Why Almost Everyone in Lima Is Low on Vitamin D

Newcomers to Lima are usually told about the food, the traffic and the ceviche. They are less often told about the sky.

For roughly half the year, Lima sits under a low, unbroken layer of coastal cloud — the garúa, which locals call panza de burro. It rarely produces actual rain. It simply sits there, grey and constant, from around May to November.

This has a specific and measurable consequence: vitamin D deficiency is remarkably common here, including in people who spend a great deal of time outdoors.

Why a tropical latitude doesn't protect you

Lima is at about 12° south. The intuition — tropical latitude, therefore abundant sun, therefore adequate vitamin D — is reasonable and, for half the year, wrong.

Between 80% and 90% of your vitamin D is synthesised in the skin from UVB radiation. UVB is the specific part of the spectrum that matters, and it is far more readily blocked than UVA. Lima's coastal cloud layer is thick enough to absorb most of it, which is why you can spend an entire grey Lima winter outdoors and produce very little vitamin D.

Add the factors that apply everywhere — office work, sunscreen, covering clothing, darker skin tones requiring longer exposure, and older skin synthesising less efficiently — and deficiency stops being surprising.

What the numbers mean

Vitamin D is measured as 25-hydroxyvitamin D, or 25(OH)D. Peruvian laboratories report it in ng/mL, as in the US. If your previous results were in nmol/L, multiply the ng/mL figure by 2.5.

The widely used categories:

• Deficiency: under 20 ng/mL (50 nmol/L)
• Insufficiency: 20–29 ng/mL (50–74 nmol/L)
• Sufficiency: 30 ng/mL (75 nmol/L) and above

There is genuine scientific disagreement about the threshold for the general population, and some bodies set the adequacy bar lower than 30. The disagreement is about the middle of the range. Almost nobody argues that a result under 20 is fine, and that is where a substantial share of the Lima patients I test actually sit.

Who should actually be tested

Population-wide screening is not recommended, and I do not order this on everyone. Testing is worthwhile if you have:

• Osteoporosis, osteopenia, or a fracture from a minor fall
• Chronic kidney or liver disease
• Malabsorption — coeliac disease, inflammatory bowel disease, or previous bariatric surgery
• Long-term corticosteroid, anticonvulsant or antiretroviral use
• Persistent bone pain or muscle weakness, particularly proximal weakness — difficulty rising from a chair or climbing stairs
• Very limited sun exposure, whether from indoor work, illness or clothing
• Hyperparathyroidism or unexplained abnormal calcium

If you are healthy, under 50, and have no risk factors, the more sensible approach is sensible sun exposure and adequate dietary intake rather than a blood test.

Getting some without wrecking your skin

Short, regular exposure of arms and legs — on the order of 10 to 20 minutes, in the middle of the day, several times a week — is what the skin needs. Longer is not better; the synthesis pathway saturates, while UV damage does not.

Two Lima-specific points. In the grey months, that exposure often is not producing much regardless of how long you stay outside, which is precisely the problem. And on clear summer days in Lima the UV index is genuinely high — the same cloud that suppresses your winter synthesis is absent, and sunburn happens quickly. Neither season is an argument for unprotected midday sunbathing.

Dietary sources are real but limited: oily fish (and Peru has superb oily fish), egg yolks, and fortified products. Diet alone rarely corrects an established deficiency.

On supplementation

If you are deficient, supplementation works, is inexpensive and is well tolerated. What it should not be is self-directed and indefinite at high doses.

Vitamin D is fat-soluble and does accumulate. Toxicity is uncommon but real, and it is essentially always caused by sustained high-dose supplementation rather than by sunlight or food. Very high intermittent "mega-doses" have also performed poorly in trials, and in some studies of older adults were associated with more falls rather than fewer.

The sensible approach is to establish the level, correct it with a defined dose over a defined period, recheck, and then settle on a maintenance dose — or none, if the level holds. That is a short conversation and one blood test, not an ongoing project.

Why an endocrinologist cares about this

Vitamin D is not really a vitamin; it is a hormone, and it governs calcium absorption and bone mineralisation. Persistent deficiency raises parathyroid hormone, which pulls calcium out of bone to keep serum calcium stable. Over years, that is bone density you do not get back.

It matters most in people already at risk of osteoporosis — postmenopausal women in particular. If that is you, and you have spent several Lima winters under the garúa, it is worth knowing your number.