HSA | FSA CARDS ARE NOW ACCEPTED

Last reviewed: May 12, 2026 Last updated: May 12, 2026

Written by: Jay Hastings , CEO of PlexusDx

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance. His work has included scaling healthcare startups, leading CLIA lab integrations, and helping expand consumer access to precision health tools.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Science Liaison with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

How SLC23A1 and Vitamin C Work Together

Vitamin C is an essential nutrient that supports immune function, acts as an antioxidant, helps make collagen, and improves iron absorption from plant foods. Because humans cannot make Vitamin C, we must get it from food or supplements. The SLC23A1 gene makes a protein called SVCT1 that plays a central role in getting Vitamin C into the body. SVCT1 helps absorb Vitamin C across the intestinal wall and reabsorb it in the kidneys to prevent loss through urine. Variations in SLC23A1 can change how well SVCT1 works, which may affect blood levels of Vitamin C and how much is available to important tissues like the brain, muscles, and adrenal glands.

Why this matters for health

  • Lower Vitamin C availability can reduce antioxidant protection and slow collagen production.
  • It may affect immune response and recovery from injury.
  • Certain life factors such as smoking, chronic stress, inflammation, and some medications increase Vitamin C need.
  • Knowing your SLC23A1 genotype can help tailor dietary, supplement, and monitoring choices to maintain healthy Vitamin C status.

Personalized Guidance Based on rs33972313 Genotype

Below are clear, genotype-specific interpretations and practical recommendations. Click each section to expand details for 2 effect alleles, 1 effect allele, and 0 effect alleles.

2 effect alleles (TT) — Higher likelihood of lower plasma Vitamin C

What this means

If your genotype is TT at rs33972313, you carry two copies of the effect allele associated with lower plasma Vitamin C. SVCT1 may function less efficiently, so your intestines may absorb less Vitamin C and your kidneys may reabsorb less, resulting in greater urinary loss. Even with a balanced diet, blood Vitamin C levels may be lower than average.

Diet and food tips

  • Eat Vitamin C rich foods daily: citrus fruits, strawberries, kiwi, bell peppers, broccoli, Brussels sprouts, and leafy greens.
  • Include a source of Vitamin C with plant-based iron sources to improve iron absorption, for example spinach with orange slices or bell peppers with lentils.
  • Favor raw or lightly cooked vegetables to preserve Vitamin C, which is heat sensitive.
  • Spread Vitamin C containing foods over the day rather than a single large serving to support steady absorption.

Supplement considerations

  • Consider a daily Vitamin C supplement if diet alone may not meet needs; common supplemental amounts range from 100 to 500 mg per day for maintenance, adjusted by individual needs.
  • If you smoke, have chronic disease, are under high stress, or have higher body weight, higher supplementation may be considered after discussing with your healthcare provider.
  • Divide larger doses into two sittings to reduce urinary loss and support continuous tissue availability.

Lifestyle recommendations

  • Quit smoking or reduce exposure to tobacco smoke, which increases Vitamin C requirements.
  • Manage chronic stress through sleep, exercise, and relaxation techniques since stress can deplete Vitamin C.
  • Maintain healthy body weight and regular physical activity to support overall nutrient handling.

Suggested tests and monitoring

  • Discuss a plasma Vitamin C test with your healthcare provider if you have symptoms of deficiency, chronic illness, or are concerned about low levels.
  • Monitor dietary intake and recheck levels if you start supplementation or make major lifestyle changes.
1 effect allele (CT) — Likely lower plasma Vitamin C

What this means

If your genotype is CT at rs33972313, you carry one copy of the effect allele. This genotype is associated with a likelihood of lower plasma Vitamin C compared to people with no effect alleles. SVCT1 may be somewhat less efficient at transporting Vitamin C, which may reduce retained Vitamin C from the diet.

Diet and food tips

  • Prioritize a variety of Vitamin C sources every day: citrus, kiwi, berries, bell peppers, tomatoes, sweet potatoes, and dark leafy greens.
  • Include Vitamin C with meals that contain plant-based iron to improve iron uptake.
  • Choose fresh produce when possible and limit long cooking times to preserve Vitamin C content.

Supplement considerations

  • If dietary intake is inconsistent, consider a daily supplement of 100 to 250 mg of Vitamin C. Higher doses may be used short term under medical advice.
  • For ongoing supplementation, lower divided doses may help maintain steady blood levels and reduce urinary loss.

