The Study On Dietary Reference Intakes Of Protein For Chinese Female Youths

1.BackgroundProtein is indispensable for human and is the base of life.Both scarce and excessive intakes of protein are harmful to health.Chinese present protein reference intakes set the intake at 65g/d for female youths at light physical activity level.However,the results of The Nutrition and Health Status Survey of Chinese Residents in 2002 pointed out that the actual protein intake of Chinese reference man was only 90%of RNI at most,and on the whole,there was no protein malnutrition in Chinese adults. The results indicate that Chinese present protein reference intakes are probably on the high side.If that is true,it will not be favor of reasonable dietary intake and food protein production in Chinese.Therefore,to study the protein requirement of Chinese residents under current economic situation and dietary mode,the present research applied stable isotope tracer technique to explore the protein physiological requirement of Chinese female youths.There are two approaches to administrate tracer amino acids-venous administration and oral administration.The former is invasive to subjects and complicated to operate,while the latter is safe and easy to applicate and the metabolism of tracer amino acids resembles physiological states inside.Therefore,the present study applied two stable isotope-labling amino acids to explore the protein requirement of Chinese young women by oral administration.2.MethodsThe present study designed six dietary protein levels:0.70g/kg/d,0.78g/kg/d, 0.86g/kg/d,0.94g/kg/d,1.02g/kg/d,1.10g/kg/d,including three consecutive periods, seven days each.Twenty healthy young women participated in the experiment.They were divided evenly into 2 groups,receiving 3 dietary protein levels each.The first day to the sixth day of each period was adaptation days,and the seventh day was the stable isotop day.Each subject was given ¹³C-leucine and ¹⁵N-lysine oral dose in feeding state to trace the metabolism of the two amino acids in body.The end-products of the two amino acids were ¹³CO₂ and ¹⁵N-urea respectively,excreted mainly through expiration and urine.Breath,blood and urine samples were collected, enrichment of stable isotopes in samples was measured,and the rate of endproduction of tracer lecuine,flux and rate of tracer oxidation at different dietary protein levels were caculated according to given formula.Breakpoint analysis of tracer oxidation rate at six dietary protein levels using a two-phase linear crossover model was used to calculate protein requirement.The experimental diet was designed according to Chinese traditional diet, providing accurate protein level to ensure expected protein intake and sufficient nutrient supply.The nutrient components of diet were measured to know the subjects situation of nutrition and energy.Daily physical activity level of each subject was controlled through pedometer and weight was measured to observe the energy equilibrium of subjects.3.Results and Conclusions(1).The actual dietary protein intake levels were:0.80g/kg/d,0.88 g/kg/d,0.94 g/kg/d,1.00 g/kg/d,1.08 g/kg/d,1.19 g/kg/d.The protein levels were quantitively different and could meet the study needs.Three macro nutrients of experimental diets provided reasonable ratio of total energy,good-quality protein had an appropriate proportion,during the whole experiment all subjects had unchanged weights.(2).The metabolism of both leucine and lysine could reach steady state 210min after the initial dose of tracer through oral administration of isotope tracers every 20min in feeding state,and the steady state could last for 30min after the end of tracer administration.(3).The protein physiological requirement resulted from the rate of ¹³CO₂ production of leucine is 1.04g/kg/d,the value resulted from the oxidation rate of leucine is 1.00g/kg/d.(4).The protein physiological requirement resulted from the oxidation rate of lysine is 0.95g/kg/d.(5).Considering the results of both leucine and lysine and population safety,we decided the maximum 1.04g/kg/d as the average protein physiological requirement, which was plused double standard deviation to get the recommended protein intake of 1.06g/kg/d.For the average weight of Chinese young women is 55.0kg,we proposed the protein reference intake as 60g/d for Chinese female youths at light physical activity level.The results of both The Nutrition and Health Status Survey of Chinese Residents in 2002 and the present study indicate that the present protein reference intake for Chinese female youths is on the high side.