Education

Think you're underhydrated? Your gut probably noticed first

Jun 12 2026
Florrie Tingle

By Lara Mills

Whether you're training for a Hyrox, playing padel, chasing a new PB on your morning run, or simply trying to survive a packed summer social calendar, most of us think about hydration in fairly simple terms: move, sweat, drink water. But there's more to hydration than just feeling thirsty.

Beyond helping us perform physically, adequate hydration influences digestion, cognition, skin health, energy levels and recovery. Even relatively mild fluid deficits of around 1–2% of body mass can impair concentration, mood, reaction time and exercise performance, often before thirst becomes a reliable signal.¹

Although "dehydrated" is often used as an umbrella term, many of us actually tend to mean “underhydrated”. Underhydration describes the mild, chronic state of regularly not consuming enough fluids day-to-day, whereas dehydration is the more acute clinical condition that occurs when fluid and mineral losses significantly exceed intake.²

The effects of underhydration

Summer creates the perfect conditions for underhydration. We sweat more, travel more, exercise more, and often drink more alcohol, meaning the body has to work harder to maintain fluid balance.

With temperatures recently exceeding 35°C in parts of the UK during summer heatwaves, staying adequately hydrated becomes even more important.³ Despite all the health tech innovations on the market, automated hydration tracking isn’t integrated within the main players like Whoop, Oura or FitBit yet—you’d have to look to specialist monitors FLOWBIO or Nix Hydration Biosensor. So how accurate can your readiness score be without accounting for one of the most fundamental aspects of human physiology?

The gut–skin connection

Luckily, our bodies often provide their own clues. Often referred to as “the mirror to the gut”, the skin can act as a useful indicator of hydration status. Increased sensitivity to sun exposure, dullness, more noticeable creases around the eyes, or skin that suddenly feels tighter and less resilient can all be signs that fluid levels are out of balance.⁴ While ageing, UV exposure and natural collagen decline all play a role, underhydration can exaggerate these effects by impairing skin barrier function and elasticity.⁵

The gut–skin axis describes the ongoing communication between the digestive system, immune system, microbiome and skin. When gut barrier function becomes compromised, inflammatory compounds can circulate more freely throughout the body, influencing skin barrier function, recovery and sebum production.⁶

For men, this connection may be particularly relevant. Male skin is typically thicker, contains greater collagen density, and produces more sebum than female skin due to the influence of androgens such as testosterone. While these characteristics can help maintain skin structure, higher sebum production may also increase susceptibility to inflammatory skin concerns when factors such as stress, poor sleep, dietary changes and impaired gut health disrupt normal skin function.⁷

Periods of poor sleep, stress, disrupted eating habits, travel and increased alcohol intake often coincide with breakouts, irritation, or skin that simply looks more tired and less resilient. Add underhydration into the mix and the effects can become more visible, particularly during summer when UV exposure, sweat loss and physical activity are all elevated.⁴˒⁵

Your skin isn't necessarily the problem—it's often the messenger, telling you what changes are occurring much deeper within the gut.

The gut’s role in hydration

Emerging research suggests that hydration status may directly influence the gut microbiome—the trillions of microorganisms living within the digestive tract that help regulate digestion, immunity, metabolism, and inflammation.⁸

One study examined hydration biomarkers alongside gut microbiome composition in healthy adults aged 25–45. Researchers found that individuals with lower hydration status showed distinct differences in several bacterial groups associated with the intestinal mucus layer—the protective barrier separating gut microbes from the gut wall.⁸

The same study also identified associations between higher levels of copeptin—a marker of physiological water-conservation stress—and elevated concentrations of lipopolysaccharide-binding protein (LBP), a biomarker linked to gut barrier disruption and low-grade systemic inflammation. Essentially, this means that when the body is consistently trying to conserve water, the gut environment may begin to shift in ways that affect gut barrier integrity.

As a cross-sectional study, it can only demonstrate associations rather than prove direct cause and effect. However, the findings highlight a biologically plausible relationship between hydration status, gut health, and inflammation that warrants further investigation.

The impact of stress

One of the most overlooked influences on gut health is stress. Travel, poor sleep, busy social schedules, heat, alcohol, demanding jobs, ultra-processed foods and intense exercise all place additional pressure on the nervous system, which in turn affects gut motility, digestion, and microbial balance through the gut–brain axis.⁹ It's why digestive symptoms often appear during holidays, business trips, festivals, or particularly busy periods of life.

Alongside hydration and nutrition, supporting the nervous system can help maintain digestive resilience. Ingredients such as chamomile and lemon balm have traditionally been used to support digestive comfort during periods of stress, while ashwagandha has been studied for its potential to enhance resilience to physical and psychological stressors.¹⁰˒¹¹

Signs you need to drink more water

While the old "eight glasses a day" recommendation offers a useful starting point, hydration requirements vary significantly according to body size, activity levels, climate, alcohol intake, and overall health status.¹²

Practical signs that you may need more fluids include:

Dry lips or skin

Headaches

Fatigue

Constipation

Darker or strong-smelling urine

Reduced exercise performance

Difficulty concentrating

Skin that appears dull or less elastic

Getting the foundations right for good hydration

Good hydration isn't simply about drinking more water. The gut plays an important role in how efficiently the body absorbs and utilises fluids, making the microbiome an often-overlooked part of hydration. Think of it like watering a lawn: the water matters, but so do the soil conditions that allow everything to thrive.

