Vegetable peels: the secret prebiotic you’re throwing away

By Laura Campbell

Our gut microbiota (the beneficial microorganisms which live in our body) feed off prebiotics, the fibre in vegetables. With 1/3 of global emissions coming from food waste, 30% of which, according to (Business Waste, Uk), is from parts of highly palatable vegetables which we currently throw away. Currently, the gut microbiota is very fashionable, with probiotics and prebiotic supplements flying off the shelves. Yet it seems utterly illogical that we disregard a natural prebiotic, to buy another, made of the same ingredients. Why not boost your gut microbiota, save money, and save the planet at the same time by consuming more of the plants you purchase?

What is in plants we eat?

With the world now crazy for plant-based diets, it is important to learn what is in the plants we eat and the health-giving benefits of these. Plants contain vitamins such as vitamin K, C, D, A, E and B vitamins and minerals including magnesium, selenium, calcium, potassium, and iron. Some (such as beans, peas, lentils), contain protein (needed for growth and repair). Others, (such as Brussel sprouts), can contain Omega-3 (for cognition), 6 and 9 fatty acid fats. Plants also contain three types of carbohydrates: simple sugars, starch, and cellulose.

The simple sugars in plants are sucrose, fructose and glucose sugars which sweeten fruit and vegetables. The starch is what the simple sugars are stored as in the plant and the cellulose is the branched chains of glucose that makeup fibers in the plant (providing the plant structure when living). These indigestible fibers are also called fibre, and it is this fibre, both soluble and insoluble fibre, that our gut microbiota feeds off as prebiotics, providing a health benefit to us.

What is a prebiotic?

Prebiotics are defined as “non­ digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria already resident in the colon” (Gibson et al., 2017)

When the gut microbiota consumes fibre, the microbiota themselves produce short-chain fatty acids (SCFAs), principally Acetate, Propionate and Butyrate. It is these SCFA that have beneficial health properties. SCFA have a role in helping to regulate the immune system, helping to improve heart health, improving mental health through the brain-gut-axis, and helping regulate blood sugar and metabolism, helping you regulate your weight and energy levels.

Prebiotics also work with probiotics (substances containing live cultures of microbiota microorganisms) to form synbiotics. Synbiotics are defined as “a mixture comprising live microorganisms and substrates, selectively utilized by host microorganisms to confer a health benefit” (Swanson et al., 2020).

Why should you eat your peels?

If the peels are one of the parts of a plant highest in fibre, it is then so surprising that most of us waste this as is the most prebiotic. The majority of the prebiotic fibre in plants is found in the (Hao et al, 2021) peels of fruits and vegetables. We throw away essential prebiotics which could help us lose weight and stay disease and allergy-free.

Prebiotic peels:

1) Potato and parsnip peel:

So many people leave the sweet potato peels behind after a meal and scrape off their potato, parsnip, and new potato peels when they make a roast, mash or chips, reducing the fibre of their food. Fibre is prebiotic, feeding the microbiota. In return, your gut microbiota also regulates melatonin (which helps you sleep) and serotonin (the happiness chemical) via the brain-gut axis. Peeling potatoes reduces fibre and wastes the Serotonin-boosting, happiness neurotransmitter (brain chemical) potential of your potatoes via the microbiota (Stasi et al, 2019). The peel again also adds flavour. A cottage pie or chicken hotpot is so much tastier with peels in it. It makes it crispier and adds volume, keeping you feeling fuller for longer so you are less likely to snack and so lose weight. Throwing the peels away makes no sense!!

2) Watermelon, Papaya and Honeydew melon rind:

This sounds unusual, but it is more cultural ignorance and a lack of knowledge of the benefits that stops us all from consuming melon peel. It is biochemically similar in structure and taste to cucumber and thus completely edible and highly palatable and satisfying. The melon peel contains up to 7g fibre, more than flaxseed. Recent studies by Chan Hao and team (Hao et al, 2021) at the Institute of Food Science in Malaysia have shown that these peels are highly prebiotic. Hao writes “According to the Department of Agriculture of Malaysia (2017), the total production of watermelon, honeydew, and papaya for the year 2016 reached 192 909.8 metric tons.” As only the flesh is consumed, the delish cucumber flavour and fantastic prebiotic potential of the peels are lost. Instead, these peels are taken to landfills where they decompose and give off carbon dioxide emissions, which contribute to global warming. Why not try swapping tuna mayo and cucumber sandwiches for a tuna mayo and melon peel sandwich? Be food adventurous, try it! Have a go. You might even like it.

