Are recommended daily allowances for vitamin C adequate?

Area of Science:

• Biochemistry
• Human Physiology
• Nutritional Science

Background:

• Recommended daily allowances (RDAs) for ascorbic acid (Vitamin C) in the U.S.A. significantly exceed the minimum daily requirement of approximately 10 mg needed to prevent clinical deficiency symptoms in adults.
• RDAs are established to achieve tissue saturation, leading to prompt urinary excretion of excess amounts.
• Previous research indicates that responses to ascorbic acid may not occur at intakes below RDAs but above the threshold for preventing deficiency symptoms.

Purpose of the Study:

• To investigate the rationale behind current Recommended Daily Allowances (RDAs) for ascorbic acid (Vitamin C).
• To explore the discrepancy between minimum requirements for preventing deficiency and levels needed for tissue saturation.
• To analyze the limitations of using animal models, like guinea pigs, for extrapolating human ascorbic acid requirements.

Main Methods:

• Review of existing literature on ascorbic acid metabolism and requirements.
• Analysis of data comparing clinical deficiency thresholds with tissue saturation levels.
• Comparative assessment of metabolic rates between humans and guinea pigs concerning ascorbic acid.

Main Results:

• Ascorbic acid RDAs are set higher than the minimum 10 mg required to prevent clinical deficiency in adults.
• Tissue saturation, targeted by RDAs, results in rapid excretion of excess ascorbic acid.
• Human ascorbic acid requirements cannot be accurately extrapolated from guinea pig data due to metabolic rate differences.

Conclusions:

• The current RDAs for ascorbic acid are designed to achieve tissue saturation, not merely prevent deficiency.
• Extrapolation of ascorbic acid needs from smaller animals like guinea pigs is scientifically invalid due to metabolic scaling differences.
• The evolutionary loss of ascorbic acid synthesis in humans may be a result of neutral genetic drift.