Championing potatoes, leftover pasta and beans on toast, is the ‘resistance starch’ diet too good to be true? Hattie Garlick looks at the evidence.
I am about to embark on a radical new weight-loss diet. Want to hear the rules? Beans on toast, leftover pasta and potatoes, and reheated rice. I sense you raising an eyebrow, but let me assure you. The regime is backed by science. Or so reports on new research would have us believe.
This week, a randomised, placebo-controlled study published in the journal Nature Metabolism found that supplementing the daily diets of overweight and obese people with 40 grams of resistant starch improved their insulin resistance, boosted their gut microbiome and helped them shed weight.
What do we have to do to get our hands on this magical medicinal compound? Well, that’s the best bit. No need to hand over half your income for an acrid powder ground from roots in a remote Amazonian village. It’s already in your kitchen. Resistant starch occurs naturally in some of our most everyday, unglamorous staples like beans, lentils and wholegrain bread, and in some of our cheapest and laziest meals, like leftover potatoes, pasta and rice.
So how exactly does this diet work, and is it really the one we have all been waiting for?
What is resistance starch?
The study participants received the same diet for 16 weeks, but lost an average 2.8 kilograms during the eight in which they were also fed resistant starch (in powdered form, mixed with water). How? “Because they absorbed less glucose,” explains Simon Gaisford, a professor of pharmaceutics and microbiome expert at University College London.
Let’s get back to basics. All carbohydrates and sugars contain the same thing, Gaisford explains: glucose. The difference lies in how your body processes them once swallowed.
Sugar is composed of just two molecules, so is easily broken down – the glucose absorbing rapidly from your small intestine into your blood, leading to rapid blood sugar spikes. When that glucose cannot be removed from the blood, the body stores it in tissues as fat. Carbohydrates, including fibre and starch, are constructed differently. Their glucose molecules are joined in long chains – some are easy to dismantle, others less so. “The more branched, cross-linked or tightly wrapped the carbohydrate, the harder it is for the body to break them down and utilise them for fuel,” says Gaisford.
Simple starches are easily broken down and absorbed in your small intestine. But resistant starch is tightly bound, making it act more like fibre, which cannot be broken down, so passes instead into your lower intestine. This has immediate upsides if you are trying to control, or decrease, your weight, suggests Rhian Stephenson, a nutritional therapist and the founder of the Artah nutrition company. “Studies have shown that resistant starch consumption can reduce postprandial glucose – the rise in blood sugar levels after eating – by 33 per cent.”
This, she explains, has important and positive impacts on our appetite, mood and energy: “We want a sloping rise and fall in blood sugar, not a dramatic spike and subsequent crash. The slower, more measured release of carbohydrate into the blood helps us stay fuller for longer and feel better in general.”
The guts of the issue
The latest study, however, points towards yet another potential benefit. Gut health in the subjects who took resistant starch also improved, likely because of what happens in the next stage of its journey, inside your large intestine, says Gaisford.
“Although we can’t digest these materials, our gut bacteria can,” he explains. “They break it down to release glucose, but then they use that glucose for fuel, so we don’t get blood glucose spikes.” When we feed our “good bacteria” with resistant starch, we are repaid bountifully. “Different types of bacteria produce different waste products from metabolism, so in effect the bacteria in our gut turn something we would ordinarily waste into a myriad of useful compounds.”
The study, Gaisford stresses, was small. But the science broadly stacks up. So how can you get more resistant starch into your meals?
Baked beans on toast (but hold the sugar)
“There are three types of naturally occurring resistant starch,” explains Stephenson. Type one is inaccessible – or resistant to digestion – because it is trapped within the fibrous cell walls of the food itself. This form is found in beans, lentils and grains.
So is it time to rehabilitate the reputation of my biggest (and hitherto guilty) pleasure: beans on toast? “It’s all about the nuance here,” says Stephenson. “Homemade or more natural forms of baked beans, on high-quality toast like wholegrain sourdough, could be a great choice. Beans with lots of added sugar and thickeners like corn starch on ultra-processed bread would not confer the same benefits.” Drat.
Overnight oats
Oats are among the cheapest items on the supermarket shelf. Raw, they are also a good source of resistant starch, says Stephenson, but the benefits fade once they are cooked, “so to maximise resistant starch, you can make overnight oats using jumbo oats”.
