How does combining foods change how they're digested in the body?

The process of dining, from picking what’s good for you to plating it up, may feel like the end of a journey, but it’s actually the start of another. 


Digestion begins as soon as you put food in your mouth, as enzymes in your saliva (or salivary amylase, as it’s known amongst biology bods) work their magic while you chew. The food’s chemical structure starts breaking down before you swallow, then it travels to the stomach where powerful acids break it down further.     


Next destination is your intestine: home to villi, tiny finger-like brushes which act like combs by taking out the important nutrients and shuttling them to the bloodstream. And the final step? Carrier proteins usher all those nutrients - things like glucose, fats and vitamins - from your bloodstream into the cells they’re needed.


So that’s absorption in a nutshell. But before amylase, villi, and carrier proteins crack on, there’s actually a fair bit of evidence to show that certain food combinations can impact how nutrients are absorbed in the body. Today, we’ll dig into the myths and facts around food combining - and the things you can do to ensure you’re eating right and sending the good stuff to where it’s needed. 


The science of food combining - or is it food separating?


Lots of diets—both old and new—are built upon the idea that eating certain foods separately from each other can help with digestion, speed up weight loss, and contribute to overall health. The Ayurvedic diet, for example, prohibits the eating of raw and cooked foods together, while the Hay Diet separates food into two groups - acidic (meat, seafood and other protein-heavy foods) and alkaline (carbs and starchy foods) - and advises that they should never be combined. Some others recommend eating fruit on an empty stomach, or only drinking water between meals. Ironically, the practice is known as food combining, when in reality, it’s more about separating things out.


Not only are they pretty restrictive, but these sorts of diets have little evidence of efficacy. Research shows that a balanced diet vs. a ‘dissociated’ diet (or one that separates out certain foods) showed no difference in weight loss or body fat changes [1].


So—we’ve established that food separating doesn’t deliver on all its promises, but there’s a lot to suggest that actual food combining (quite literally, eating one thing alongside another) can help your body absorb nutrients better. 


Carotenoids, which are a group of antioxidants found in dark leafy greens, as well as orange and deep red vegetables, are best absorbed by the body when munched with fat. But not just any kind of fat - it has to be a monounsaturated fat, rather than a saturated one... for example, snacking on carrots with hummus (preferably made with olive oil) instead of a cheesy French Onion dip. Interestingly, the carotenoid plus healthy fats combination means that full-fat, homemade salad dressing would trump a shop-bought low-fat dressing, as one study found. [2]

Everyday application: Whisk together extra virgin olive oil with a little mustard and apple cider vinegar to enjoy on a leafy salad made with rocket, kale, or spinach. Totally blows that fat-free Thousand Island out of the water! 


Iron is an essential mineral that our bodies require in order to function. As anyone with anaemia will know, without the right amounts of iron, you may experience fatigue, dizziness, and reduced mental function. You can get iron from meats such as beef, chicken, and fish, as they contain the protein haemoglobin, which is present in red blood cells and helps move oxygen around the body. 


Plant-based foods can be rich in iron too, even though they don’t contain haemoglobin. But eating them with vitamin C captures what’s called ‘non-heme iron’ and stores it in a way that’s more easily absorbed by your body. [3] Some people also choose to take a Vitamin C supplement alongside plant-based sources of iron, which has been shown to boost iron absorption by 67%. [4]


Everyday application: squeeze lemon juice on a lentil or chickpea stew or add segments of grapefruit or orange to dark green leafy salad


There are two main types of carbohydrates - complex (which have a low glycaemic index) and simple (which have high glycaemic index). For people with type 2 diabetes, complex carbohydrates do a stellar job at keeping blood sugar levels stable and help you feel fuller for longer. To boost their benefits even further though, evidence suggests that eating carbohydrates with a healthy protein could slow digestion and reduce glucose response. [5] 


But there’s also evidence that suggests the order you eat your protein and carbohydrates could impact your blood glucose response. Participants who ate a chicken breast, steamed broccoli with butter, lettuce and tomato salad with low-fat dressing, then waited 20 minutes before eating ciabatta bread and orange juice found they had much lower blood glucose and insulin levels than those who’d done the reverse. [6] So either eat carbs with protein, or eat your protein just before the carbs. It’s not a complete dietary overhaul, but one of these strategies could be helpful to adopt when it’s convenient for you.

Everyday application: swap strawberry jam for nut butter with wholegrain toast or top a jacket potato with cottage cheese and tuna, rather than butter and mayo 


If you monitor your blood glucose, you might find it interesting to try some different combinations - everybody has a unique glucose response to various foods, so there is no one perfect way to eat. You may already have adopted elements of the food combining approach without even realising! And the ones you haven’t might even add variety and vibrancy to your cooking and flavour combinations. With some deeper knowledge about how the body digests food and absorbs nutrients, perhaps you feel more confident experimenting and finding what’s right for you from here on in.

 

References


[1] Golay, A., Allaz, AF., Ybarra, J. et al. (2000). Similar weight loss with low-energy food combining or balanced diets. Int J Obes 24, 492–496. Accessible here.

[2] Brown MJ., Ferruzzi MG., Nguyen ML., Cooper DA., Eldridge AL., Schwartz SJ., White WS. (2004) Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings as measured with electrochemical detection. Am J Clin Nutr, 80(2), 396-403. Accessible here

[3] Teucher B., Olivares M., Cori H. (2004) Enhancers of iron absorption: ascorbic acid and other organic acids. Int J Vitam Nutr Res, 74(6), 403-19. Accessible here

[4] Hallberg L, Hulthén L. (2000) Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability of dietary iron. Am J Clin Nutr, 71(5),1147-60. Accessible here

[5] Moghaddam, E., Vogt, J., Wolever, T. (2006) The Effects of Fat and Protein on Glycemic Responses in Nondiabetic Humans Vary with Waist Circumference, Fasting Plasma Insulin, and Dietary Fiber Intake, The Journal of Nutrition, 136(10), 2506–2511. Accessible here

[6] Shukla, A., Iliescu, R., Thomas, C., Aronne, L. (2015) Food Order Has a Significant Impact on Postprandial Glucose and Insulin Levels. Diabetes Care, 38(7), 98-99. Accessible here


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