Lifestyle recommendations

  • Avoid smoking and limit alcohol, which can both increase Vitamin C needs.
  • Prioritize sleep, stress reduction, and consistent physical activity.

Suggested tests and monitoring

  • Consider testing plasma Vitamin C if you have risk factors for low Vitamin C or symptoms such as easy bruising, slow wound healing, or fatigue.
0 effect alleles (CC) — Typical Vitamin C absorption and retention

What this means

If your genotype is CC at rs33972313, you carry two copies of the non-effect allele. SVCT1 is expected to function normally, supporting typical Vitamin C absorption in the intestines and reabsorption in the kidneys. Your genetic capacity to transport and retain Vitamin C is considered standard.

Diet and food tips

  • Continue eating a variety of Vitamin C rich foods regularly: citrus fruit, berries, peppers, cruciferous vegetables, and leafy greens.
  • Use Vitamin C rich foods to boost iron absorption from plant-based meals.

Supplement considerations

  • Most people with a balanced diet do not need routine supplementation, but short term supplements can be useful during illness or increased need periods.

Lifestyle recommendations

  • Maintain a healthy lifestyle including not smoking, managing stress, and regular exercise to support nutrient needs.

Suggested tests and monitoring

  • Routine testing is not typically required unless you have symptoms, dietary restrictions, or risk factors that could reduce Vitamin C levels.

Practical next steps

  • Review your typical daily diet and look for consistent sources of Vitamin C across the day.
  • If you suspect low intake or have risk factors, discuss a plasma Vitamin C test with your healthcare provider.
  • Consider a modest daily supplement if diet is inadequate or if you have the CT or TT genotype and additional risk factors.
  • Address modifiable lifestyle factors like smoking, poor sleep, and chronic stress that increase Vitamin C requirements.

PlexusDx does not provide medical advice. This information is educational and about genetic predispositions only. Always consult your healthcare provider before changing your diet, starting supplements, or ordering health tests. Your provider can help integrate genetic information with your personal medical history, current medications, and overall health goals.

If this genetic variant is present in your PlexusDx results, the following tests and reports are commonly used to explore it further:

🧬 Genetic Tests:

🧪 Blood Tests:

📄 Genetic Report:

Frequently Asked Questions About Vitamin C and SLC23A1 rs33972313

How does the SLC23A1 gene affect my Vitamin C levels?

The SLC23A1 gene helps produce SVCT1, a transporter that absorbs Vitamin C from the intestines and reabsorbs it in the kidneys to reduce urinary loss. If you have variations in SLC23A1, SVCT1 may work less efficiently, which can lower blood Vitamin C availability and reduce how much reaches key tissues such as the brain, muscles, and adrenal glands.

What does the rs33972313 genotype mean for plasma Vitamin C?

At rs33972313, your genotype can influence the likelihood of lower plasma Vitamin C: TT (two effect alleles) is linked to a higher likelihood of lower Vitamin C due to reduced SVCT1 performance, CT (one effect allele) is also associated with a likelihood of lower Vitamin C compared with no effect alleles, and CC (no effect alleles) is expected to support typical Vitamin C absorption and retention.

What should I do nutritionally if I have the CT or TT rs33972313 genotype?

If you have CT or TT, consider prioritizing Vitamin C–rich foods daily (citrus, strawberries, kiwi, bell peppers, broccoli, Brussels sprouts, and leafy greens) and spread intake across the day to support steady absorption. Pair Vitamin C with plant-based iron sources to improve iron uptake, choose raw or lightly cooked produce to preserve Vitamin C, and if diet may be insufficient, consider a daily supplement (often 100–500 mg depending on needs). Lifestyle factors that increase Vitamin C requirements—especially smoking, chronic stress, inflammation, and certain medications—should be addressed, and a plasma Vitamin C test can be discussed with your healthcare provider if you have symptoms or risk factors.

What tests can help me learn more about Vitamin C and SLC23A1 rs33972313?

The Optimal Diet and Weight Loss Genetic Test delivers over 295 genetic insights related to nutrition response, body composition, metabolism, and fitness. The Diet and Nutrition Genetic Report translates your results into personalized, actionable guidance. Your healthcare provider can also recommend targeted blood tests based on your specific pathway results and health history to complement your genetic insights with current biomarker data.

Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance. References are included at the end of the article when scientific, medical, or health-related claims are discussed.

Commercial transparency: PlexusDx offers genetic testing, blood biomarker testing, personalized supplement recommendations, and related precision wellness services. Product mentions are intended to help readers understand available options and should not be interpreted as medical advice.

Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.