Saturated fat opening

• Saturated fat has a straight carbon backbone, packs together tightly, and is usually solid at New Zealand room temperature; polyunsaturated fat has double bonds that create more exposure and fragility and allow easier oxidation.
• About 16 meta-analyses tested randomized trials swapping saturated fat for polyunsaturated fat; 15 showed no effect from reducing saturated fat on all-cause mortality, cardiovascular mortality, or cardiovascular events, while the Hooper Cochrane review showed no mortality effect but did show an effect on cardiovascular events. [1]
• The raw data behind that review contained about 988 cardiovascular events among people who reduced saturated fat and 986 among those who did not; the positive meta-analytic result came from random-effects methods overweighting small, bias-prone studies.
• Simon Thornley's 2019 inverse-heterogeneity reanalysis found no effect, and a Lancet publication said dietary guidelines should be changed. [2][3]

Why sunshine and vitamin D became the focus

• New Zealand has the highest per-capita rate of melanoma and fatal melanoma in the world, ahead of Australia, and getting sunburnt and overexposed to UVB is a public-health risk.
• In the low-carb and keto community, especially with strict keto, carnivore, or longer fasting, people somehow seem to improve their sunburn resistance; there is surprisingly little data, so the later evidence is mainly mechanistic and animal work.
• Age compounds the vitamin D problem: the ability to produce vitamin D from the same sun exposure falls about 13% per decade, so at 70 it is about half as effective as in a younger person. [4]
• Skin colour, ancestry, latitude, and season therefore matter: an Indian living in Melbourne and an Irishman living in Cairns should behave very differently in the sun, with phenotypically appropriate latitude affecting what is suitable.

Vitamin D metabolism and supplementation

• Vitamin D3 is hydroxylated in the liver to calcifediol, which is what the usual blood test measures, and the kidney then processes it to calcitriol, the active form used throughout the body, including glucose uptake and bone health.
• Chronic kidney disease can block the kidney step and lead to calcitriol use as a medicine; fatty liver disease and metabolic syndrome impair the earlier D3-to-calcifediol hydroxylation step, helping explain disappointing vitamin D trials.
• Low vitamin D is implicated in many chronic diseases and bone health, and Australia and New Zealand have very low vitamin D levels, which may speak to the success of SunSmart campaigns; some groups, such as obese Korean boys and older Middle Eastern women, are essentially universally deficient.
• Supplementation works at quite high levels: a single 50,000-IU dose in a week improves vitamin D status, and a single high dose after excess sun exposure can mitigate many negative aspects of experimental sunburn, although the practical instruction remains not to get sunburnt. [5]
• People with obesity have received doses up to 500,000 IU a day without toxicity in this example; that dose is not recommended here, but supplementation can probably go higher than the usual 1,000-2,000 IU while toxicity is monitored.
• Calcifediol bypasses the liver step and is much more effective than normal D3 in this comparison: among frankly deficient people with metabolic syndrome, some became sufficient after one month while none on normal D3 did, and after three months more than a third were sufficient versus about 8% on normal D3. [6]

Immune function and metabolic trials

• With sufficient or optimal vitamin D, the chance of viral or bacterial infection is about half as high and recovery is about twice as fast, and supplementation trials in children improve this as well.
• In the Spanish severe-COVID trial, 76 patients were randomized so that 50 received calcifediol and 26 received standard care; 1 of 50 in the calcifediol group went to ICU versus 13 of 26 in the other group. The trial is small, but the effect size is astonishing. [7]
• A recent meta-analysis of randomized vitamin D supplementation trials across metabolic health shows no change in weight, but small effects on blood pressure, fasting glucose, fasting insulin, and CRP; the effects are encouraging but small, with high doses around 4,000-5,000 IU and low doses around 1,000 IU. [8]
• Adding vitamin D during weight loss shows no overall effect, but subgroup analysis shows more weight loss when someone starts frankly deficient and supplementation actually moves them into sufficiency. [9]
• A cancer trial shows a small favorable difference in cancer mortality that is not statistically significant; benefit is still plausible and dosing may be part of the problem. [10]

