Dehydration in summer: the signals your body sends

When it is hot, the body's primary goal is to cool down (1). It activates two mechanisms: perspiration, which releases water and mineral salts through the skin, and vasodilation, meaning the widening of blood vessels. Those in the legs in particular dilate to bring more blood close to the skin, where heat can be released to the outside.

When blood vessels are dilated in this way, blood tends to pool in the lower legs and has more difficulty returning to the heart. This is one of the explanations for the sensation of heavy legs, ankles that swell or feet that feel tight by the end of the day — a common phenomenon in summer, particularly during pregnancy.

One point that is often misunderstood deserves clarification: drinking plenty of water is not always enough. Staying well hydrated depends not only on how much fluid you drink, but also on where that water ends up. There are broadly two situations to distinguish:

• Intracellular hydration (the kind we're aiming for). Water enters the cells themselves: it is what allows muscles, the brain, and the skin to function properly. This movement depends primarily on sodium and potassium (2). Without them, some of the water you drink is eliminated without having truly hydrated the cells.
• Oedematous water retention. This time, water stagnates between cells, in the tissues, which manifests as swelling or oedema. Vasodilation and venous stagnation, both encouraged by high temperatures, intensify this phenomenon.

The aim in summer is therefore to direct water to the right compartment. After physical exertion or heavy sweating, drinking water alone is best supplemented with minerals.

Dehydration sets in gradually, and the body sends several signals before it becomes severe. The main symptoms (3):

• Thirst. This is a late signal: by the time it appears, the body has already begun to lack water. It is better to drink regularly, without waiting until you feel thirsty.
• Unusual fatigue. A drop in energy with no obvious cause is often linked to a fluid deficit.
• Headaches. Inadequate hydration is a common cause of headaches.
• Dark urine. This is the most reliable indicator: clear and plentiful, urine signals good hydration; dark and infrequent, it signals the opposite.
• Tight-feeling skin, dry lips, dark circles under the eyes. Skin loses its suppleness when water is lacking.
• Dizziness or light-headedness when standing up. When blood volume decreases, pressure in the vessels drops: the heart struggles to send enough blood to the brain, which causes that dizzy, light-headed feeling.

Taken individually, these symptoms are not alarming. But if they accumulate or persist, they warrant attention, as there is a risk that mild dehydration may worsen.

Pregnancy and breastfeeding: increased needs

During pregnancy, the blood volume increases progressively, up to around 45–50% by the end of pregnancy (4), to meet the needs of the mother, the placenta and the foetus. This requires a higher water intake. Combined with high summer temperatures, these needs increase further, which also explains the frequency of heavy legs and oedema in pregnant women.

Duringbreastfeeding, breast milk is made up of nearly 90% water. Daily production, in the order of 750 ml to 1 litre, draws directly on the mother's reserves (5): the losses are real. The thirst felt at the time of feeding reflects this need. Inadequate hydration can contribute to fatigue, headaches and, over time, affect milk production.

During both of these periods, the priority is to drink regularly without waiting until you feel thirsty, increasing intake slightly and ensuring an adequate mineral intake.

The case of babies and young children

Infants and young children are more vulnerable to dehydration: their bodies contain proportionally more water and they dehydrate more quickly. The most common causes in summer are ambient temperature, fever, diarrhoea and vomiting.

Signs to watch for in a baby (6):

• a nappy that has been dry for several hours (reduced urination);
• a dry mouth and lips, no tears when crying, slightly sunken eyes;
• a child who is unusually limp and apathetic, or conversely very irritable;
• in infants, a sunken fontanelle (turned inward) or rapid weight loss.

Preventive measures: avoid the hottest hours of the day, offer the breast or bottle more frequently (milk remains the reference liquid before 6 months), keep the child cool and in the shade. In the event of significant losses (gastroenteritis, intense heat), a doctor may prescribe an oral rehydration solution (ORS) (7), given in small amounts, sometimes by spoon. If in any doubt, seek medical advice.

In summer, it is better to focus on how you hydrate rather than simply drinking more. A few tips for effective rehydration:

• Plan ahead: keep water within easy reach and drink small amounts throughout the day.
• Favour water-rich foods: cucumber, watermelon, courgette, tomato, melon, cold soup and blended vegetable soup provide both water and minerals.
• Top up with electrolytes: not all drinks hydrate in the same way, and plain water is not always the best retained by the body. During periods of heavy sweating, exercise, pregnancy or breastfeeding, adding electrolytes (particularly sodium) helps water to be better retained rather than eliminated too quickly (8). 

Most situations can be managed with rest, shade, and keeping well hydrated. However, some symptoms require immediate medical attention:

• severe dizziness, confusion, difficulty staying awake;
• no urine output for more than 8 to 12 hours;
• rapid heart rate associated with a drop in blood pressure;
• vomiting or diarrhoea preventing you from drinking and retaining fluids, a situation that may require medical rehydration or appropriate treatment;
• in babies: lethargy, refusal to drink, sunken eyes, sunken fontanelle, prolonged dry nappy, persistent fever. In this case, seek medical advice or call emergency services without delay.

Identifying the early signs of dehydration and responding with adequate fluid intake remains the best way to protect your health and your baby's during the summer.