Nitrogen balance studies are challenging to achieve because valid 24-hour urine collections are difficult to obtain unless the patient has a catheter. In addition, changes in renal function are common in patients with inflammatory metabolism, making standard nitrogen balance calculations inaccurate without calculation of nitrogen retention. In these patients, urea kinetics provides a more accurate estimation of nitrogen balance.
Urinary Creatinine
Creatinine is formed from creatine, a compound found almost exclusively in muscle tissue. Creatine is synthesized from the amino acids glycine and arginine with addition of a methyl group. It is a high-energy phosphate buffer, maintaining a constant supply of ATP for muscle contraction. Creatinine has no specific biologic function. It is continuously released from the muscle cells and excreted by the kidneys with little reabsorption. When a patient follows a meat-restricted diet, the size of the patient’s somatic (muscle) protein pool is directly proportional to the amount of creatinine excreted. The clinical significance is that men generally excrete larger amounts of creatinine than women do and that individuals with greater muscular development excrete larger amounts than those who are less muscular. Total body weight is not proportional to creatinine excretion. Creatinine excretion rate is related to muscle mass in healthy individuals.
The use of urinary creatinine to assess somatic protein status in a healthcare setting in which patients are consuming a mixed diet has its limitations. Creatine is a component of muscle meats which is converted to creatinine. The body can not distinguish between the two sources of creatinine. In addition, urinary creatinine can vary significantly within individuals, probably due to sweat losses. Urinary creatinine concentration as a biomarker of muscle mass is typically used only in research.
Nutritional Anemias
The prevalence of anemia increases with each decade of life over age 70 and is associated with both frailty and mobility impairment (AMDA, 2007). Macrocytic anemia in older adults is often due to insufficient dietary vitamin B12 or folic acid. Other causes of macrocytic anemia include kidney disease, hemolytic anemia, hypothyroidism, or certain medications.
With aging, there is a decrease in iron in RBC (Fischbach, 2008; Corbett, 2000). The mechanism is not known, although iron seems to be absorbed from the intestine. However, there is a decreased incorporation of iron into the RBC resulting in lower hemoglobin levels (Beers, 2005). The significance of this drop in hemoglobin is not known. In addition, MCV (MCV) increases slightly with age.
Anemia is a deficiency in the erythrocyte mass and hemoglobin contents. A low hemoglobin or hematocrit needs to be evaluated further by a complete blood cell count (CBC), including hemoglobin concentration, hematocrit, red blood cell mass, and MCV (Blackwell, 2001). Norms are assuming adequate hydration. If the patient is dehydrated, the values will be falsely high. Overhydration will result in falsely low values.
Patients with an infection or other inflammatory processes present often have low hemoglobins and hematocrits related to redistribution of iron rather than a change in nutritional status. With infection, the cytokine interleukin-1β (IL-1β) inhibits the production and release of transferrin while stimulating the synthesis of ferritin. The net result is that iron is moved from transferrin and hemoglobin to ferritin. The redistribution of iron stores is a protective mechanism since many virulent bacterium have specialized receptors for iron uptake. The survival of the bacterium within the host is dependent upon its ability to extract nutrients from the surrounding environment.
Classification of Anemias
Anemias are classified by etiology. The causes include:
•Blood loss
•Deficient erythropoiesis
•Excessive hemolysis
Blood Loss
Anemias can result from acute or chronic blood loss. Identification of the cause of the blood loss and resolution of the loss will most likely resolve the anemia. The lost erythrocyte mass and hemoglobin content will be replaced via transfusion or erythropoiesis (Blackwell, 2001).
Deficient Erythropoiesis
Anemias due to a deficient erythropoiesis include microcytic anemias, normochromic-normocytic anemias and macrocytic anemias. All of these anemias are characterized by low hemoglobins and hematocrits (Blackwell, 2001). The distinction is made by examining the MCV (Coulter, 1991). This test provides the average size of the patient’s red blood cell. In microcytic anemias the heme or globin synthesis is deficient or defective resulting in a lower than normal MCV. In normocytic anemia the bone marrow failure prevents the erythroid mass from expanding as needed, but the volume is normal, so the MCV is normal. Megaloblastic erythropoiesis results when DNA or RNA synthesis is impaired. The MCV then exceeds normal values (Blackwell, 2001). Other tests used to evaluate erythropoiesis include reticulocyte count, red blood cell count, erythrocyte count and red blood cell width (RDW).
Excessive Hemolysis
Anemia due to destruction of RBC is much less common and rarely associated with blood loss or bone marrow failure. These anemias are caused by defects that are either extrinsic or intrinsic to the RBC. For example, an anemia with extrinsic defects is autoimmune hemolysis. An anemia with intrinsic defects is sickle cell disease.
Categories of Nutritional Anemias
Nutritional anemias are categorized using red blood cell indices quantifying the size, weight and hemoglobin concentration of red blood cells. These tests are used to categorize anemias. Table 6 categorizes nutritional anemias by red blood cell indices.
The four types of nutritional anemias are:
•Iron Deficiency Anemia
•Megaloblastic Anemia
•Pernicious Anemia
•Anemia of Chronic and Inflammatory Diseases
Iron Deficiency Anemia
Iron deficiency anemia is most commonly seen in children with low iron intakes. However, approximately 20 percent of women, 50 percent of pregnant women, and 3 percent of men are iron deficient. The DRI for iron for adult males and females 51 years and older is 8 mg/d. For females under the age of 50, the DRI is 18 mg/d. The NHANES data reports that median intakes of iron for adults aged 40 to 59 and 60 years and older are 15.5 mg/day and 14.8 mg/d respectively (Ervin, 2004).
Iron deficiency anemia may be the result