National Council on Strength & Fitness
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The Scientific Reality of High Protein Diets

The evidence is clear that high protein, low carbohydrate diets can lead to significant short-term weight loss. It is equally clear that high protein, low carbohydrate diets can cause serious health difficulties for the consumer. What are consumers to make of this conflicting information? Low carbohydrate intake results in glycogen storage depletion in muscle tissue. Glycogen is the storage form of glucose in muscle. Each gram of glycogen requires 2 grams of water in order to be deposited in the muscle tissue. Without the glycogen, the water is not needed and is excreted in the urine. Therefore, the majority of early weight loss due to high protein diets is from water loss. After returning to a balanced diet, the weight lost from water will return. This is due to the muscle tissue’s ability to store glycogen and the associated water necessary for glycogen storage once carbohydrates are returned to the diet.


On the surface, water loss for weight loss doesn’t seem to be such a big deal. However, systems in the body are complex and interactive. A modification in food intake, specifically emphasizing a single nutrient will have many different effects. The first one to be examined will be the loss of glycogen. The preferred source of energy for muscle contraction is glucose, which is stored in the muscle for the most part as a much larger molecule known as glycogen. As one exercises, glucose molecules are cleaved off the glycogen and are used to make ATP, the molecule which is used directly as the energy source for muscle contraction. During exercise, these glycogen stores may be depleted by as much as 75%. In order to replace the glycogen, a carbohydrate source is necessary. Digestive enzymes break the carbohydrate compounds down into glucose which is then used to replenish glycogen stores in the muscle. Without a glucose source, the glycogen is not replaced. The practical implication for glycogen loss is a feeling of tiredness, lethargy, and the inability to exercise at a high level on a day to day basis. In other words the individual becomes “stale”.


Glycogen depletion may be the least of the problems caused by high protein diets. Whenever carbohydrates are not available, lipids become a primary fuel source (which could be desirable for weight loss, if what follows did not occur). Unfortunately, lipids cannot be used for a long period of time without carbohydrate ingestion. Glucose is metabolized during glycolysis, a process resulting in the production of pyruvic acid. If an adequate amount of oxygen is present, pyruvic acid is used in the Kreb’s Cycle to generate a large amount of ATP. The first step in this process is to transfer pyruvic acid into the mitochondrion and remove a carbon dioxide. The process removes a couple of hydrogen atoms, and adds a Co-enzyme A, forming acetyl Co-A. The two carbon acetate moiety is then tacked on to an oxaloacetic acid to make citric acid, the first step in the Kreb’s or Citric Acid Cycle. When lipids are used, enzymes allow them to enter the cycle as acetyl Co-A, as well as in a process known as Beta-oxidation. In order to keep this sequence continuing, adequate amounts of oxaloacetic acid must be present. The source of oxaloacetic acid in this case is pyruvic acid. Remember that pyruvic acid is obtained from the metabolism of glucose. If there is no glucose, there will not be adequate levels of pyruvic acid which means that oxaloacetic acid levels are low. Therefore, the lipids will not be used in the Kreb’s Cycle. The irony is that by using lipids as a primary energy source an individual is creating an environment in which the body cannot utilize lipids as an energy source.


As a result of the inability of muscle cells to utilize lipids for energy, free fatty acids (a lipid component) begin to accumulate in the intercellular fluid and move into the blood stream. Upon reaching the liver, free fatty acids are converted into ketone bodies. Some ketone bodies are excreted in the urine, but not all. The ketone bodies leftover accumulate in the blood (ketosis) and change the pH of the blood causing it to become more acidic, in extreme cases this can lead to acidosis. Enzyme systems in the body require a narrow range of pH to function well. Even the slightest of pH changes can have significant detrimental effects. With an increasingly acidic environment, enzymes fail to allow normal cellular reactions to proceed resulting in decreased performance. Minimally the results are tiredness and nausea; maximally this toxicity may result in death.


Other serious effects have also been found as a result of high protein consumption at the expense of carbohydrates. Acidosis leads to calciuria, the excretion of calcium in the urine, due to an increase of bone reabsorption into its constituent parts due to increased osteoclastic activity. Bone density loss is the result. An examination of blood profiles for cardiac risk factors show lipid profiles deteriorate, with increasing LDL and decreasing HDL levels accompanied by an increase in triglycerides. This translates into an advanced rate in the progression of coronary artery disease. For individuals with mildly reduced kidney function the consumption of animal proteins in the diet has been shown to significantly accelerate further decline. A compounding factor may be increased levels of dehydration due to the necessity of using larger amounts of water in the metabolism of proteins at levels above 30% of caloric intake.


In addition to the many serious medical problems that may result from long-term high protein diets, perhaps the effects with the greatest impact are the counterproductive issues related to weight loss. Most individuals on high protein diets psychologically find it difficult to remain true to their program. They constantly fantasize about returning to their normal dietary habits. Furthermore, they cannot keep the weight off effectively long term. In the end, they experience the adverse psychological effects of another failed diet plan and their long-term health may be compromised as a result. For the majority of individuals, proper nutrition requires the intake of a high percentage of calories primarily from the consumption of complex carbohydrates; with an emphasis on fruits, vegetables, and whole grains. Rational nutrition coupled with appropriate exercise is the way to achieve a healthy and productive lifestyle. If carbohydrates are to be reduced the reduction should be from sugars and refined carbohydrates.