Swollen ankles and heavy legs: drinking less won't help.

It's seven in the evening and you kick off your shoes. You've got swollen ankles and heavy legs, and your socks have left a groove above the ankle. It wasn't there this morning.

It happens after a day at a desk, a flight, hours on your feet at a trade show. Many people call it water retention and respond by drinking less and cutting out salt. I see it often in the people I work with, and it's usually the wrong move.

Swollen feet in the evening: how to spot it

This swelling keeps to a timetable. In the morning your ankles are fine. By evening they aren't.

• The sock mark. A clear groove above the ankle.
• Tight shoes. Fine this morning, pinching by late afternoon.
• Legs like lead. The stairs are harder work in the evening.

Why do legs swell in the evening?

Because they've been still and hanging down for hours. Blood climbs back up mostly on a push from your feet and calves, and without it fluid tends to settle at the ankles.

The push starts in your foot. When you stand, over half the blood in your veins is in your legs and has to climb against gravity. Under the sole sits a mesh of veins as dense as a sponge. Every step squeezes it, then the calf contracts and sends the blood higher. Small valves stop it falling back.

Sitting or standing, still is still. Your legs hang down and no step squeezes the sponge. Blood pools and fluid builds up in the tissues. Heat widens the vessels and makes pooling easier, and tight trousers can act like a tourniquet.

Salt and water cut both ways. Sodium governs the water outside your cells, so the more you eat, the more water you tend to hold. Cut it out completely, though, and your body raises the hormone that hangs on to sodium, and water with it. Drinking works the same way. When water runs short your kidneys save it, as I explain in the article on water.

Swollen ankles and heavy legs: what to do

Toes and heels, sitting down. The rule is still two minutes on your feet every hour. Stuck in a meeting or mid-flight, put your feet to work. Heels down, lift your toes. Then toes down, lift your heels, until your calves feel warm. On your feet for hours? Rise onto your toes now and then.

Walk, and train your legs. Brisk walking is the exercise to go for. Your seven to eight thousand steps a day and ten to fifteen minutes after meals count here too. In your two weekly strength sessions, include thighs and calves. A toned muscle pushes better with every step.

The salt you can't see. Before the salt cellar come cured meats, cheese, crisps and ready meals, so start there. At home, salt food at the end of cooking and flavour it with herbs, lemon or vinegar. And have fruit or vegetables with every meal. They bring potassium, which works inside your cells and balances sodium.

Water, in the daytime. About 30ml per kilo and never under a litre and a half, more in the heat, and little after 5pm.

Tonight, look at your ankles as you take your shoes off. Tomorrow change one thing only, then look again in a few evenings.