Title: Vegetarianism and protein deficiency
Key words: protein intake, recommended nutrient intake, RNI, protein balance, vegetarians, protein status, omnivores, iron, zinc, deficiency, wheat, cereals, lysine, leucine, homeostasis, amino acids, oxidation, bioavailability, microbial fixation, protein metabolism, gene transcription, diurnal, post-prandial, protein assimilation,
Date: Sept 2006
Author: Morgan, G
Vegetarianism and protein deficiency
Although protein intakes have remained remarkably constant over the years and average UK consumption remains above the recommended nutrient intake (RNI) to maintain protein balance (MAFF 1991a, Gregory 1990), recent revisions of the 1985 FAO/UN/WHO recommendations, based on Rose’s early seminal nitrogen balance studies (Rose 1957), have called into question whether certain groups such as vegetarians are now able to meet the new revised RNI’s for protein intake. Thus, although the average protein intake in the UK, at 1.5 g/kg/day, remains well above the established RNI of 0.75 g/kg/day, it is estimated that 20% of such men and 32% of women still fall below the minimum RNI (Millward 2002).
Concern over the protein status of the vegetarian diet has focused on both its quantitive and qualitative aspects. It is well known that vegetarian’s protein and energy consumption is on average less than that of omnivores and that such diets are associated with iron, zinc and other deficiencies (Donovan 1996). In addition, wheat and other cereals are deficient in the indispensable amino acids lysine and leucine, leading to fears that their intake might be seriously compromised in vegetarians. In the case of lysine, revisions to Rose’s original RNI estimate of 12 mg/day by statistical reanalysis and using isotope tracer techniques have yielded results ranging from 19 mgs/day to 30 mgs/day (e.g. Millward 1999, Kurpad 2001), most studies being at the higher end of this range and fuelling speculation that vegetarians might not be able to meet these targets.
Against a background where long-term studies of protein homeostasis have not been carried out and clinical markers of individual amino acid deficiencies have not been identified, it is important to make the following observations:
1) Cross-culturally, protein requirements for populations in nitrogen balance range from some 0.4 to 1.1 g. protein/kg/day (FAO/UN/WHO 2002). This to a large extent reflects dietary adaptation to differing patterns of protein consumption, excessive intake of protein in, for example, the American population being balanced by increased amino acid oxidation. The FAO/UN/WHO estimate of a recommended intake of 0.6 g. protein/kg/ day (+25% as a safe margin of error) amounted to a cautious statistical estimate based on a biased data base and took no account of adaptation occurring at lower protein intakes (Millward 2002).
2) The bioavailability of individual amino acids such as lysine and leucine have not been exhaustively described. It is known that lycine, for example, is more bioavailable, being less oxidised post-prandially than other amino acids (Millward 1995). It has also been shown that endogenous production of lysine by microbial fixation of non-protein nitrogen in the gut occurs (Metges 1999). Leucine, a key amino acid modulating protein metabolism by gene transcription, has also been shown to exhibit variable bio-availability (Beaufrere 2000).
3) Diurnal and post-prandial factors have been shown to influence protein metabolism (Millward 1995). One study (El-Khoury 1995) showed that in humans giving hourly protein portions rather than three times a day increased the rate of protein deposition. Another study in the elderly (Arnal 1999) showed that protein assimilation was higher when 80% of the daily protein intake was given at midday rather than spread between four meals.
The above facts highlight the difficulty of making prognostications regarding the adequacy of a vegetarian diet. Vegetarians as a group tend to be more health conscious than their meat-eating cousins and, as a consequence, intakes of fats, carbohydrates, food additives and other micro- and macro-nutrients would be different in such a group. How these factors interact and effect protein metabolism has yet to be determined. Given the present re-evaluation of protein requirements, however, for vegetarians such as adolescent girls on restricted diets and thought to be at risk of protein deficiency, it would seem prudent at this stage for nutritionists to make additional dietary recommendations for such groups. Further research targeting such groups, would also help to provide a greater understanding of protein metabolism.
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