This is where nutrition becomes particularly relevant. Polyphenols—plant compounds found in berries, cacao, coffee, green tea, herbs, spices, pomegranate, grapes and extra virgin olive oil—have been shown to interact closely with the microbiome. Research suggests they can help support beneficial bacteria, short-chain fatty acid production and gut barrier integrity.¹³ In summer, when many of us naturally gravitate towards lighter, more plant-rich meals, there is a perfect opportunity to support the microbiome through simple dietary habits such as adding fresh herbs generously, choosing berries as snacks, drizzling olive oil over salads or swapping an iced coffee for green tea.

Alongside supporting gut health, maintaining fluid balance remains essential. Rather than waiting until you're thirsty, try to drink water consistently throughout the day, particularly after exercise, travel, alcohol consumption or long periods in the heat. Electrolyte-rich foods such as bananas, avocados, leafy greens and coconut water can also help replenish minerals lost through sweat.

For those looking for additional support, formulations such as Bio.Me Essential can complement a food-first approach. Essential combines a diverse blend of polyphenol-rich plant compounds—including citrus bioflavonoids, cocoa, pomegranate, green tea and grapeseed extracts—with botanicals such as chamomile, lemon balm and ashwagandha, selected to support the microbiome, gut barrier function and digestive resilience during periods of stress, travel and lifestyle disruption.

Whether your goal is sharper focus at work, better recovery after training, healthier-looking skin or simply feeling more energised through the summer months, hydration remains one of the most important foundations of health. Increasingly, research suggests that hydration, gut health and skin health are intrinsically connected.

References

Wittbrodt, M. T., & Millard-Stafford, M. (2018). Dehydration impairs cognitive performance: A meta-analysis. Medicine & Science in Sports & Exercise, 50(11), 2360–2368. https://doi.org/10.1249/MSS.0000000000001682

Kavouras, S.A. Hydration, dehydration, underhydration, optimal hydration: are we barking up the wrong tree?. Eur J Nutr 58, 471–473 (2019). https://doi.org/10.1007/s00394-018-01889-z

Met Office. (2026). Record-breaking heat rewrites May temperature records across the UK. https://www.metoffice.gov.uk/blog/2026/record-breaking-heat-rewrites-may-temperature-records-across-the-uk

Akdeniz, M., Tomova-Simitchieva, T., Dobos, G., Blume-Peytavi, U., & Kottner, J. (2018). Does dietary fluid intake affect skin hydration in healthy humans? A systematic literature review. Skin Research and Technology, 24(4), 459–465. https://doi.org/10.1111/srt.12454

Palma, L., Marques, L. T., Bujan, J., & Rodrigues, L. M. (2015). Dietary water affects human skin hydration and biomechanics. Clinical, Cosmetic and Investigational Dermatology, 8, 413–421. https://doi.org/10.2147/CCID.S86822

Willis, H. J., Slavin, J. L., & colleagues. (2022). Hydration biomarkers and gut microbiota in healthy adults. Annals of Nutrition and Metabolism, 77(Suppl. 4), 37–45. https://doi.org/10.1159/000520478

O'Neill, C. A., Monteleone, G., McLaughlin, J. T., & Paus, R. (2016). The gut-skin axis in health and disease: A paradigm with therapeutic implications. BioEssays, 38(11), 1167–1176. https://doi.org/10.1002/bies.201600008

Rahrovan, S., Fanian, F., Mehryan, P., Humbert, P., & Firooz, A. (2018). Male versus female skin: What dermatologists and cosmeticians should know. International Journal of Women's Dermatology, 4(3), 122–130. https://doi.org/10.1016/j.ijwd.2018.03.002

Cryan, J. F., O'Riordan, K. J., Cowan, C. S. M., Sandhu, K. V., Bastiaanssen, T. F. S., Boehme, M., ... Dinan, T. G. (2019). The microbiota-gut-brain axis. Physiological Reviews, 99(4), 1877–2013. https://doi.org/10.1152/physrev.00018.2018

Pizarro Meléndez, G. P., Valero-Jara, V., Acevedo-Hernández, P., & Thomas-Valdés, S. (2024). Impact of polyphenols on stress and anxiety: a systematic review of molecular mechanisms and clinical evidence. Critical reviews in food science and nutrition, 64(8), 2340–2357. https://doi.org/10.1080/10408398.2022.2122925

Lopresti, A. L., Smith, S. J., & Drummond, P. D. (2022). Modulation of the hypothalamic–pituitary–adrenal (HPA) axis by plants and phytonutrients: a systematic review of human trials. Nutritional Neuroscience, 25(8), 1704–1730. https://doi.org/10.1080/1028415X.2021.1892253

Dmitrieva, N. I., Boehm, M., Yancey, P. H., & Enhörning, S. (2024). Long-term health outcomes associated with hydration status. Nature Reviews Nephrology, 20, 275–294. https://doi.org/10.1038/s41581-024-00817-1

Plamada, D., & Vodnar, D. C. (2021). Polyphenols-Gut Microbiota Interrelationship: A Transition to a New Generation of Prebiotics. Nutrients, 14(1), 137. https://doi.org/10.3390/nu14010137