Why Leftover Christmas Dinner is Better for You

21st December 2021

By Dr Fred Warren, Group Leader at the Quadram Institute

Serving Christmas Leftovers

We all like to overindulge at Christmas with a traditional festive dinner with all the trimmings, roast potatoes and parsnips, Brussels sprouts, and pigs (or no-pigs)-in blankets. The aim of the game certainly isn’t to be healthy, and most of us will have Christmas leftovers lasting for days. Did you know, though, that those Christmas leftovers could hold a secret which might make them just a little bit better for you than you expected?

Research at the Quadram Institute aims to uncover the secrets of leftovers, unravelling the combination of chemistry, biology, and microbiology, which can lead to some surprising health benefits.

You’ve Got to Starch Somewhere

This festive story starts with starch – an essential component of our food and one of our diet’s significant sources of calories. Plants make their own food, converting the energy of the sun into a type of sugar called glucose. Starch is a convenient way for plants to store this energy. Starch stored by plants in storage organs makes up the bulk of many foods in our diet, including the roast potatoes, parsnips, Yorkshire puddings and the breadcrumbs in the stuffing on our Christmas table. Plants generate starch in the form of tightly packed microscope granules around the width of human hair.

Starch multiscale structure

Starch multiscale structure. (a) Starch granules from normal maise (30 µm); (b) amorphous and semicrystalline growth rings (120-500 nm); (c) amorphous and crystalline lamellae (9 nm), magnified details of the semicrystalline growth ring; (d) blocklets (20-50 nm) constituting a unit of the growth rings; (e) amylopectin double helices forming the crystalline lamellae of the blocklets; (f) nanocrystals: other representation of the crystalline lamellae called starch nanocrystals when separated by acid hydrolysis; (g) amylopectin molecular structure; and (h) amylose molecular structure (0.1-1 nm).[1]

The plant packs glucose tightly into the granules to maximise energy storage efficiency. It does this by linking thousands of individual glucose molecules together into long, branched chains called amylose and amylopectin. Due to the compact form of starch in the plant, our digestive system has difficulty accessing the glucose in the starch. Think about eating a raw potato; it will not make you feel very well and definitely isn’t advisable. To overcome this, around 1 to 1.5 million years ago, early human ancestors developed a magical process – cooking!

The Magic of Christmas Cooking

Through the combination of water and heat applied during cooking, the glucose chains that starch is made of unwind, causing the starch granule to swell and take in water. These softened starch granules result in the delicious texture of fluffy roast potatoes, soft buttery parsnips, and moist stuffing.

Christmas Leftover Potatoes

Cooking also dramatically increases the digestibility of the starch after we eat it, making the glucose it contains much more available. This can be a problem for our health as the glucose from our food is absorbed through our digestive system into the bloodstream, where it causes peaks in blood glucose. Repeated large peaks in blood glucose from rapidly digested starchy foods can ultimately lead to the development of type II diabetes.

While this might not be too much of a concern on Christmas day, we all want to look for something a bit healthier after Christmas. Research at Quadram is exploring what happens in the starch of those leftover spuds left in the fridge for a day or two. Using advanced spectroscopic techniques, we have modelled how the chains of glucose can partially re-associate in the cool temperatures of the fridge, forming a gel-like network structure.[2]

The formation of this structure is why cooled potatoes feel firmer than freshly cooked potatoes and also reduces the accessibility of starch for digestion. In turn, this helps reduce the peak in blood glucose following consumption – making those Christmas leftovers a little healthier for you than the freshly cooked food.

 

A Gift for Your Gut Microbiome

There is a further benefit from the reassociation of starch chains in leftovers kept in the fridge. A small proportion of this starch will become ‘resistant starch’. Resistant starch is starch that completely escapes digestion and reaches the large intestine. Here it acts as an energy source for the complex and vital bacterial community in our large intestines, termed the gut microbiota.

The effect of starch on the gut microbiota is a central theme of research at Quadram. In collaboration with colleagues at Imperial College London, the John Innes Centre, and the University of Glasgow, we are uncovering the mechanisms by which resistant starch can benefit our health. We have discovered that delivering starch to the gut microbiota in peas can produce beneficial molecules called short-chain fatty acids in the large intestine.[3] These molecules act as signals, telling our body that we feel full and should eat less and that we should store more glucose, potentially protecting against a range of metabolic diseases.

After the indulgence of Christmas, we are all looking for something a little healthier. Cutting edge research at the Quadram is revealing how those Christmas leftovers might be a bit healthier than you thought they were.

 

References

[1] Bozic, Natasa & Loncar, Nikola & Slavić, Marinela & Vujčić, Zoran. (2017). Raw starch degrading α-amylases: an unsolved riddle. Amylase. 1. 10.1515/amylase-2017-0002. Reprinted with permission from the 2010 American Chemical Society, Elsevier, and 1997 Science and Technology Facilities Council. https://www.researchgate.net/figure/Starch-multiscale-structure-a-Starch-granules-from-normal-maize-30-m-b-amorphous_fig1_314132048

[2] Koev, T.T., Muñoz-García, J.C., Iuga, D., Khimyak, Y.Z. and Warren, F.J., 2020. Structural heterogeneities in starch hydrogels. Carbohydrate polymers249, p.116834.

[3] Petropoulou, K., Salt, L.J., Edwards, C.H., Warren, F.J., Garcia-Perez, I., Chambers, E.S., Alshaalan, R., Khatib, M., Perez-Moral, N., Cross, K.L. and Kellingray, L., 2020. A natural mutation in Pisum sativum L.(pea) alters starch assembly and improves glucose homeostasis in humans. Nature Food1(11), pp.693-704.