Vitamin D and Northern New England: The Complete Picture

Stand on a dock in Eastport in January and watch the sun. It comes up late, slides along the southern horizon without ever climbing very high, and quits by mid-afternoon. That low arc is not just a matter of short days and cold toes. For roughly four months of the year, the angle of that sun is the reason your skin makes almost no vitamin D, no matter how many cloudless afternoons you spend outside.

Vitamin D has become one of the most tested, most supplemented, and most argued-about nutrients in American medicine. For people living in Maine and most of New Hampshire, the conversation carries extra weight, because geography stacks the deck against us every winter. Here is what the research actually establishes, where it gets murky, and why the latitude printed on a map matters for what is in your blood.

What vitamin D is, and the two ways you get it

Vitamin D is a fat-soluble vitamin that acts more like a hormone once your body processes it. Its best-known job is helping the gut absorb calcium and keeping bones from going soft and brittle. It also shows up in immune cells, muscle, and brain tissue, which is part of why researchers keep poking at it for effects well beyond the skeleton.

You get it two ways. The first is sunlight: ultraviolet B rays hit a cholesterol-derived compound in your skin and kick off a chain reaction that ends in usable vitamin D. The second is what you eat or swallow as a pill. For most of human history in northern places, the summer sun did the heavy lifting and the body coasted on stored reserves through the dark months. That arrangement works until the latitude gets high enough that the summer surplus runs short.

The 44-to-45-degrees-north problem

This is where Maine and New Hampshire become a case study rather than a footnote. The classic experiment came from Michael Holick’s lab in 1988. Webb, Kline, and Holick set out tubes of 7-dehydrocholesterol, the skin compound that converts to vitamin D, and exposed them to real winter sunlight at different latitudes. In Boston, at about 42 degrees north, the skin made no usable previtamin D3 from November through February. Farther north in Edmonton, at 52 degrees, the dead zone stretched from October through March. Cities at lower latitudes kept producing vitamin D right through midwinter. The authors gave the phenomenon a name that stuck: the vitamin D winter.

Now look at a map. Maine and most of New Hampshire sit between roughly 43 and 47 degrees north, all of it above Boston. The sun in December is too low and the atmosphere it passes through too thick for the right wavelengths to reach the ground in any useful amount. You can ski all day at Sugarloaf under a brilliant sky and make essentially zero vitamin D from it. This is not about sunscreen or cloud cover or staying indoors. It is about the geometry of the planet, and you cannot supplement your way around it with more sunshine. For a closer look at the mechanism, our piece on why latitude shuts off winter vitamin D production walks through the UVB angle in detail.

How low Mainers actually go

The most direct local evidence comes from the University of Maine. Between 2000 and 2003, researchers tracked 23 girls aged 9 to 11 in the Orono and Bangor area, latitude about 44 degrees north. Their average blood level of 25-hydroxyvitamin D fell 28 percent between September and March. Eleven of the 23, just under half, dropped into the insufficient range at least once during the year. Four of them were insufficient in both September and March, meaning they never fully recovered even after a Maine summer (Sullivan et al., Journal of the American Dietetic Association, 2005).

That is a small study of one age group in one corner of the state, so read it as a flag, not a census. But it lines up with the national picture. A 2022 analysis in Frontiers in Nutrition pooled blood samples from nearly 72,000 Americans collected between 2001 and 2018. About 22 percent had moderate deficiency and another 41 percent fell into the insufficient range. The seasonal swing was stark: moderate deficiency ran 28.9 percent in winter versus 17.0 percent in summer, and severe deficiency more than doubled in the cold months. Maine and New Hampshire, sitting where they do, land on the rough end of that seasonal curve.

What the blood test measures

If you have ever had your vitamin D checked, the lab drew blood and measured 25-hydroxyvitamin D, abbreviated 25(OH)D. The NIH considers it the best single marker of your overall vitamin D status, because it reflects both what you eat and what your skin makes, and it sticks around in the blood long enough to give a stable reading.

