Issue 20: Should You Take Vitamin D?

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Welcome to Weekly.health’s 20th issue. Every week, we explore cutting-edge research, actionable advice, and science-backed nutrition insights to help you live longer and healthier.

Our aim is to help you live another 10 healthy years and cut your risk of age-related disease.

🗒️ In This Issue:

• 📚 Books We’re Reading Right Now

• 📈 Research Digest: What’s new in nutrition science

• 💊 Supplement Deep Dive: Vitamin D3

• 🛍️ What We Suggest

• ❤️ Support Weekly.health: Help us keep the research flowing

• 📘 Glossary of Terms: Definitions for technical terms in this issue

📚 Books We’re Reading Right Now

📈 Research Digest: What’s New in Nutrition Science

Here’s the best of recent nutrition research:

🍎 Short, fun lessons can shift kids’ food choices
In Mexican 9–12-year-olds, a 14-week school programme with weekly 30-minute sessions on healthy eating, emotions, and body image led to a small but measurable rise in fruit and dairy intake (effect size ~0.2–0.24) but no change in weight or mood. For real impact, pair school or home-taught lessons with active play and healthier options at home and in lunchboxes. (source)

✨ Eat your way to younger-looking skin
This 2025 meta-analysis of 61 studies found that specific nutrients can visibly slow skin ageing. Collagen reduced wrinkles and lightened pigment spots, while lipids and fatty acids improved elasticity and hydration. Polyphenols (from tea, cocoa, and berries) strengthened the skin barrier, and carotenoids (from tomatoes and mangoes) reduced redness. For supple, glowing skin, combine colourful fruits, oily fish, and collagen-rich foods daily. (source)

🍇 Dealcoholised red wine boosts antioxidants without the hangover
A meta-analysis of 36 trials found that dealcoholised red wine increases blood antioxidant levels by 72% and boosts gut microbiome diversity compared to water. However, it’s slightly less effective for gut health than alcoholic red wine—and offers none of the risks linked to alcohol. In short, you can sip your polyphenols and skip the hangover with dealcoholised reds. (source)

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💊 Supplement Deep Dive: Vitamin D3

Should you be taking the sunshine vitamin this winter?

1-minute verdict

Try for most adults in winter. From October to March, sunlight is usually too weak to maintain vitamin D status. A simple daily dose of 25–50 µg (1,000–2,000 IU) of vitamin D3 (cholecalciferol) is a practical baseline.

• Outcomes that matter: reduces risk of deficiency; supports bone health; plausible benefits for muscle function and mood.

• Time to benefit: allow 8–12 weeks for blood levels to stabilise.

• Who might skip or adjust: if you have hypercalcaemia, sarcoidosis, certain kidney or parathyroid disorders, or you take medicines that affect calcium (e.g., thiazide diuretics), speak to your doctor first. Pregnant or breastfeeding? Use products suitable for pregnancy and follow medical advice.

• Upper limit: do not exceed 100 µg (4,000 IU) per day unless advised by a clinician.

Does it actually do anything?

Evidence shows that Vitamin D is synthesised solely in the skin when exposed to UV-B rays from sunlight. Those of us who avoid the sun and use sunscreen daily are at highest risk of deficiency, but almost everyone living above 37° N latitude will not maintain adequate Vitamin D levels in the winter.

This means that if you are in the UK, Ireland, most of Europe (France, Germany, most of Italy, northern Spain), eastern Europe, every US state north of the northern border of Texas, or anywhere else that is further north - you may become deficient over winter.

Taking Vitamin D3 supplements does increase blood levels of the hormone, and this appears to improve the absorption of calcium which contributes to healthy bones. The evidence suggests that taking a combined Vitamin D3 and Vitamin K supplement may be slightly more beneficial.

Deficiency can present as bone pain, muscle weakness and aches, fatigue, and increased risk of fractures. In children it can caused rickets and bone deformities.

Evidence is mixed for immunity. A 2017 meta-analysis suggested a small reduction in acute respiratory infections with vitamin D (strongest if deficient), but a 2025 update found no significant overall protection. (source) Treat deficiency for musculoskeletal reasons; don’t rely on vitamin D to prevent colds.

Those deficient in Vitamin D may also experience low mood and energy, although the evidence is mixed for this and difficult to measure accurately. Several meta-analyses suggest small improvements in depressive symptoms.

There are also many anecdotal reports online of improved mood with supplementation, but we suggest to treat these reports with caution, especially if they are attached to a particular branded product.

Dose & form

• Daily dose: 25–50 µg (1,000–2,000 IU) D3 as a baseline or up to double this (but no more than double unless directed by a clinician).

• Form: D3 (cholecalciferol) is preferred; vegan D3 from lichen is widely available. If D3 is not specified on the label, it could be D2, which has poorer absorption.

• With food? Absorption is a little better with a meal containing fat, but timing is flexible if you take it consistently.

• Testing: If you’ve had deficiency before, consider a 25-OH vitamin D blood test after 8–12 weeks.

Our picks - what to buy

Stack sensibly (optional)

• D3 + K2 (MK-7, 90–200 µg/day): Some readers might prefer pairing, especially if calcium intake is high. Evidence on hard outcomes is still developing. Do not combine with vitamin K if you use vitamin K-antagonist anticoagulants unless your clinician agrees.

• Bone basics: Aim for 700–1,000 mg/day dietary calcium (from food if possible), resistance training 2–3×/week, and regular walking.

Mechanism minute (for the curious)

D3 → liver → 25-hydroxyvitamin D (the blood marker) → kidney and other tissues → 1,25-dihydroxyvitamin D (active form). The active hormone binds the vitamin D receptor (VDR) and increases intestinal calcium and phosphate absorption. That’s the practical reason status matters in winter.

Reader lab (tell us what you do)

Safety & cautions

• Do not exceed 100 µg (4,000 IU) per day without medical advice.

• Stop and speak to a clinician if you develop symptoms of high calcium (nausea, constipation, unusual thirst, confusion).

• Check interactions if you take thiazide diuretics, orlistat, antiepileptics, or vitamin K-antagonist anticoagulants.

🛍️ What we suggest

Discover the products, services, and retailers we’ve mentioned in past Weekly Health issues, all in one place so you can explore at your own pace.

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🙏 Thanks for Reading!

That’s the end of this issue of Weekly.health.

This newsletter is written by a small, independent team, led by James — who’s been following nutrition science for nearly 20 years and is now working towards a formal, industry-recognised qualification.

We’re based in England, so if you’re over the pond, you might notice a few strange spellings.

Our goal is to make cutting-edge, evidence-based nutrition advice simple, useful, and genuinely applicable to everyday life.

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We’ll keep improving with every issue. If you’ve got any feedback or suggestions, we’d love to hear them (just reply to this email).

See you next week!

📖 Glossary of Terms in This Issue (Alphabetical Order)

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