Mediterranean, Pesco-Mediterranean, Keto, Paleo, Atkins, DASH, vegetarian, lacto- vegetarian, ovo-vegetarian, vegan. There are a staggering number of diets, but which one is right? Each has their proponents and their opponents. All of these diets tout weight loss, cholesterol lowering, prevention of heart disease and longevity. Unfortunately, the research behind the claims of these diets is shaky. Nutritional research is plagued by poorly conducted scientific studies. What is lacking in dietary studies? First, the method used by most studies is an observational model rather than the more rigorous randomized controlled trial. In addition, outcomes such as heart disease or death take a long time to develop, so diet studies must be carried on for years to see a possible effect. Lastly, nutritional studies are often funded by industry and thus subject to bias. It is therefore very difficult to recommend a particular diet, with scientific certainty, to promote health and reduce heart disease. However, what if the key to health lies not in the food we eat, but in not eating at all?
Of course we need to eat to survive and to live a long and healthy life. However the benefits of fasting have recently come into focus. Early in the course of human evolution, energy rich food was not always available. We evolved to handle periods of feast and famine. We adapted by storing energy as fat. During times of fasting, the stored fat was metabolized into energy. Nowadays, with the round-the-clock availability of energy dense food coupled with inactivity, fat stores in the body grow, are not mobilized and obesity, diabetes, heart disease and early death are the result. Not eating, fasting, may be a good thing. In fact, many studies have shown that reduced calorie intake over the course of a lifetime, increases the life span. For example, the exceptional longevity of the people on Okinawa can be partially attributed to a constant low level of calorie intake.
Three types of fasting have been studied: alternate day fasting, 5:2 fasting (fasting 2 days per week) and time restricted eating. Time restricted eating, limiting the intake of calories to a short window between six and twelve hours per day, is the best studied and the most in vogue in the medical literature. This strategy is attractive in that it does not require time consuming measurements and adherence to a strict diet. After a twelve hour overnight fast, the body mobilizes fat stores. This decreases intra-abdominal fat tissue and leads to weight loss. The stress of fasting challenges the body to come up with ways to tolerate or overcome the period of fasting. Most of the organs in the body are able to do this. With repeated bouts of fasting, these adaptive changes allow the body to resist a broad range of potentially damaging stresses. Fasting does stress the body, but low levels of stress are good. Consider the analogy with exercise. Everyone agrees that exercise stresses the body and can make people feel uncomfortable. No Pain No Gain! However that stress produces positive benefits by improving fitness, helping with weight loss, lowering blood pressure, decreasing the risk for diabetes and heart disease. If the stress of exercise is good, so too is the stress of fasting. Time restricted eating has been shown to prevent obesity as well as lower blood pressure, decrease heart rate, lower cholesterol and triglycerides, reduce inflammation and prevent diabetes. In addition, intermittent fasting may prevent dementia. In trials with older adults, intermittent fasting improved memory. Whether these beneficial effects translate into a lower risk for heart disease and death has not yet been determined.
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There are downsides to time restricted eating. First, the timing of meals is often dictated by work schedule or cultural norms or even weather (Mediterranean countries often eat at 9 or 10 PM, after the sun is down and it is cooler). In addition, fasting for prolonged periods makes people jittery and irritable. It makes them hangry! We have to rationalize that that uncomfortable feeling does stress the body, but it produces positive adaptations. Intermittent fasting results in weight loss. However the amount of weight lost is similar to the weight loss on a reduced calorie diet. Fasting can lead to the loss of muscle mass and bone density in people who are not obese. In fact, the overall effects of intermittent fasting in nonobese patients is not known. Lastly, the studies on time restricted eating were mostly in animals, with some observational human trials. In addition, no studies have determined the optimal time window for eating. Clearly more research is needed before the widespread prescription of time restricted eating.
Until further research is done, what recommendations can be made? Certainly an easy adaptation would be to restrict food intake to a tight window. There are a couple of possible options. One is unrestricted eating but only between 9 AM and 6 PM, with no nighttime snacking. The other approach is eating only between 12 PM and 8 PM, while skipping breakfast. Both methods provide more than a twelve-hour window of fasting. In addition, restricting total calories intake is always a good recommendation. Remember: Eat less, live longer