QUOTE from Dr Fung (emphasis mine):
“What can I take on fasting days? ALL CALORIE-CONTAINING FOODS and beverages are withheld during fasting. However, you must stay well hydrated throughout your fast. Water, both still and sparkling, is always a good choice. Aim to drink two liters of water daily. As a good practice, start every day with eight ounces of cool water to ensure adequate hydration as the day begins.” 
Adding a squeeze of lemon or lime flavors the water. Alternatively, you can add some slices of orange or cucumber to a pitcher of water for an infusion of flavor, and then enjoy the water throughout the day. You can dilute apple-cider vinegar in water and then drink it, which may help with your blood sugars. However, artificial flavors or sweeteners are prohibited. Kool-Aid, Crystal Light or Tang should not be added to the water. All types of tea are excellent, including green, black, oolong and herbal. Teas can often be blended together for variety, and can be enjoyed hot or cold. You can use spices such as cinnamon or nutmeg to add flavor to your tea. Adding a small amount of cream or milk is also acceptable. Sugar, artificial sweeteners or flavors are not allowed. Green tea is an especially good choice here. The catechins in green tea are believed to help suppress appetite. Coffee, caffeinated or decaffeinated, is also permitted. A small amount of cream or milk is acceptable, although these do contain some calories. Spices such as cinnamon may be added, but not sweeteners, sugar or artificial flavors. On hot days, iced coffee is a great choice. Coffee has many health benefits, as previously detailed. 
Homemade bone broth, made from beef, pork, chicken or fish bones, is a good choice for fasting days. Vegetable broth is a suitable alternative, although bone broth contains more nutrients. Adding a good pinch of sea salt to the broth will help you stay hydrated. The other fluids—coffee, tea, and water—do not contain sodium, so during longer fasting periods, it is possible to become salt-depleted. Although many fear the added sodium, there is a far greater danger in becoming salt depleted. For shorter fasts such as the twenty-four- and thirty-six-hour variety, it probably makes little difference. All vegetables, herbs, or spices are great additions to broth but do not add bouillon cubes, which are full of artificial flavors and monosodium glutamate. Beware of canned broths: they are poor imitations of the homemade kinds. 
Given its health benefits (see chapter 19), there is no reason to limit coffee intake. The caffeine in coffee may also raise your metabolism further boosting fat burning. 
NOTE (my commentary)
You must stay well hydrated throughout your fast.
Coffee and tea work well, but water is probably the best. I drink iced tea instead of coffee on fasting days, as I prefer my coffee with milk – about 2 cups of milk for a pot, which is beyond the appropriate level for fasting.
A dash of salt, sprinkled on my palm and licked off my skin will help prevent salt depletion during a long fast. (Skip the lime and tequila!) Avoiding sodium during a fast for medical reasons is less important as my body will decrease water retention during a fast.
Hydration in the morning counteracts Dawn Phenomenon transitory high blood sugar test results as it will dilute the concentration of blood sugar.
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SOURCE – Footnotes:
 Quote from The Obesity Code: Unlocking the Secrets of Weight Loss by Dr. Jason Fung; Greystone Books (March 3, 2016). Page 252.
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Key: Fung-05 Shipped: 05/04/2021. Updated 01/11/2022.