The numbers come in two flavors of units, which causes endless confusion. American labs usually report nanograms per milliliter (ng/mL). Research papers and much of the rest of the world use nanomoles per liter (nmol/L). One ng/mL equals 2.5 nmol/L. The NIH Office of Dietary Supplements describes a level of 20 ng/mL (50 nmol/L) or higher as adequate for most people, below 12 ng/mL (30 nmol/L) as too low, and above 50 ng/mL (125 nmol/L) as possibly too high.

A word of caution before you fixate on a single figure. “Adequate” here means adequate for bone health and general wellbeing in most people, not a target everyone needs to chase to the decimal point. If you want the full breakdown of what each range means and who actually benefits from testing, we cover that in getting tested for vitamin D and what the numbers mean.

Bones, immunity, mood: what the evidence supports

Vitamin D’s effect on bone is the part nobody really disputes. Too little of it, sustained over time, leads to weaker bones, and the official intake recommendations exist mostly to protect skeletal health. The recommended dietary allowance is 600 IU per day for most children and adults, rising to 800 IU for adults over 70, figures that trace back to the Institute of Medicine.

Beyond bone, the picture gets more interesting and more contested. Take respiratory infections. In 2017, a team led by Adrian Martineau pooled data from 25 randomized trials covering more than 11,000 people. Vitamin D supplementation cut the risk of acute respiratory infection by a modest amount overall. The benefit clustered in two groups: people who took it daily or weekly rather than in occasional big doses, and people who were genuinely deficient to begin with. For those starting out very low, the protective effect was substantial. For everyone else, it was small. That is a useful pattern to remember about this nutrient. It tends to help the people who are short on it and do little for the people who already have enough.

Mood is the newest frontier. A 2026 dose-response meta-analysis in Frontiers in Nutrition pooled 15 randomized trials of 962 people who had been diagnosed with depression. Supplementation improved their symptoms, with the largest effect around 5,000 IU per day, alongside measurable drops in two markers tied to inflammation and bone turnover. The catch, and it is a real one, is that the studies varied enormously in design and quality, and the finding applies to people already diagnosed with depression, not to the general population looking for a winter pick-me-up. The honest read: vitamin D may be a contributor to mood for some people, not a stand-alone fix. If you are wrestling with the long dark, our coverage of sun exposure at 44 north puts the seasonal piece in context.

There is also a tantalizing line of brain research. A 2026 study from the long-running Framingham Heart Study followed nearly 800 dementia-free adults whose vitamin D was measured around age 39 and whose brains were scanned roughly 16 years later. Higher midlife vitamin D was associated with lower levels of tau, a protein tangled up in Alzheimer’s disease. No link turned up for amyloid, the other Alzheimer’s marker. It is a genuinely interesting result, written up in detail in our piece on the midlife vitamin D and Alzheimer’s study. But it is observational, which means it can show that two things travel together without proving one causes the other. People with higher vitamin D tend to exercise more, eat better, and spend more time outdoors, and any of those could be doing the real work.

Food sources that hold up in a Maine kitchen

Here is some regional good luck. The single best natural food source of vitamin D is fatty fish, and Maine has plenty of it. A three-ounce serving of cooked sockeye salmon delivers about 570 IU, roughly your full daily target in one piece. A tablespoon of cod liver oil, that bracing relic of the old Yankee medicine cabinet, packs around 1,360 IU. Those numbers come from the NIH Office of Dietary Supplements.

After fish, the drop-off is steep. An egg yolk contributes about 44 IU. Most milk sold in the United States is fortified, adding roughly 100 IU per cup, a practice MedlinePlus traces back decades. Fortified cereals, orange juice, soy beverages, and some yogurts pitch in smaller amounts. You can hit your daily requirement through food, but for most people it means eating salmon or its cousins more often than they actually do. Lobster and clams, for all their Maine glory, are not meaningful sources.

Testing, supplements, and the limits of the evidence

This is where the experts stop agreeing, and it is worth knowing why. In 2021, the U.S. Preventive Services Task Force reviewed the evidence on screening healthy adults with no symptoms and concluded there was not enough to recommend for or against it. Their review found that treating asymptomatic low vitamin D did not reduce deaths, fractures in people selected only by low blood levels, or type 2 diabetes, and that the evidence on falls, cancer, heart disease, and infection was too thin to judge.

