November 20, 2007
Volume 2007, No. 9
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IN THIS EDITION
New technology sheds light on current protein recommendations
Iron Bioavailability of foods in the diet is more important than total Iron Intake
New technology sheds light on current protein recommendations
The importance of protein in the diet has been well established, however there are differing opinions of what level is the correct amount for optimal health for the general public. In 2002, the Food and Nutrition Board of Canada reviewed a meta-analysis of 19 nitrogen balance studies and concluded a “safe” intake level of 0.66 - 0.80 g/kg/day. However, the linear analysis regression model used has limitations. In addition, the limitations of nitrogen balance also complicate the issue. Accuracy depends on the preciseness of information which generally includes overestimating nitrogen intake and underestimating nitrogen excretion creating a false positive nitrogen balance. Researchers believe current protein recommendations are below optimal levels and have developed a method for testing their theory.
The indicator amino acid oxidation (IAAO) technique uses a more direct approach to measuring protein requirements by tracking changes in oxidation of orally administered amino acids in subjects. For this study, eight healthy adult males were observed on an out-patient basis 7 times over the course of 3 months. Two days prior to the study day, participants ate a maintenance diet of 1.0 g/kg/day of protein consisting of premeasured shakes with added protein and energy based on their individual calories needs. Intake on the day of the study consisted strictly of 1 of 7 dietary protein intakes (0.10, 0.30, 0.60, 0.90, 1.2, 1.5, and 1.8 g/kg) in a crystalline amino acid mixture which was consumed in 8 isocaloric hourly meals over the course of the day. The tracer consisted of a primer dose of NaHC-13O-3 and L- [1-C-13] phenylalanine with the fifth meal and continuing with the phenylalanine for the remaining meals. Breath and urine samples were taken and analyzed and results from this study indicated intake between 0.9 and 1.8 g/kg/day did not differ significantly when it came to phenylalanine oxidation. The mean and population-safe protein requirements were estimated to be 0.93 and 1.2 g/kg/day.
Based on several factors, researchers believe the better mechanism for determining nitrogen balance is a biphase linear regression analysis rather than the previous linear model. Current EAR and RDA protein recommendations appear to be underestimating protein needs by 29 and 33 percent. When calculating out a diet with the RDA of protein at 1.2 g/kg/day, the percentage of protein in the diet is 17 percent. That is well within the recommended range of energy intake from protein, 10 – 35 percent. However, when calculating out the same diet with the current recommended RDA for protein at 0.8 g/kg/day, the energy intake from protein is eight percent which is below the suggested range. Researchers believe this study indicates the need for more accurate testing of protein requirements which will show the need to increase current recommended levels of protein intake.
Citation:
Am J Clin Nutr, 2007; 86:995-1002.
To read the abstract, go to Reevaluation of the protein requirement in young men with the indicator amino acid oxidation technique.
Iron Bioavailability of foods in the diet is more important than total Iron Intake
Iron deficiency is one of the most prevalent nutritional concerns around the world. Severe iron deficiency resulting in anemia can have lifelong consequences especially on the development of a child. Anemia in children less than two years of age can result in adverse cognitive and psychomotor development, physical capacity, decreased immunity and infant mortality. In addition, anemia of a pregnant woman can also result in low birth weight. It is estimated 48 percent of preschool children worldwide are anemic and at least half of that number is caused by iron deficiency. Factors contributing to iron deficiency anemia in children less than five years include insufficient dietary intake of iron or low bioavailability from foods consumed, coupled with high requirements due to rapid growth. Few studies have looked at the bioavailability of iron in the diet in relation to concentrations of iron in the blood. Researchers in Mexico used a population study to conduct a secondary analysis to look at that relationship.
The 2nd Mexican National Nutrition Survey is a representative national survey of 17,944 households conducted between October 1998 and March 1999. A random sample of children between the ages of 12 months and 59 months was used resulting in 919 children with a 24 hour recall of dietary information obtained from the adult caretaker. Additional information used in the analysis was hemoglobin (Hb) levels in the blood and the dietary intake of total iron, heme and nonheme iron, phytate, vitamin C, meat (red meat, poultry, fish) and cow's milk. Total bioavailable iron (FeBIO) was estimated based on the individual’s iron status, dietary iron content and dietary factors that influence iron absorption (i.e. vitamin C and phytate). A multiple linear regression model was created in order to study the relationship between FeBIO and HB concentrations and variables included age, high milk consumption, presence of diarrhea, altitude of residence, height for age and socioeconomic factors.
Breaking the children into groups based on age, sex, and geographical location showed no differences in the rate of anemia, which was about 23 percent. Four out of seven variables were significantly associated with Hb concentrations (FeBIO intake, age, altitude, and socioeconomic factors) and two (diarrhea and height-for-age) were marginally associated. The children’s estimate intake of FeBIO was significantly lower than recommended and seemed to be due to consumption of low iron bioavailability foods rather than total iron consumption. The association between FeBIO and Hb concentrations was greater in children 12-23 months old which seem to indicate diet is a bigger contributing factor to younger children than older. Looking at the data from a different angle showed that total iron intake was not significantly associated with Hb which highlights the importance of looking at iron bioavailability in the diet versus total iron consumption. This study seems to be further proof that it is important to look at the whole diet rather than individual nutrients to get the complete picture.
Citation:
J. Nutr. 137:2304-2310, 2007.
To read the abstract, go to Bioavailability Dietary Iron is Associated with Hemoglobin Concentration in Mexican Preschool Children.
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