You can eat all the right foods and still miss out on their goodness. What your body absorbs depends on your gut, your food pairings, when you eat and how you eat.
You eat your greens. You choose lentils over takeaway. You swapped soft drinks for green tea years ago. And yet you're tired, your iron keeps coming back low, and meals leave you bloated.
You may not be doing anything wrong. You might just be missing the other half of nutrition: absorption.
Food only helps once it's broken down, taken in through your gut and carried to your cells. Plenty can get in the way along that journey.
Digestion starts before your first bite.
The moment you see and smell food, your body prepares. Saliva flows, and your stomach begins releasing acid and enzymes.
That response works best when you're relaxed. Stress shifts the body away from 'rest and digest', and can slow digestion and change how the gut works (Konturek et al., 2011).
Sit down to eat, even for 10 minutes.
Chew well. Your stomach has no teeth.
Take a few slow breaths before your first bite.
Stomach acid is your friend.
Stomach acid is often seen as a problem. In fact, you need it to break down protein and to absorb vitamin B12, iron, calcium and zinc.
Acid levels can fall with age and with some long-term medicines. In a large US study, people who took common reflux medicines for two years or more were more likely to be diagnosed with B12 deficiency (Lam et al., 2013). Long-term use of metformin, a common diabetes medicine, has also been linked with lower B12 levels (de Jager et al., 2010).
On reflux or diabetes medication long-term? Don't stop it. Ask your GP whether your B12 and iron levels should be checked.
Your gut does the real work.
Most nutrients are absorbed in the small intestine, through a lining just one cell thick. Alongside it live trillions of gut bacteria that help digest fibre, produce some vitamins and support the gut lining.
Low fibre, frequent antibiotics, stress, alcohol and highly processed diets can all disturb this system. Conditions such as coeliac disease can also damage the gut lining. Ongoing gut symptoms are worth getting checked.
Go for variety. In the American Gut Project, one of the largest studies of the gut microbiome, people who ate more than 30 different plant foods a week had more diverse gut bacteria than those who ate 10 or fewer (McDonald et al., 2018). Herbs, spices, nuts, seeds and legumes all count.
Include fermented foods such as yoghurt, kefir, sauerkraut, kimchi, idli, dosa and miso.
Eat fibre every day from vegetables, fruit, legumes, oats and whole grains.
Food pairings: friends and foes.
Some foods help each other get absorbed. Others get in each other's way.
Food pairings: friends and foes
The same healthy foods, absorbed better or worse depending on what they're eaten with
Pairings that help
Iron-rich plants + vitamin C
Vitamin C can substantially increase the iron absorbed from plant foods, as in lentils with lemon (Hallberg et al., 1989).
Colourful vegetables + a little fat
Carotenoids are absorbed far better from salad with a full-fat dressing than a fat-free one (Brown et al., 2004).
Cooked tomatoes + oil
Lycopene is more available from cooked, processed tomatoes than raw ones (Gärtner et al., 1997).
Turmeric + black pepper
In one small study, piperine raised curcumin absorption about 20-fold (Shoba et al., 1998).
Pairings that hold you back
Tea or coffee with iron-rich meals
Black tea served with a meal cut iron absorption by well over half (Hurrell et al., 1999). Enjoy it between meals.
Calcium and iron together
Calcium taken with a meal reduces iron absorption from that meal (Hallberg et al., 1991). Space supplements apart.
Relying on spinach for calcium
People absorbed only about a fifth as much calcium from spinach as from milk (Heaney et al., 1988).
Spinach is still an excellent vegetable. For calcium, though, rely on dairy, calcium-set tofu, fortified plant milks, almonds or sesame seeds.
How you prepare food matters.
Soak, sprout or ferment grains and legumes. This reduces phytates, which bind iron, zinc and calcium (Gupta et al., 2015).
Steam rather than boil vegetables. Boiling causes larger losses of vitamin C and other water-soluble nutrients (Yuan et al., 2009).
Cook some foods, keep some raw. Cooking releases more carotenoids from carrots and tomatoes, while raw fruit and salads protect heat-sensitive vitamin C.
Timing: when you eat may matter.
Your body runs on a 24-hour clock. Blood sugar tends to be handled better earlier in the day and less well late in the evening (Poggiogalle et al., 2018). Large, late meals may push blood sugar higher and disturb sleep.
Front-load where you can. Make breakfast and lunch substantial, and keep dinner lighter and earlier.
Spread protein across the day. One study found muscle protein synthesis was about 25% higher when protein was spread evenly across meals rather than loaded into dinner (Mamerow et al., 2014).
Take vitamin D with a meal that contains fat. Fat in the meal improves its absorption (Dawson-Hughes et al., 2015).
Other things that quietly get in the way.
Stress and poor sleep change how your gut works. Alcohol irritates the gut and interferes with B-vitamin absorption. Not drinking enough water limits what fibre can do. And with age, absorbing B12 and calcium becomes less efficient.
Signs you may not be absorbing well.
Ongoing tiredness despite eating well. Bloating, reflux or irregular bowel habits. Brittle nails, hair shedding or frequent cramps. Low iron, B12 or vitamin D on blood tests.
If this sounds familiar, talk to your GP before adding more supplements. More isn't better if your body can't absorb what it already gets.
The bottom line.
Healthy eating isn't only about what's on your plate. It's about helping your body use it. Slow down at meals, feed your gut with variety and fibre, pair foods wisely and pay attention to timing. These small changes help the food you already eat work harder for you.
References
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- Dawson-Hughes B et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225–230.
- de Jager J et al. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency. BMJ. 2010;340:c2181.
- Gärtner C, Stahl W, Sies H. Lycopene is more bioavailable from tomato paste than from fresh tomatoes. Am J Clin Nutr. 1997;66(1):116–122.
- Gupta RK, Gangoliya SS, Singh NK. Reduction of phytic acid and enhancement of bioavailable micronutrients in food grains. J Food Sci Technol. 2015;52(2):676–684.
- Hallberg L, Brune M, Rossander L. The role of vitamin C in iron absorption. Int J Vitam Nutr Res Suppl. 1989;30:103–108.
- Hallberg L et al. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. Am J Clin Nutr. 1991;53(1):112–119.
- Heaney RP, Weaver CM, Recker RR. Calcium absorbability from spinach. Am J Clin Nutr. 1988;47(4):707–709.
- Hurrell RF, Reddy M, Cook JD. Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 1999;81(4):289–295.
- Konturek PC, Brzozowski T, Konturek SJ. Stress and the gut: pathophysiology, clinical consequences, diagnostic approach and treatment options. J Physiol Pharmacol. 2011;62(6):591–599.
- Lam JR et al. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013;310(22):2435–2442.
- Mamerow MM et al. Dietary protein distribution positively influences 24-h muscle protein synthesis in healthy adults. J Nutr. 2014;144(6):876–880.
- McDonald D et al. American Gut: an open platform for citizen science microbiome research. mSystems. 2018;3(3):e00031-18.
- Poggiogalle E, Jamshed H, Peterson CM. Circadian regulation of glucose, lipid, and energy metabolism in humans. Metabolism. 2018;84:11–27.
- Shoba G et al. Influence of piperine on the pharmacokinetics of curcumin in animals and human volunteers. Planta Med. 1998;64(4):353–356.
- Yuan GF et al. Effects of different cooking methods on health-promoting compounds of broccoli. J Zhejiang Univ Sci B. 2009;10(8):580–588.
Written by Pratha
Nationally Recognised Nutrition Practitioner (Australia) — HWC, SNC
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