Someone said something nice about white bread

A really interesting paper came out recently looking at the impact of diet and nutrition on “microbiota” — the complex ecosystem of bacteria that form the heart of our gastrointestinal tract.

Keep in mind that in terms of sheer numbers, the bacteria living inside of up vastly outnumber the cells in our body, so from a certain perspective we’re as much defined by our microbiota as we are by our cell structure.]

Intestinal microbiota not only play a critical role in our digestion of food, but studies have shown that they have wide impact upon health and disease in general in our bodies, including direct effect on our immune system.  But there has been little work on understanding how both individual foods and combinations of foods in one’s diet influences the number, health and function of gut bacteria.

This new study asked adults to keep an extensive diary of their food intake over a year, and then checked for levels of specific well-known gut “good” bacteria in their GI tract (through DNA analysis of fecal samples — there’s a grad student job you don’t want).

The bad news is that they found a correlation between eating oranges and depressed levels of two important gut bacteria. Now it’s worth cautioning that this is an early study that needs to be replicated before we give it a lot of faith, and it was trying to explore effects that came from combinations of foods or specific chemicals in foods. There has been some past evidence that pectins might depress gut bacteria levels; oranges are a source of pectins, but they are also a source of several other compounds and there is a hypothesis that pectins PLUS flavanones is what is causing the drop in gut bacteria levels, not pectins alone. Oranges contain both pectins and flavanones, but there are other foods that you could get either or both from. There is a lot more work to do in order to understand what this finding means, and oranges provide many other important nutrients, so you should not take this as a sign to stop eating oranges. For now, just take it as a curiosity in an area where we are trying to learn more.

The other unusual finding, and I suppose you might call this good news, is that white bread had a strong positive correlation with increased levels of lactobacillus, another important “good” gut bacteria. The refined grains in white bread, and most pasta, break down into simple sugars quickly and easily and so they tend not to provide much nutritional value on their own, but this study has strengthened an existing hypothesis that two compounds in white bread — hemicellulose and resistant starch — can raise lactobacillus levels.

There is a real limit to this study because of its approach: it is a classic multivariate study, with lots of people eating lots of different combinations of foods over a long period of time. The fact that no two diets are alike allows for some ability to find correlations between gut bacteria levels in that population and specific foods that they all ate, but it gets fuzzier as you try to look for the effect of combinations of foods.  So ideally, scientists develop a new set of hypotheses from this study, and then go off and test them individually under properly controlled circumstances so that a clearer effect (or lack of effect) can be observed.  For instance, they can take the hypothesis “white bread raises lactobacillus levels”  and they can build an experiment around that, where white bread is added or removed from individuals’ diet where everything else is controlled for (and a separate control group maintains the same diet over the same period of time) to see if that one change actually makes a difference.  Science!

What this does remind us, though, is that diet, nutrition and digestive health are complicated beasts, in which individual foods (including oranges and white bread) all have benefits and liabilities. And we are much better off eating a wide variety of foods than narrowing our diet to a smaller set where the effects of individual foods would be more concentrated. Our ancestors did this naturally as the seasons controlled which foods were available; there was no way they could eat the same food all year. By contrast, with modern global transportation and agricultural hot houses, our grocery stores stock most of the same foods year-round, so it’s much easier for us to stick to the same things all the time. So the next time you go to the grocery store, grab something different for a change.


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