories, that excess is more efficiently turned into fat. This is why chronic dieters so often develop more and more depot fat, which is precisely what they were trying to lose.
The deficit side of the equation
What diets ignore is that weight loss is not just restricting the calories ingested. Weight loss is also dependent upon the numbers of calories expended. It is worth noting that Mosley has admitted he'd rather do anything (including starve) than exercise.
Exercise can achieve the same results as restrictive dieting. Using myself as an example, as I said above "fat stores get depleted, body temperature lowers, heart rate slows" and I'm at 6% body fat, and upon awakening, my resting heart rate is 36 beats per minute and body temperature is often 95.5 degrees. These numbers are so low as to mimic hypothyroidism, but are the result of decades of strenuous exercise. I need to stress that these numbers are not obtainable by all - there are world-class endurance athletes who do not have these numbers - but a combination of proper diet and exercise will improve the health of anyone. You knew that already; it's common sense.
The pay-off for runners
The idea of depleting one's energy stores twice a week in order to lose weight and improve health is a good one. I suggest one do it through exercise, rather than dieting. If you're running regularly (as I expect anyone reading this blog is), run far enough and fast enough twice a week to at least partially deplete your glycogen stores. Two hard workouts per week - that's completely reasonable.