The 2024 Endocrine Society guideline landed in a similar place with more nuance. It advised against routine testing in healthy adults and against supplementing above the standard recommended intake for healthy people under 75. It did suggest extra vitamin D for specific groups: children and teens, adults 75 and older, people who are pregnant, and adults with high-risk prediabetes. Every one of those was a weak recommendation built on low-to-moderate evidence, which is the guideline’s polite way of saying the science is still arguing.

So what does a reasonable northern New Englander do with all that? The pattern across the evidence is consistent even when the details are not. Vitamin D reliably helps people who are deficient and does little for people who are not. Living above 43 degrees north genuinely raises your odds of being short, especially by late winter. Whether to test, and what to do about a low result, is a conversation for you and your own clinician, who can weigh your age, your health history, and your actual numbers. This article cannot do that, and neither can a supplement label.

How much is too much

One more thing, because the supplement aisle never mentions it. More is not better, and at the extreme it is dangerous. The tolerable upper intake level for vitamin D is 4,000 IU per day for anyone 9 and older. Push well past that for long enough and you can develop hypercalcemia, too much calcium in the blood.

That is not a vague worry. Excess vitamin D drives the body to absorb more calcium than it can use, and the surplus has to go somewhere. According to MedlinePlus, it can form deposits in soft tissue including the heart and lungs, trigger kidney stones and kidney damage, and produce confusion, nausea, vomiting, constipation, poor appetite, weakness, and weight loss. The toxicity comes almost entirely from over-supplementing, never from sunlight or food. Your skin has a built-in shutoff that food cannot override, but a daily megadose pill has no such brake. In a state where half the population may run low in winter, the temptation to overcorrect is real, and worth resisting without a number to aim for and a clinician helping you read it.

The short version

Geography is not on our side here, and pretending otherwise helps no one. From late fall through early spring, the sun over Bangor or Berlin simply cannot make vitamin D in your skin. Plenty of Mainers run low by March. The blood test is real and useful, the deficiency is worth taking seriously, and the evidence is strongest for bone health and for helping people who are genuinely deficient. The rest, mood and immunity and the brain, is promising and incomplete. Eat the salmon, know your latitude, and let your own clinician help you decide what, if anything, the number on your lab slip should change.

Sources

• Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Journal of Clinical Endocrinology & Metabolism. 1988. https://pubmed.ncbi.nlm.nih.gov/2839537/
• Sullivan SS, et al. Adolescent girls in Maine are at risk for vitamin D insufficiency. Journal of the American Dietetic Association. 2005. https://pubmed.ncbi.nlm.nih.gov/15942551/
• NIH / U.S. National Library of Medicine. Vitamin D. MedlinePlus Medical Encyclopedia. 2024. https://medlineplus.gov/ency/article/002405.htm
• NIH Office of Dietary Supplements. Vitamin D, Consumer Fact Sheet. 2024. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/
• NIH Office of Dietary Supplements. Vitamin D, Health Professional Fact Sheet. 2024. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
• Cui A, et al. Prevalence, trend, and predictor analyses of vitamin D deficiency in the US population, 2001-2018. Frontiers in Nutrition. 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC9573946/
• Martineau AR, Jolliffe DA, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017. https://pmc.ncbi.nlm.nih.gov/articles/PMC5310969/
• Efficacy of vitamin D supplementation in patients diagnosed with depression: a dose-response meta-analysis of randomized controlled trials. Frontiers in Nutrition. 2026. https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2026.1772451/full
• U.S. Preventive Services Task Force. Screening for Vitamin D Deficiency in Adults: Recommendation Statement. JAMA. 2021. https://jamanetwork.com/journals/jama/fullarticle/2778487
• The Endocrine Society. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. JCEM. 2024. https://www.endocrine.org/clinical-practice-guidelines/vitamin-d-for-prevention-of-disease
• Association of Circulating Vitamin D in Midlife With Tau-PET Burden in Dementia-Free Adults (Framingham Heart Study). Neurology: Open Access. 2026. https://www.neurology.org/doi/10.1212/WN9.0000000000000057
• Sci.News. Higher Vitamin D in Midlife May Be Associated with Lower Levels of Alzheimer’s Biomarker Years Later. 2026. https://www.sci.news/medicine/alzheimers-vitamin-d-14669.html

This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making any health decisions.