She suggests soaking two cups of oats in the same quantity of milk (or half milk, half water). Add a teaspoon of vanilla extract, some mashed berries and a pinch of salt. Stir, and leave in the fridge overnight. In the morning, top with seeds.
Banana and almond butter
Cheap, sweet and naturally pre-wrapped, the humble banana is one of the easiest snacks around. It is also high in ‘type two’ resistant starches, indigestible because they are so compact. But there is a catch, suggests Stephenson. You have to eat them while they’re still relatively green. As bananas ripen and yellow, the resistant starch turns into simple sugars.
Slice yours up before they get to this stage, then top them with almond butter to add a moreish creamy texture (as well as protein, fibre and healthy unsaturated fats that will also help to keep you feeling full for longer).
Potato salad (with leftover spuds - and without mayo)
There is, Stephenson explains, one final form of naturally occurring resistant starch: “Retrograde resistant starch forms when certain starches have been cooked and cooled.”
When you cool cooked pasta, potatoes or rice, the process turns some of the food’s starches into resistant ones. In 2013, American research concluded that a baked potato contained 3.6 grams of resistant starch per 100g and a boiled potato 2.4g. But if you chilled either one, that figure rose to 4.3g.
So, Stephenson suggests, a simple potato salad is not just a good way of stretching last night’s supper into today’s cheap leftovers lunch. It is also a great way of enjoying potatoes while reducing their glycemic load (the amount it will raise your blood glucose levels).
Roughly chop some leftover boiled potatoes. Add some boiled eggs, spring onions, celery, tomatoes, and deseeded cucumber (all chopped). Dress with olive oil, Dijon mustard, apple cider vinegar, a squeeze of lemon and chopped dill and parsley. Toss and enjoy.
Toast straight from the freezer
“The same effect happens with fresh bread kept in the freezer,” says Dr Emily Leeming, a scientific researcher at King’s College London and the author of Genius Gut: How to Eat to Superpower Your Second Brain. As it freezes, starches are turned into resistant ones.
So use your loaf by keeping some bread in the freezer – not just to save money (by preventing it from moulding and wasting), but also to boost your health.
Microwaved rice
In 2015, Sri Lankan researchers found that white rice, boiled in water with a teaspoon of coconut oil, drained, then left in the fridge for 12 hours, contained at least 10 times the resistant starch to that conventionally prepared. Reheating it had no adverse effect on these boosted properties.
In fact, other studies have suggested that microwave reheating, in particular, can slow the pace of starch digestion (when compared to other forms of heating).
... with dhal
Legumes and pulses like lentils and chickpeas are one of the best sources of resistant starch. Nearly half of the starch in raw legumes is resistant to digestion. In 2022, a scientific review conducted at Florida State University found that working the resistant starch into your diet via pulses “fosters the growth of beneficial gut bacteria and significantly enhances the production of short-chain fatty acids in the colon”.
Lucky, then, that they are also ridiculously cheap, easy to have hanging around in either tinned or dried form, and simple to transform into a simple dhal.
Jamie Oliver’s recipe calls for a pack of red lentils, turmeric, butter, cumin seeds, garlic and green chillies. Add garam masala, tomatoes, ginger and coriander if you can.
How much is too much?
Ready to join the resistant starch revolution? Well, there are a few final details to consider. Participants in the new study were fed 40g of resistant starch a day, in the form of a powder. “It’s highly unlikely you would eat 40g of resistant starch without taking a supplement,” says Leeming. For reference, 100g of cooked beans is likely to contain between 1-5g of resistant starch. The same quantity of cooled and then reheated white rice will serve up around 1.65g.
In any case, it is best not to focus exclusively on your resistant starch intake, says Dr Federica Amati, the head nutritionist at Zoe: “Too much of any one type of prebiotic can be problematic. It’s best to have a diverse range of fibres and prebiotic foods in our diet.”
Leeming and Stephenson agree that the more useful goal would be to increase your daily fibre intake, with resistant starches counting towards that target. The majority of us fail to get anywhere near the minimum 30g that adults should aim for daily, yet fibre lowers your risk of heart disease, stroke, type two diabetes and bowel cancer, as well as helping you feel fuller for longer (and thus deterring you from reaching into the biscuit drawer).
So maybe I won’t be embarking on a beans and toast diet after all. Still, says Stephenson, “considering how to increase the resistant starch of foods that traditionally have less fibre and more of an effect on blood sugar, like white rice and potatoes, can be a great way to tweak your nutrition, get more fibre, and make these foods work harder for you”.