Fasting tips from Magen David Adom:

Before the Fast: Make sure to drink a lot of water throughout the day. Eat a proper meal before the fast with carbohydrates, fats, and proteins, but do not overeat. Avoid salty or spicy foods, since extra water is required to absorb all the salt. 

Break Fast: It is recommended to break the fast with a light meal: a lukewarm (not too hot) drink with a slice of cake, or two slices of bread with cheese is best. After about an hour you can have a light meal.

Those who are not used to fasting and are not planning on resting during the day, should be extra vigilant about drinking before fasting, and staying in shaded areas at all times. Tisha B'Av is a long fast in the summer heat.

Dehydration: Avoiding food and drink can endanger your health. Special attention should be paid to young fasters and the elderly, and any medical needs should be promptly attended to. Magen David Adom urges the public to keep tabs on older family members, to make sure they feel good throughout the fast.

Kidney Damage: Due to refraining from liquids during the fast, kidney stones may form in our bodies. The first signs of damage to the body's systems due to lack of fluids are headache, blurred vision, more severe urinary difficulties, and damage to other body systems.

Do not hesitate to call for medical assistance immediately and call MADA paramedics and paramedics at 101 or through the MADA app.

Please consult your Rabbinic authority before making any Halachic decisions.