by Jason Fung
When people lose weight, the lost weight isn’t entirely body fat; some of it is muscle. Although overweight and obese people tend to carry more muscle than people of normal weight, in general, we want to avoid losing muscle. Muscle mass and strength correlates with health and long life.35 As much as 25 percent of the weight lost using low-calorie diets is muscle,36 which can sometimes be mitigated by eating more protein.37
Adding resistance exercise and more protein to a low-calorie diet not only can abolish muscle loss but actually can help you add muscle as you lose fat. Whey protein has been well studied, but casein supplements might be more effective, with some studies showing more fat loss, more lean mass gain, and a greater increase in strength with casein.38 Whey is a fast-digesting protein, whereas casein is a slow-digesting protein; therefore, each protein supplement has unique advantages. Whey causes a spike in plasma amino acids that are useful for athletes immediately after training. Casein taken just before bedtime slowly releases amino acids, which prevents muscle breakdown and promotes muscle protein synthesis throughout the night.
Higher protein intake is often useful in weight loss because of its satiating effect. Eating protein increases satiety hormones like peptide YY. Think about eating a small piece of steak or chicken in comparison to drinking an equal-calorie portion of soda. You can drink the soda without feeling the slightest bit fuller than when you started, but the steak or chicken will make you feel full and keep you feeling full for longer. This feeling of satiety is beneficial in weight-loss efforts. Protein’s role in obesity has been a relatively neglected area of study because protein typically makes up only about 15 percent of food energy, and protein consumption has remained nearly the same throughout the obesity epidemic.
The protein leverage hypothesis of obesity suggests that eating food relatively dilute in protein leads to obesity.39 Eating foods low in protein may result in an innate physiological drive to eat more food overall to get adequate protein, which leads to weight gain. Junk foods like chips and soda are low in protein. Official dietary recommendations to eat a low-fat diet coincided with the start of the obesity epidemic. Eating low-fat foods also might have led to eating low-protein foods because many foods that are high in fat, such as meat, are also high in protein.
The Optimal Amount of Protein
Optimal protein intake varies depending on goals and underlying health conditions. People who are older, ill, or immobilized need more protein to maintain good muscle tone and health. Athletes need more protein than nonathletes, but not nearly as much as common lore suggests.
Consuming more protein can reduce hunger and increase muscle growth. Although protein restriction doesn’t make much sense for those who exercise on a regular basis, consuming too much protein may also have some negative consequences.
PROTEIN RECOMMENDATIONS FOR HEALTH AND ACTIVITY CONDITIONS40
Maximal stimulation of muscle protein synthesis may require at least 0.24 gram of protein per kilogram per meal for younger adults (20 to 29 years old) and 0.40 gram per kilogram for older adults (older than 50).
PROTEIN RECOMMENDATIONS BASED ON TYPE OF WORKOUT
Fasting, the voluntary abstinence from eating food, has been used throughout human history for various purposes, including religious, health, and spiritual. Whether you call it a fast, a cleanse, or detoxification, the idea is that periodic restriction of all foods is a healthy habit. In fact, all major religions embrace fasting as a cornerstone of healthy living.
Fasting is one of the keys to longevity because it improves all the dietary factors we’ve talked about so far. It restricts calories and protein. It reduces insulin and mTOR and activates AMPK and autophagy. These benefits are delivered at no cost and without taking any time. Fasting is not something you do; it’s something you do NOT do. It can both simplify and enrich your life. So why has this ancient tradition been largely abandoned? People practiced fasting for millennia, but it has been only recently that many people have started to believe that fasting is harmful because it may cause malnutrition as the body metabolizes its own protein for energy.
You should not confuse malnutrition from not eating, which is called wasting, with fasting. Wasting is a pathological situation in which the body has inadequate stores of body fat and is therefore forced to burn functional tissue such as muscle to provide the energy needed to survive. Metabolism of muscle for energy can cause weakness and, in extreme situations, death. These severe outcomes typically happen when a person has a body fat percentage lower than 4 percent. By contrast, a typical American male carries 25 to 30 percent body fat and females carry 35 to 40 percent body fat, although this varies by age. Even an elite marathon runner, who might not have any obvious visible fat, still carries approximately 10 percent body fat.
We can use a quick calculation to see at what point the body is in danger of wasting. A man who weighs 180 pounds and stands 5 feet 11 inches has a body mass index of 25 (normal). With average body fat of 25 percent, he carries 45 pounds of body fat (180 lbs x 0.25 = 45 pounds of body fat). A pound of fat supplies approximately 3,500 calories of energy—enough for two days. With 45 pounds of body fat, this man carries enough body fat to sustain him for ninety days, or almost three months straight, without eating any food whatsoever before he is in danger of wasting. As we discuss in more detail later, during extended periods without food, the body burns mostly fat, with some protein breakdown. In general, as long as an individual’s micronutrient intake is adequate, the loss of protein (much of it coming from the skin and damaged proteins) does not appear to be life-threatening—even when it continues for months.
The three-month estimate is an underestimate because the body’s basal metabolism, or the rate of consumption of energy, will fall as the person loses weight. An obese or overweight person can survive even longer before wasting is a real concern. By contrast, most people are concerned if they go more than three hours between breakfast and lunch without eating.
