According to Gram Research analysis, vitamin D levels in tropical monsoon climates follow the opposite seasonal pattern compared to temperate regions, peaking in December and dropping lowest in July. A cross-sectional study of 1,202 adults in Dhaka, Bangladesh found that vitamin D deficiency was twice as common during monsoon months (May-August) compared to January. Rather than UV sunlight availability, the pattern appears driven by behavioral changes—people spending less time outdoors during hot, humid summers—suggesting that seasonal vitamin D recommendations need to be customized for tropical climates.
A new study from Bangladesh reveals that vitamin D levels follow a completely different seasonal pattern in tropical monsoon climates compared to temperate regions. Researchers tracked vitamin D levels in over 1,200 adults in Dhaka and found that levels peaked in December and dropped lowest in July—the opposite of what happens in cooler climates. The surprising discovery suggests that people in hot, humid tropical areas change their behavior during summer months (staying indoors more, wearing more protective clothing) in ways that reduce their sun exposure, which is the main way our bodies make vitamin D. This research shows that seasonal vitamin D patterns aren’t just about sunlight availability—they’re also about how people adapt to extreme heat and humidity.
Key Statistics
A 2026 cross-sectional study of 1,202 adults in Dhaka, Bangladesh found that vitamin D levels peaked in December at 22.96 ng/mL and declined to 12.94 ng/mL in July, the opposite pattern observed in temperate climates.
According to research reviewed by Gram, vitamin D deficiency prevalence doubled during monsoon months (May-August) in tropical Dhaka, with odds ratios of 2.03-2.27 compared to January.
A 2026 study of 1,202 tropical monsoon residents found that humidity was negatively associated with vitamin D levels (β = -0.764), suggesting behavioral responses to heat and humidity drive seasonal vitamin D patterns more than UV availability.
Research from Dhaka showed that age was protective against vitamin D deficiency, with each additional year reducing deficiency odds by 2.5%, indicating older adults maintain better vitamin D status year-round.
The Quick Take
- What they studied: Whether vitamin D levels in the blood follow seasonal patterns in tropical monsoon climates, and what weather factors drive those patterns.
- Who participated: 1,202 adult patients visiting clinics in Dhaka, Bangladesh, a tropical megacity with hot, humid monsoon seasons.
- Key finding: Vitamin D levels peaked in December (22.96 ng/mL) and hit their lowest point in July (12.94 ng/mL)—the opposite pattern seen in temperate climates. During monsoon months (May-August), vitamin D deficiency was twice as common as in January.
- What it means for you: If you live in a tropical monsoon climate, you may need to pay special attention to vitamin D levels during hot, humid summer months when people naturally spend less time outdoors. This finding doesn’t apply to people in temperate climates with traditional winter vitamin D drops.
The Research Details
Researchers conducted a retrospective cross-sectional study, meaning they looked back at blood test results already collected from 1,202 patients in Dhaka, Bangladesh. They measured vitamin D levels using a precise laboratory method called chemiluminescence immunoassay. For each blood sample, they matched it with weather data from that same date, including UV index, temperature, humidity, cloud cover, and air quality. They then used statistical analysis to determine which weather factors were most strongly connected to vitamin D levels.
This approach allowed researchers to see the natural pattern of vitamin D across an entire year without manipulating anything. They could observe how real people in a tropical monsoon climate naturally changed their sun exposure based on seasonal weather conditions. The study captured the complete annual cycle, making it possible to identify seasonal peaks and valleys.
Understanding seasonal vitamin D patterns in different climates is important because vitamin D deficiency can affect bone health, immune function, and mood. Previous research had only documented seasonal vitamin D patterns in temperate climates with cold winters. This study fills a critical gap by showing that tropical populations experience completely different seasonal patterns. The findings suggest that public health recommendations about vitamin D supplementation need to be customized for different climates rather than using one-size-fits-all guidance.
This study has several strengths: it included a large sample size (1,202 people), used precise laboratory methods to measure vitamin D, and collected detailed weather data for each blood sample date. However, it’s a retrospective study, meaning researchers couldn’t control which patients were included or what they were doing. The study was conducted in one city (Dhaka), so results may not apply to all tropical monsoon regions. Additionally, the researchers didn’t directly measure sun exposure or behavior—they inferred behavioral changes from the weather patterns and vitamin D results.
What the Results Show
The most striking finding was that vitamin D levels in Dhaka follow the opposite seasonal pattern compared to temperate climates. Instead of peaking in summer and dropping in winter, vitamin D peaked in December (22.96 ng/mL) and reached its lowest point in July (12.94 ng/mL). This represents a significant difference of about 10 ng/mL between seasons.
During the monsoon months (May through August), vitamin D deficiency was dramatically more common. The odds of having vitamin D deficiency were 2.03 to 2.27 times higher during monsoon months compared to January. This means that roughly twice as many people had insufficient vitamin D during the hot, humid summer season.
When researchers examined specific weather factors, they found that humidity had a strong negative relationship with vitamin D levels—higher humidity was associated with lower vitamin D. Interestingly, cloud cover showed a positive association with vitamin D, suggesting that the relationship between sunlight and vitamin D in tropical climates is more complex than previously understood. The UV index, which directly measures sun intensity, did not independently predict vitamin D levels after accounting for the calendar month.
