Answer: When it is ‘resistant starch’.
As plant-minded individuals we are probably familiar with the plant cells known as fibres*. As a member of the cell type known as sclerenchyma, they may exist on their own as long, thick-walled, lignified fibres**, or as equally thick-walled, and lignified structures of a variety of shapes – star-shaped (e.g. astrosclereids) in the leaves of such hydrophytes as water-lily, or as aptly-named stone cells in the flesh of pear (it’s the ‘grit’ you may notice when you eat the fruit). As a component of the vascular tissues, fibres provide structural support and some protection to the long-distance transport pathways in both the xylem and the phloem. Fibres may also be associated with vascular bundles as a ‘cap’ of such cells between the phloem and the epidermis (e.g. sunflower stem), where they may have a defensive role in deterring the phloem sap-sucking intentions of insects.
As those who understand the importance of plant materials as food for animals, we might also be aware of the part played by plant-derived fibre in the mammalian diet. Fibre *** is an important component of a healthy, balanced diet and can prevent heart disease, diabetes, weight gain, some cancers, and can improve digestive health. Whilst, and importantly, plants are the principal source of fibre in the diet, the term ‘fibre’ in this context refers not to the plant cells known as fibres but to ‘the indigestible portion of food derived from plants’. The definition of fibre therefore goes well beyond fibre cells, but because it includes material such as cellulose and lignin found in the cell walls of true fibres, at least can include those plant cell types.
Now, whilst that’s confusing enough, the confusion hinted at in this item’s title comes from the fact that some types of starch, which polysaccharide is also an important plant-derived component of the human diet, are resistant to digestion in the small intestine – unlike ‘normal’ starch. Such resistant starches (RSs) are classed as fibres from a dietary perspective. Resistant starches are, however, fermented within the large intestine by microbes producing short chain fatty acids (SCFAs), that are thought to be beneficial to health by helping to prevent the development of abnormal cells in the gut. The health benefits of RS have been reviewed by Stacey Lockyer and Anne Nugent.
Who’d’ve thought food science would be so different from the plant science – upon which so much of it is based – in terms of terminology? A situation that’s a little remiscent of the United Kingdom and the United States of America, two great nations divided by a common language?
* A word on spelling is in order at this point. Apparently, ‘there is no difference in meaning between fiber and fibre. Fiber is the preferred spelling in American English, and fibre is preferred in all the other main varieties of English [Ed. – which presumably includes English (UK)]. Which do you prefer?
** And which have a long history of exploitation by humans as jute, ramie, hemp, flax, etc..
*** Even fibre in the diet can be further categorised as ‘soluble’, which can be digested, and ‘insoluble’ (also known as roughage), which is hardly digested at all. This insoluble fibre is also known as dietary fibre by some sources…
[Ed. – the keen-eyed amongst you may have noticed that Mr Cuttings has – somewhat mischievously – chosen to illustrate this item with an image of kapok ‘fibres’. Such fibres are actually hairs that extend from the seed surfaces. They are therefore not fibres in the true sense of the word. This erroneous term is also used for the seed hairs of cotton. So, it seems that even botanical uses of the word fibre are not without their problems. Hence, is a fibre a fibre, a hair, or a type of starch? English, a language with at least 171,476 words, but still – clearly – not enough!].
Catian, G., & Scremin-Dias, E. (2013). Compared leaf anatomy of Nymphaea (Nymphaeaceae) species from Brazilian flood plain. Brazilian Journal of Biology, 73(4), 809–817. https://doi.org/10.1590/S1519-69842013000400018
Smith, W. W. (1935). THE COURSE OF STONE CELL FORMATION IN PEAR FRUITS. PLANT PHYSIOLOGY, 10(4), 587–611. https://doi.org/10.1104/pp.10.4.587
Lockyer, S., & Nugent, A. P. (2017). Health effects of resistant starch. Nutrition Bulletin, 42(1), 10–41. https://doi.org/10.1111/nbu.12244