Fasting should also not be confused with starvation, which is an involuntary condition. There’s a stark contrast between starvation and fasting. During starvation, no food is available, regardless of whether the person wants to eat. Fasting is a fully conscious decision to abstain from food, even though food is readily available. The photos of children without adequate nutrition in Africa show starvation. These children do not eat because no food is available; going without eating is not a choice for them. Because fasting is an entirely voluntary process, a person may stop fasting at any point.
The possibilities for fasting are infinite. We offer some suggestions in Chapter 13, but if you’re looking for a more detailed plan for starting a fasting regimen, Dr. Fung’s book The Complete Guide to Fasting is a great resource.
Furthermore, when fasting for health reasons, remember that you should not start a fasting regimen unless your medical doctor is closely monitoring your health status and states that you are healthy enough to fast. And if you do not feel well for any reason at any time during your fast, you should stop immediately and seek help from an experienced practitioner.
Fears about fasting are highly prevalent. We constantly hear that we must eat, eat, and eat—even when we want to lose weight! If neither malnutrition nor being underweight is a concern, the fear of “burning muscle” (which mostly comes from nitrogen balance studies) during periods of fasting has not been backed up by practical experience, the known physiology of fasting, or clinical studies. (Nitrogen loss does not necessarily mean muscle loss; it can be due to the loss of excess skin and damaged proteins that comes with fasting.)
Let’s examine the physiology of fasting and what happens to protein metabolism when you fast.
The Physiology of Fasting
During fasting, the body must rely upon its stores of food energy for basal metabolic needs. Although we mostly think about the body’s energy expenditure in terms of exercise, our body requires a significant amount of energy to keep our vital organs (the brain, heart, lungs, and kidneys) running properly. All that functionality
is largely controlled unconsciously by our autonomic nervous system. Even when we’re bed-bound, our bodies still need energy to keep this machinery going. If we eat nothing (fasting), there is no incoming food energy, and we must rely completely on our stored food energy to survive.
HOW DOES THE BODY STORE FOOD ENERGY?
The body stores food energy in two main ways.
• Glycogen in the liver
• Body fat
When we eat, the level of the hormone insulin goes up and instructs our body to store some of the incoming food energy. Glucose from carbohydrates is strung together into long chains called glycogen, which is stored in the liver. When we eat protein, we break down and absorb the component amino acids. These are used to build any necessary new proteins. However, if we eat more than we need, the body has no way of storing excess amino acids.
Fig. 7.1: Differences between a fed state and a fasted state on the body’s use of food energy
Fed versus fasted state: storing versus burning energy
Instead, the body turns those excess proteins into glucose to store that food energy. The average American who eats a standard diet is estimated to turn 50 to 70 percent of the protein they ingest into new glucose molecules.1 In other words, the average diet exceeds the protein needs of the body by a fairly large margin.
Glycogen is a useful storage form of energy, but the liver has limited storage space. Once glycogen has maxed out, the body converts the excess glucose into triglycerides, or fat, by a process known as de novo lipogenesis. These newly created fat molecules can then be exported out of the liver and into the fat cells for long-term storage.
The two storage systems are complementary. The glycogen system is simple but limited in storage capacity. The body fat system is much more complex and requires the body to change molecules of carbohydrates and proteins to fat (triglycerides). However, the advantage is that the body fat system is almost limitless in its capacity.
The two systems are analogous to the way we use a refrigerator (the glycogen system) and a freezer (the body fat system). We store excess food in two different ways. We can store food in the refrigerator and both putting food in and taking it out are easy. However, a refrigerator has limited capacity. When the refrigerator is full, we can freeze the food. Storing food in a freezer is more difficult because we must package it properly and freeze it; however, we have unlimited storage because we can always add another freezer in the basement of our house.
WHAT HAPPENS DURING FASTING?
During fasting, the food energy storage process reverses. Insulin falls, and that decline is the signal that the body should start using some of the stored food energy to power the body. Dr. George Cahill described the five stages between eating and prolonged starvation/fasting. (Four stages are shown in Figure 7.2.2) In the first four hours after you have eaten, insulin is high, and you are still mostly drawing from the glucose you have eaten. All the tissues of the body can use this glucose, and you are still storing food energy as glycogen. Once the glycogen store is full, any excess must be turned into body fat.
By stage 2 (four to sixteen hours after eating), the exogenous glucose is no longer available as a source of energy, so you must rely on body stores. The most readily available source of energy is glycogen in the liver. You break down the glycogen into its component glucose molecules and send it out to the body for energy. Those glycogen stores last approximately twenty-four hours. So, if you are not exercising, even up to twenty-four hours of fasting does not necessarily force the body to burn either fat or protein.
By stage 3 (sixteen to thirty hours after eating), the stores of glycogen are starting to run out. Body fat is not yet available, so you bridge the gap by producing glucose from protein in a process called gluconeogenesis, which means “the creation of new glucose.” During this stage, your body transitions from using glucose, which is increasingly becoming scarce, to using fat and protein, which is becoming available from body stores. This area is what concerns many people because they believe that this breakdown of protein constitutes muscle loss. These concerns are largely misplaced for reasons we discuss in more detail later in this chapter.