3) Lemon zest:

Lemon slices are used to flavour fish, in alcoholic beverages like Gin and Tonics, in salad dressings and in meat marinades. The juice is used, and the flesh and peel go in the bin. Yet it is these parts that are highest in fibre, with 1g fibre per spoon of peel. Lemon peel also contains loads of vitamin C (which boosts your immune system), providing a staggering 6g in 1 tablespoon (Guolin et al, 2012). Why not grate it into cakes and use it in salads, nut roasts and plant-based burgers and boost the flavour and fibre of your food.

4) Mango peel:

Most people are fussy about mango peel. Pre-prepared mango in supermarkets comes de-skinned. Yet most of the fibre lies in the peel. It tastes nice too, having the sweetness of the mango on it and yet a bitterness that brings out the sweetness of the mango even more. Why not add it to sweet potato mash or use it in your smoothies. Add fibre and flavour.

5) Spring onion and leek peel:

Again these peels are thrown away and yet they are full of magnesium (which helps you sleep and boosts your metabolism) and fibre. They also taste lovely and are a great salad addition. They again fill you up, making you less likely to snack and lose weight.

6) Apple and carrot peel:

Apple and carrot peels are high in vitamin C (for immunity) and A (for eyes and skin) and contain loads of fibre. Why not try a baked apple and skin with ice cream for a great dessert or baked apple in porridge to add fibre to your breakfast? Why not try baking an apple Danish with apple peels on to add fibre to your low fibre pastry or a carrot cake with peels to add fibre to your cake? Peels have so much potential!

7) Broccoli and Cauliflower stalks and leaves: The stem and stalk of broccoli and cauliflower taste so delish. The leaves taste like romaine or gem lettuce so are great in salads and the stem is great boiled where it tastes fluffy (it makes a great mash) like potato or eaten raw where it tastes crunchy and sweet like a carrot. Try dipping the stems in humous as a snack at work or grating them into a carrot and sultana salad. They are very high in fibre and very prebiotic, adding a whopping 10g fibre to your meal. They also improve insulin sensitivity (Zandani et al, 2021) and taste utterly delicious.

Peels are prebiotic. They are palatable. They are free as they are a currently wasted byproduct of our society. Why not eat your precious prebiotic peels, boost your gut microbiota, and save money and the planet at the same time!

References

1) Food waste 2022 The facts. [online] Business Waste. Available at: <https://www.businesswaste.co.uk/food-waste-2022-the-facts/> [Accessed 26 February 2022].

2) Hao, C.L., Esah, E.M., Tajarudin, H.A., Akter, B. and Salleh, R.M., 2021. Effect of potential prebiotics from selected fruits peel on the growth of probiotics. Journal of Food Processing and Preservation, 45(6), p.e15581.

3) Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J Nutr. 1995 Jun;125(6):1401–12. doi: 10.1093/jn/125.6.1401. PMID: 7782892.

4) Swanson KS, Gibson GR, Hutkins R, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of synbiotics. Nat Rev Gastroenterol Hepatol. 2020;17(11):687–701. doi:10.1038/s41575–020–0344–2

5) Guolin H, Jeffrey S, Kai Z, Xiaolan H. Application of ionic liquids in the microwave-assisted extraction of pectin from lemon peels. J Anal Methods Chem. 2012;2012:302059. doi:10.1155/2012/302059

6) Stasi C, Sadalla S, Milani S. The Relationship Between the Serotonin Metabolism, Gut-Microbiota and the Gut-Brain Axis. Curr Drug Metab. 2019;20(8):646–655. doi:10.2174/1389200220666190725115503

7) Rivas, M. Á., Benito, M. J., Ruíz-Moyano, S., Martín, A., Córdoba, M. G., Merchán, A. V., & Casquete, R. (2021). Improving the Viability and Metabolism of Intestinal Probiotic Bacteria Using Fibre Obtained from Vegetable By-Products. Foods (Basel, Switzerland), 10(9), 2113. https://doi.org/10.3390/foods10092113

8) Jovanovic-Malinovska, R., Kuzmanova, S. and Winkelhausen, E., 2014. Oligosaccharide profile in fruits and vegetables as sources of prebiotics and functional foods. International journal of food properties, 17(5), pp.949–965.

9) Zandani, G., Anavi-Cohen, S., Tsybina-Shimshilashvili, N., Sela, N., Nyska, A. and Madar, Z., 2021. Broccoli Florets Supplementation Improves Insulin Sensitivity and Alters Gut Microbiome Population — A Steatosis Mice Model Induced by High-Fat Diet. Frontiers in nutrition, 8.