The use of amino acid infusions to provide nutrition to individuals incapable of eating a sufficient amount of dietary protein to meet nutritional requirements dates back to the 1960s. An amino acid infusion can either provide partial nutritional support or may be given as part of total parenteral nutrition (TPN). TPN refers to providing all nutrition, including amino acids, carbohydrate, fat, and vitamin and minerals via intravenous infusion. But is there significant advantage to taking amino acids intravenously over taking amino acids orally as a supplement?
Intravenous Amino Acids
Intravenous amino acids can provide adequate support for protein metabolism (the breaking down and building up of proteins). Individuals who have had large portions of their intestines removed surgically can live indefinitely on TPN. In this case, all amino acid intake is via intravenous infusion. TPN can promote muscle protein synthesis, as well as meet other requirements for amino acids, including immune function and brain neurotransmitter synthesis.
The primary drawback to amino acid infusion as the sole source of amino acids from the physiological perspective is that the intestine atrophies. This is because some of the amino acid requirements of the gut are satisfied by direct uptake of amino acids from the digestion of orally ingested protein or amino acids. Atrophy makes the intestines more permeable to diffusion of bacteria and bacterial products from the gut into the body. Of course, if TPN is used because of removal of the gut, this point is moot. If intravenous amino acids are used alongside orally ingested protein, gut integrity will be maintained so long as the oral intake is sufficient.
Benefits of Amino Acid Infusion
The clinical benefits of amino acid infusion in patients who are incapable of ingesting adequate nutrition orally are well established. In addition, amino acid infusion in individuals fully capable of ingesting amino acids orally is gaining popularity. The “selling point” of this therapy is that the exact concentrations of individual amino acids in the blood can be precisely controlled. This is promoted as particularly important to the relative concentrations of amino acid precursors of specific neurotransmitters. This approach is used in recovery from addiction, as well as for general support of protein metabolism in the body.
The premise of intravenous amino acid therapy is that the individual cannot get the full benefit from orally ingested amino acids, or that the intravenous infusion somehow confers special beneficial effects.
Advantages of Oral Ingestion of Amino Acids
The oral ingestion of free amino acids has advantages over amino acid infusion. There is no risk associated with oral ingestion, whereas intravenous infusion carries with it the risk of vein irritation. A health care provider is needed to perform the intravenous infusion, whereas free amino acids for oral consumption are relatively cheap and readily available.
It is possible to obtain a mixture of essential amino acids for oral consumption specifically designed for a particular condition, whereas the options for intravenous infusion are much more limited due to the arduous procedure to obtain approval from the FDA.
There are also physiological advantages to the oral ingestion of amino acids. Most important of these is that the uptake and utilization of orally ingested amino acids by the intestines provides a direct source of essential nutrition.
Orally ingested amino acids are directly absorbed by the intestines and require no digestion. The amount of amino acids provided to the body by the two routes (oral and intravenous) are thus similar. Since orally ingested amino acids pass through the intestines, liver, and other organs before reaching the peripheral circulation, the peripheral plasma concentrations of amino acids reach higher levels when infused intravenously. Conversely, orally ingested amino acids provide more direct support of the internal organs.
Orally ingested and intravenously infused amino acids both stimulate muscle protein synthesis, which is the metabolic basis for muscle growth and strengthening. A clinical study of older individuals showed that muscle protein synthesis was stimulated in response to both intravenously infused as well as orally ingested amino acids. In addition, the profile of amino acid concentrations in the blood reflected the profile of the orally ingested amino acids, so there is no advantage to intravenous infusion in terms of supplying the proper balance of amino acids needed to synthesize neurotransmitters in the brain.
The intravenous infusion procedure has risks and costs that are avoided by oral ingestion, and the amino acid profiles in available intravenous infusion mixtures are limited. For these reasons, amino acid infusion should be used only in individuals who are incapable of ingesting amino acids or who have a limited capacity for absorption. Oral ingestion is the preferred route of delivery of amino acids in most circumstances.