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If you are feeling sick, one of the first things you usually do to take care of yourself is go to bed. With more serious conditions, bed rest is almost inevitable.
In an intensive care unit, virtually all patients are in bed. It is common to enforce bed rest for elderly patients in the hospital because of the risk of falls. Orthopedic surgery may not require bed rest, but usually a serious orthopedic problem is associated with inactivity, both before surgery and during recovery. What many people do not realize is that the effects of inactivity itself, and consequent muscle loss, can compound the problems caused by the pathological condition they are suffering from.
Our understanding of the effects of inactivity on muscle mass and function initially stem from the space program. Even though astronauts may be quite busy in space, the absence of gravity in effect creates a condition of inactivity. This is because on earth our actions are working against gravity, but the resistance caused by gravity is missing in the gravity-free environment of space. This results in a rapid loss of muscle mass and strength during space flight.
Astronauts returning from the early space flights had lost so much muscle function that it was not unusual for them to have difficulty walking. There was concern that the loss of muscle mass and strength would limit the ability to perform more prolonged flights in space. Consequently, NASA began sponsoring a series of studies to identify the reasons for the loss of muscle mass and strength in space flight, and the most effective countermeasures.
It was not practical for researchers on earth to study the effects of weightlessness, so a model was needed. Complete bed rest causes the same kind of muscle loss and strength declines as astronauts experienced in space. The NASA program, therefore, focused on bed rest. The results of those studies have not only identified effective countermeasures in space flight but have also given us insights into the broader issue of the effects of bed rest, per se, in clinical settings.
Due to a reduced rate of muscle protein synthesis, bed rest causes a loss of lean body mass, leg strength, and decreased muscle fiber function. Bed rest also induces a variety of metabolic problems, including insulin resistance and increased fat within muscle cells.
If you know about the changes in muscle mass and function that occur with aging, the responses to bed rest will sound familiar. Bed rest can be looked at as accelerated aging—all of the same aspects of deterioration of physical function that we see with aging also occur with bed rest, but at an even faster pace.
Bed rest in older individuals is particularly problematic. Older individuals lost as much muscle mass and function in just 10 days of bed rest as younger individuals lost in 30 days, as evidenced by a study my research team and I conducted. This study was published in JAMA: the Journal of the American Medical Association.
People usually have a good reason for taking to complete bed rest. In most cases there is a stressful condition layered on the effects of bed rest. The combined effect of stress and bed rest has been examined experimentally by infusing the stress hormone cortisol throughout bed rest. While stressful clinical conditions induce many responses, an increase in cortisol is common to virtually all clinical conditions. The infusion of cortisol triples the rate of muscle loss during bed rest. There is an interactive effect between bed rest and stress, such that bed rest amplifies the catabolic response to stress.
The most obvious countermeasure for inactivity is to be active. This is why patients are encouraged to get active as soon as possible after surgery or serious illness. However, in most cases bed rest is not a choice, but goes along with the clinical condition. It is, therefore, necessary to look at dietary options that will amplify the beneficial effects of activity, but that will also work in the absence of activity.
The requirement for dietary protein to curtail muscle loss during bed rest is significantly greater than when activity level is normal. However, adding sufficient protein to the diet to eliminate muscle loss due to inactivity is usually impractical.
In space flight, an increase in the protein content of the diet is limited by constraints on weight and food preparation. In bed-rest-restricted patients, a loss of appetite usually accompanies the catabolic response to the clinical condition. Consequently, we need to focus on a nutritional approach that provides greater benefit per gram consumed than dietary protein does.
Essential amino acids (EAAs) are the dietary amino acids that are not produced in the body but are needed for the synthesis of all proteins in the body, including muscle protein. EAAs constitute about 30-45% of dietary proteins and are the “active components” of the protein.
Consuming EAAs stimulates muscle protein synthesis. When taken daily throughout bed rest, EAAs lessen the loss of muscle protein mass and physical strength. We discovered that the improvement in muscle strength with EAAs is in part due to an improvement in the functional capacity of the individual muscle fibers. Strikingly, the deterioration in physical function as a result of 10 days of bed rest in older individuals was completely reversed with EAA supplementation.
EAA supplementation offers a compact delivery system of the most potent stimuli of muscle protein synthesis. When taken in conjunction with exercise the effects of EAAs are amplified, but when inactivity is dictated by a clinical condition EAA supplementation still is an effective approach to minimizing the loss in muscle mass and function that would otherwise occur.
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