For many people, excess weight creeps on slowly. U.S. research, for example, indicates that, after age 20, most Americans put on one or two pounds each year.
Canadians, too, are getting heavier. Over the past 30 years, the number of overweight adults rose by 21 per cent and obesity jumped by 200 per cent, to 18 per cent from 6 per cent.
While the usual culprits – too much food, too little exercise – account for most weight gain, research published earlier this month in the American Journal of Clinical Nutrition suggests that a diet lacking prebiotic-packed foods can also contribute to excess pounds over time.
Prebiotics are fibrous, non-digestible carbohydrates that, once consumed, make their way to the colon where they fuel the growth of beneficial, probiotic bacteria (e.g., bifiodobacteria and lactobacilli). Feeding probiotic bacteria in the gut is believed to promote better overall health.
Certain strains of probiotic bacteria are thought to to enhance the immune system, treat traveller's diarrhea, ease lactose intolerance, reduce the severity of inflammatory bowel disease and, possibly, lower the risk of colorectal cancer.
The idea that gut bacteria also play a role in weight control is being increasingly recognized by scientists.
The most common type of prebiotics are called fructans, carbohydrates found in artichokes, asparagus, bananas, chicory, dandelion root, garlic, jicama, leeks, onions and whole grains (barley, rye, wheat). Inulin, a fructan extracted from chicory root, is added to many food products such as breads, pastas (such as Catelli Smart Pasta), fruit juices and yogurt to boost fibre content.
Another member of the prebiotic family are galacto-oligosaccharides, or GOS, carbohydrates that occur naturally in breast milk and can also be produced from the milk sugar lactose. Fermented dairy products such as yogurt, buttermilk and kefir contain GOS prebiotics.
For the new study, Spanish researchers followed 8,569 normal weight adults, average age 37, for an average of nine years to evaluate the link between prebiotic consumption and the risk of becoming overweight.
Participants reported their body weight at the beginning of the study and every two years during the nine-year follow-up. Consumption of fructans and GOS was measured at baseline and at study completion.
People with the highest intake of prebiotics – both fructans and GOS – were significantly less likely to become overweight over time than those who consumed the least, even after adjusting for diet and lifestyle factors related to weight gain. (e.g. physical activity, sleep hours, daily calorie intake, fast food consumption).
This longitudinal study – one of the first to examine prebiotic intake and weight gain – suggests that eating more prebiotic-containing foods can mitigate adult weight gain, presumably by altering the composition of gut bacteria.
The study didn't collect stool samples from participants and, as a result, could not determine the composition of their gut microbiota, a collective term for the trillions of microbes that reside in our gut.
Even so, these results add to other research findings suggesting a connection between the foods you eat, your gut microbiota and body weight. Studies have shown that eating a diet low in fibre and high in fat and refined carbohydrates disrupts the makeup of gut bacteria in favour of weight gain.
When bacteria feed on prebiotics, compounds called short-chain fatty acids (SCFAs) are formed in the process. Certain SCFAs have been shown to increase the release of appetite-suppressing hormones in the gut and reduce calorie intake.
Studies conducted in obese rodents have demonstrated the ability of SCFAs to increase calorie-burning and improve insulin sensitivity.
Certainly, additional longitudinal studies are needed to confirm the role of prebiotic-rich foods in body-weight regulation. In the meantime, though, there's no reason not to add these nutritious foods to your diet. (Keep in mind, though, higher intakes of prebiotics may cause bloating and gas in certain people with irritable bowel syndrome, so add these foods gradually.)
Nutrient-dense foods that feed 'good' gut bacteria
To keep helpful gut bacteria flourishing, include these prebiotic foods in your diet. (Prebiotics are not destroyed by cooking.)
Asparagus: High in prebiotic carbohydrates called fructans, asparagus delivers plenty of potassium, vitamin A, vitamin K and anti-inflammatory phytochemicals. It's also one of the best food sources of folate, a B vitamin that keeps DNA in cells in good repair. Eight asparagus spears contain almost half a day's worth of the vitamin (179 mcg). Adults need 400 mcg of folate per day. Add it to stir-fries, pasta dishes, risotto, soups, omelettes, frittatas and vegetable platters.
Jerusalem artichokes: Not truly artichokes, these small brown-skinned tubers are packed with fructans and potassium, a mineral that helps keep blood pressure in check. Prepare Jerusalem artichokes as you would parsnips. Purée roasted artichokes with chicken or vegetable stock to make soup. Or add julienned slices of Jerusalem artichoke to salads and coleslaw.
Jicima: This inulin-containing root vegetable, cultivated in Central and South America, is a good source of fibre and vitamin C. It also offers small amounts of B vitamins and minerals. Pronounced "hee-kuh-muh," jicama looks a bit like a turnip, although the two vegetables aren't related. Its mild flavour and crisp texture make raw jicama a good addition to green salads, bean salads, salsas and crudités. It can also be added to stir-fries or sautéed on its own as a side dish.
Kefir: In addition to prebiotic galacto-oligosaccharides, Kefir serves up a hefty does of probiotic cultures – typically three times the amount found in yogurt. It's also a good source of protein and calcium. Drink kefir on its own, pour it over cereal and granola, or blend it with fruit to make a smoothie. Choose an unflavoured product to reduce added sugars.
Leeks: A milder-tasting member of the onion family, leeks deliver prebiotics along with vitamin A, flavonoids and organosulphur compounds, phytochemicals thought to have anti-cancer properties. Toss finely chopped leeks into salads. Add sliced leeks to omelettes and frittatas. Stir sautéed leeks into soups and stews for extra flavour.
Whole grains: Whole wheat (100 per cent), whole-grain rye and hulled (dehulled) barley are good sources of prebiotic fibres, protein, magnesium and manganese, a mineral that's needed for normal brain and nerve function and to regulate blood sugar. Serve a side of cooked wheat berries, bulgur (a whole grain wheat) or hulled barley as a change from rice or quinoa. When buying rye bread, look for rye berries, whole rye or rye meal on the ingredient list to be sure you're getting whole-grain rye.
Leslie Beck, a registered dietitian, is based at the Medisys clinic in Toronto.