Fig. 7.2: Four stages between eating and fasting
Most of our organs and muscles can use fat (triglycerides) directly. The brain, however, cannot use fat directly because of the blood-brain barrier. Also, because the brain requires so much energy, it would quickly deplete the available glucose. The liver compensates by producing ketone bodies from the body fat. These ketones can cross the blood-brain barrier so the brain can use them as an energy source. An estimated 60 to 75 percent of the brain’s energy can be derived from ketones, which significantly reduces the need for glucose because much of it must be produced from protein.
During stage 4 (thirty hours to twenty-four days after eating), the body mobilizes stores of body fat for energy. By this point, most tissues of the body have switched to burning triglycerides for energy. Only the brain, the red blood cells, and the inner part of the kidney must still use glucose. Some of the glycerol backbone of the triglyceride molecule is turned into glucose, and a small amount is still derived from the breakdown of protein. The only substantial difference is that the amount of protein breakdown is further reduced. During extended fasting, the body mostly burns fat.3 This is logical because the body mostly stores food energy as fat.
This entire process of extended fasting essentially describes the shift in energy metabolism from glucose (from food and glycogen) to body fat. Although there is still some breakdown of protein, as we discuss later in this chapter in the section “Glucose Requirements and Protein Breakdown,” the clinical studies show that with twenty-four hours of fasting the body does not increase the “burning” of protein for energy. Indeed, long-term studies indicate that the body doesn’t ramp up protein metabolism. However, because of the continued oxidation of protein, some people are apprehensive that this could result in muscle or organ wasting. Should you be concerned?
Clinical Studies
Involuntary periodic starvation or its voluntary counterpart (fasting) has been part of human nature since the beginning of time. Until relatively recently, food was not always available. To survive the hard times, early humans needed to store food energy as body fat when food was abundant. If humans did not have an efficient storage and retrieval method of food energy, our species would have died long ago.
After food availability became more reliable, most human cultures and religions prescribed voluntary periods of fasting. For example, Jesus was said to have fasted for forty days and forty nights, and many subsequent followers have undertaken this themselves without significant health damage. Many Muslims fast during the holy month of Ramadan, and also regularly twice a week during the rest of the year. In these situations, fasting was considered a cleansing procedure without any connotation of harmful muscle burning.
The repeated feeding-fasting cycles inherent in prehistory times did not seem to have any detrimental effect on muscle mass. Descriptions of traditional societies such as the Native Americans or Inuit of North America or tribesmen in Africa suggest they were lively and energetic rather than emaciated and weak. Descriptions of modern followers of the Greek Orthodox Church, with its many days of fasting, do not include portrayals of lethargy and weakness. It is virtually impossible that humans, who were designed to store food energy as body fat, would burn muscle when food was not available. If they did, all people throughout history until the twentieth century who followed this feast-famine cycle either through periodic starvation or fasting would be almost pure fat. Instead, they were lean and strong.
Recent clinical evidence bears out the fact that alternating twenty-four-hour periods of repeated fasting/feeding does not cause muscle loss. In a 2010 study of alternate-day fasting, patients were able to lose significant fat mass with no change in lean mass. In this schedule, subjects eat on a normal schedule on feeding days; on the alternate days, they fast. Also, the researchers noted numerous metabolic
benefits—such as reduced cholesterol, triglycerides, and waist circumference—along with the weight loss.4
A more recent 2016 study compares a strategy of intermittent fasting with daily calorie restriction (the conventional method of weight loss suggested by most health professionals).5 Both groups lost a comparable amount of weight, but the intermittent fasting group lost only 1.2 kilogram of lean mass compared to 1.6 kilograms in the calorie restriction group. When we compare the percentage increase in lean mass, we see that the fasting group increased by 2.2 percent compared to 0.5 percent in the calorie restriction group, implying that fasting may be up to four times better at preserving lean mass according to this measure. Importantly, the fasting group lost more than double the amount of the more dangerous visceral fat.
The same study highlighted some other important benefits, too. Chronic calorie restriction reduced basal metabolic rate, where intermittent fasting did not. Because fasting (but not chronic calorie restriction) induces the counter-regulatory hormones, the body is switching fuel sources, rather than shutting itself down. Further, chronic calorie restriction increases ghrelin, the hunger hormone, where fasting did not. If you are less hungry with fasting compared to calorie restriction, you are more likely to stick to the diet. Both of these are overwhelming advantages for weight loss.
Despite the concerns that fasting may cause loss of muscle, our long human experience and multiple human clinical trials show the exact opposite. Intermittent fasting preserves lean tissue better than conventional weight-loss methods. Thinking again about gluconeogenesis, at first glance, this seems counterintuitive. If intermittent fasting causes gluconeogenesis (turning protein into glucose), how can it possibly be better at preserving muscle? Part of the answer lies in the fact that gluconeogenesis does not start until approximately 24 hours after the last meal. The other part of the answer lies in the hormonal adaptation to fasting—the counter-regulatory surge.
