On a food pack, prebiotic is a term used for fibre. What you can check is the grams of fibre printed next to that word. Some foods and packets print the word prebiotic and the grams beside it. A packet can list fibre and never use that word. You still add the grams it prints, whether or not the pack uses the word prebiotic. From the age of 17, the UK intake for dietary fibre is 30 g a day. If a pack prints fibre in grams, you can add those grams toward the 30 g.2
A prebiotic is food for the body's microorganisms
ISAPP says prebiotics provide a health benefit by changing the microorganisms in or on the body, most commonly in the digestive tract. They act as food for those microorganisms. The wording on that page is a substrate that is selectively utilized by host microorganisms, conferring a health benefit.1 Not every fibre fits that description. When a pack uses the word, you still read the fibre grams printed beside it.
Fibre printed on the pack
When the pack says prebiotic, add the fibre grams printed beside that word. A Horlicks Protein powder sachet prints 4 g of prebiotic fibre in 36 g, and the same sachet lists 24 g of protein. You stir it into 200 ml of cold water or milk. If you use only half the sachet, you add about 2 g of fibre and about 12 g of protein. The bottled Horlicks Protein milkshake is a full serving as it comes. It lists 3.5 g of fibre in 240 ml, and 20 g of ultra-filtered milk protein. If you drink the whole bottle, that is 3.5 g of fibre and 20 g of protein. If you drink about half, that is about 1.8 g of fibre and 10 g of protein. Fibre from anything else that day is whatever that food prints on its label.
Fibre in one serving
If you use more of the pack, you get more fibre. If you use less, you get less. Another food that day adds whatever fibre its own label prints.
Foods with small amounts of prebiotics
ISAPP names onions, garlic, bananas, chicory root and Jerusalem artichokes as foods where prebiotics show up, usually in small amounts. Not every fibre fits what they describe. On a pack, you still add the grams of fibre that are printed.1 If the label gives 6 g of fibre per 100 g and you eat 40 g, multiply 6 by 40 and then divide by 100. That is 2.4 g. If the label already says 3 g for the serving, use 3 g. One sachet gives you the 4 g of fibre and the 24 g of protein printed on it. Two sachets would be 8 g of fibre and 48 g of protein. Fibre from the rest of the meal is on those foods' own labels. Add that to the sachet's 4 g. A food can print fibre and never say prebiotic. If you ate that food and the sachet, add its grams to the 4 g. A banana that lists 2 g of fibre, eaten with a serving that lists 4 g, comes to 6 g of fibre from the two of them. If another food lists 3 g of fibre per 100 g and you eat 30 g of it, that is about 0.9 g. Added to the 4 g, you are at about 4.9 g of fibre. The 24 g of protein is a separate count from the fibre.