Age emerged as a protective factor against vitamin D deficiency. For each additional year of age, the odds of having vitamin D deficiency decreased by about 2.5%. This suggests that older adults in Dhaka may have more consistent sun exposure habits or different behavioral patterns compared to younger adults. The study also revealed that the relationship between weather variables and vitamin D is not straightforward—humidity and cloud cover were better predictors than direct UV measurements, indicating that behavioral factors (how people respond to weather) may be more important than raw sunlight availability.
Previous research in temperate climates consistently showed that vitamin D levels peak in late summer (August-September) and decline in winter (December-February). This pattern was attributed to increased sun exposure during summer months and reduced sun exposure during winter. The Dhaka study reveals that this model doesn’t apply to tropical monsoon climates. Instead of UV availability driving the pattern, behavioral responses to extreme heat and humidity appear to be the primary driver. People in tropical regions likely reduce outdoor activities during the hottest, most humid months, which explains the opposite seasonal pattern. This finding suggests that existing chronobiological models of seasonal vitamin D rhythms need to be revised to include climate-specific factors.
The study was conducted in a single city (Dhaka, Bangladesh), so results may not apply to other tropical monsoon regions with different climates or cultural practices. The research was retrospective, relying on blood samples already collected for other purposes, which means researchers couldn’t control for important factors like sun exposure habits, clothing choices, or dietary vitamin D intake. The study didn’t directly measure these behavioral factors—researchers inferred them from weather patterns. Additionally, the study population consisted of clinic outpatients, who may not represent the general population. The study also didn’t account for potential differences in skin pigmentation or other genetic factors that affect vitamin D synthesis.
The Bottom Line
People living in tropical monsoon climates should consider vitamin D supplementation or increased dietary intake during the hot, humid summer months (May-August), when vitamin D deficiency is most common. This is the opposite recommendation from temperate climates, where winter supplementation is typically advised. The evidence for this recommendation is strong (based on a large, well-designed study), but individual needs vary. Consult with a healthcare provider about appropriate vitamin D levels for your specific situation. Older adults may have naturally better vitamin D status and may require less aggressive supplementation.
This research is most relevant to people living in tropical monsoon climates, particularly in South Asia. Healthcare providers in these regions should adjust their vitamin D screening and supplementation recommendations based on these seasonal patterns. People in temperate climates should not change their vitamin D practices based on this study, as the seasonal patterns are opposite. The findings are particularly important for public health officials designing vitamin D supplementation programs in tropical countries.
If you start vitamin D supplementation during monsoon months, you should expect to see improvements in blood vitamin D levels within 4-8 weeks, depending on the dose and your individual absorption. However, the seasonal pattern suggests that vitamin D levels naturally improve again starting in September-October as weather conditions change, so supplementation may only be needed during the peak deficiency months (May-August).
Frequently Asked Questions
Why is vitamin D lower in summer in tropical countries but higher in winter?
In tropical monsoon climates, people spend less time outdoors during hot, humid summer months due to behavioral adaptations to extreme heat. This reduces sun exposure and vitamin D production, causing summer deficiency. The opposite occurs in temperate climates where winter cold reduces outdoor time.
Should I take vitamin D supplements year-round if I live in a tropical monsoon climate?
The research suggests focusing supplementation during monsoon months (May-August) when deficiency is most common. However, individual needs vary based on sun exposure habits and diet. Consult your healthcare provider about appropriate supplementation timing for your specific situation.
Does this vitamin D research apply to people living in temperate climates?
No, this study is specific to tropical monsoon climates. Temperate regions experience opposite seasonal patterns, with higher vitamin D in summer and lower in winter. Existing winter supplementation recommendations for temperate climates remain appropriate.
How much does humidity affect vitamin D levels according to this study?
The study found a significant negative relationship between humidity and vitamin D levels (β = -0.764, p = 0.005), meaning higher humidity was associated with substantially lower vitamin D. This suggests humidity-driven behavioral changes are a major factor in tropical vitamin D patterns.
Can I rely on UV index forecasts to know when my vitamin D levels will be high?
Not in tropical monsoon climates. This study found that UV index did not independently predict vitamin D levels after accounting for calendar month. Weather factors like humidity and cloud cover, which influence behavior, were better predictors than direct sunlight measurements.
Want to Apply This Research?
- Track your vitamin D supplementation intake monthly and note the current season/month. Set reminders to increase vitamin D intake during May-August (monsoon months) if you live in a tropical monsoon climate. Record any symptoms like fatigue or muscle weakness that might indicate deficiency.
- If you live in a tropical monsoon climate, add a vitamin D supplement or increase vitamin D-rich foods (fortified milk, fatty fish, egg yolks) to your routine specifically during May-August. Set monthly reminders to check your vitamin D status during these high-risk months. Consider scheduling blood work in July or August to assess your vitamin D levels during peak deficiency season.
- Establish a baseline vitamin D level in January or February (when levels are naturally highest in tropical climates), then retest in July or August to see if supplementation during monsoon months maintains adequate levels. Track seasonal patterns year-over-year to identify your personal vitamin D rhythm and adjust supplementation timing accordingly.
This research describes seasonal vitamin D patterns in tropical monsoon climates and should not be interpreted as medical advice. Vitamin D supplementation needs vary by individual based on age, skin tone, dietary intake, sun exposure, and health conditions. People concerned about vitamin D deficiency should consult with a qualified healthcare provider for personalized recommendations and appropriate testing. This study was conducted in Dhaka, Bangladesh, and results may not apply to all tropical regions or populations. Do not start, stop, or change vitamin D supplementation without medical guidance.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.