Sunlight, skin lipids, and dietary fat

• Keratinocytes have phospholipid bilayers, while ceramides fill the spaces between cells, waterproof the skin, and undergo a hormetic response to UV; sunlight helps put ceramides into the right chemical order and remove the ones no longer working, so sun exposure is needed to maintain that layer.
• What is eaten changes the phospholipid bilayer: more polyunsaturated fat puts more polyunsaturated fat into it, more saturated fat puts more saturated fat into it, and changing the omega-3-to-omega-6 ratio changes every cell's barrier; both saturated fats and essential polyunsaturated fats are needed, and changing the bilayer can change the cell's fragility or robustness.
• Cholesterol is also required in these bilayers, particularly in skin cells, where it confers heat and UVB resistance; cholesterol is also the beginning molecule needed to make D3, so there is no reason to avoid it here.
• Mechanistically and practically, avoiding industrial seed oils is a good idea even though the bigger epidemiology is not overwhelming; in the Health Professionals Follow-up Study and Nurses' Health Study, seed oils have an effect on skin cancer and skin-cancer mortality, with effects that are not large in these confounded cohorts. [11]
• In the 1988 hairless-mouse UV study, 5% and 20% polyunsaturated-fat groups developed skin cancers similarly, while the saturated-fat group stayed on the bottom line and none developed skin cancers; when polyunsaturated fat was introduced, the cancers caught up. This does not mean butter stops human skin cancer, but polyunsaturated fats are implicated in the mechanism. [12]
• In another hairless-mouse UVB study, higher-omega-3 and higher-omega-6 groups stayed the same weight, but by the end only about 50% of the higher-omega-3 group had developed skin cancer versus everyone in the higher-omega-6 group; tumor number and size also differed substantially, and the protected mice had more omega-3 in their skin phospholipid bilayers. [13]

Practical closing points

• Vitamin D is a crucial part of metabolic health, but liver and kidney dysfunction create a catch-22 around activation; hydroxylated D3 forms may help bypass this, while the K2 and magnesium feeding studies did not show enough effect to make those cofactors worthwhile.
• Skin health is probably important and diet probably affects it; if there were ever a reason to stay away from polyunsaturated seed oils, the mouse evidence would be it, but none of this changes the instruction not to get burnt.
• An Australian rub-in sunscreen used at the surf club once tested in a consumer magazine as having essentially no sun-protection effect; that product issue has since been resolved and it does stop burning, but it raises the question of absorbing UV-filter chemicals through active skin.
• A recent review finds roughly 18 sunscreen chemicals in tissues where they do not normally belong and implicates them in harm in both animal and human studies, particularly endocrine disruption: low testosterone in teenage boys, low sperm count and motility in males, and estrogen and other sex-hormone disruption in women. Birth-weight and IQ changes were both seen in mice, with some evidence in humans where more work needs to be done. [14]
• To avoid burning, use a shirt or hat or use a sunscreen that does not rub in; zinc oxide or titanium oxide are the appropriate examples because they do not get absorbed.

References

1. [01:49] Reduction in saturated fat intake for cardiovascular disease — https://doi.org/10.1002/14651858.CD011737
2. [03:15] How reliable is the statistical evidence for limiting saturated fat intake? A fresh look at the influential Hooper meta-analysis — https://doi.org/10.1111/imj.14325
3. [03:41] Dietary guidelines are not beyond criticism — https://doi.org/10.1016/S0140-6736(17)30278-7
4. [08:33] Vitamin D Synthesis Following a Single Bout of Sun Exposure in Older and Younger Men and Women — https://doi.org/10.3390/nu12082237
5. [15:58] Oral Vitamin D Rapidly Attenuates Inflammation from Sunburn: An Interventional Study — https://doi.org/10.1016/j.jid.2017.04.040
6. [17:18] Calcifediol is superior to cholecalciferol in improving vitamin D status in postmenopausal women: a randomized trial — https://doi.org/10.1002/jbmr.4387
7. [18:37] Effect of calcifediol treatment and best available therapy versus best available therapy on intensive care unit admission and mortality among patients hospitalized for COVID-19: A pilot randomized clinical study — https://doi.org/10.1016/j.jsbmb.2020.105751
8. [20:38] The impacts of vitamin D supplementation in adults with metabolic syndrome: A systematic review and meta-analysis of randomized controlled trials — https://doi.org/10.3389/fphar.2022.1033026
9. [21:42] Vitamin D3 supplementation during weight loss: a double-blind randomized controlled trial — https://doi.org/10.3945/ajcn.113.073734
10. [22:13] Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease — https://doi.org/10.1056/NEJMoa1809944
11. [27:57] Fat Intake and Risk of Skin Cancer in U.S. Adults — https://doi.org/10.1158/1055-9965.EPI-17-0782
12. [28:35] Effect of dietary lipid on UV light carcinogenesis in the hairless mouse — https://doi.org/10.1111/j.1751-1097.1988.tb02882.x
13. [31:09] Effects of high-fat diets rich in either omega-3 or omega-6 fatty acids on UVB-induced skin carcinogenesis in SKH-1 mice — https://doi.org/10.1093/carcin/bgr074
14. [34:13] Endocrine and Reproductive Health Considerations of Sunscreen UV Filters: Insights from a Comprehensive Review 2014-2024 — https://doi.org/10.1007/s40572-025-00492-9