I thought white bread was unhealthy. I thought white rice was unhealthy at one time. In fact, although I’ve been eating more white rice (almost daily), I have still stayed away from white bread, because of my fear that it’s fattening, spikes blood sugar, etc etc. Having a strong diet mentality is like having strict parents growing up. Sometimes, the stuff you convince yourself to be true stays with you subconsciously even if you’ve been ready to move past it. I guess I have to admit that I haven’t full recovered mentally, as I still subconsciously avoid certain foods because of something I read years ago about it that isn’t true.
A study published earlier this year contradicts what white bread haters say about its health effects. This study examined the effects of white bread vs. sourdough bread on several clinical variables as well as the gut microbiome. The study employed a crossover design, where each participant (20 total) was compared against themselves and tried both white bread and sourdough bread for one week each. The study found no major differences between bread type, challenging the idea that white bread is absolutely terrible for us and that sourdough or whole wheat breads are inherently healthier for us.
The study was particularly interested in predicting the glycemic response (blood sugar after eating) to bread based on an individual’s gut microbiome. Below, I will summarize a few different aspects of the study, bulleted below:
- Differences between white bread and sourdough bread on glucose tolerance, fasting blood glucose (FBG), CRP, and other clinical variables
- Effects of bread on the microbiome
- How the microbiome predicts the glycemic response to the two different kinds of bread
White bread vs. sourdough: results
Participants either ate white bread first for one week, then after a two week break ate sourdough bread for another week, or vice versa. They ate 145 grams and 100 grams of sourdough or white bread respectively, each morning, which each provided 50 grams of carbohydrate. During the first week, participants were instructed to eat as much bread as they liked for the rest of the day. During the second week when they switched to the other type of bread, the amount of bread they ate in the first week was matched.
Before and after each week of bread consumption, the researchers measured several variables, as well as taking a stool sample:
- Calcium, magnesium, and iron
- Liver enzymes (AST, ALT, GGT)
- C-reactive protein (CRP, a marker of inflammation)
- Total, LDL, and HDL cholesterol
- Creatinine, urea
- Lactate dehydrogenase (marker of tissue damage)
- FBG, and OGTT (oral glucose tolerance test)
- Blood pressure
- Basal metabolic rate (BMR)
There was no difference between the type of bread on any of these parameters. However, both bread-types on average decreased calcium, magnesium, and iron, as the researchers expected, and slightly decreased total and LDL cholesterol, as well as CRP, as well as decreasing liver enzymes. LDH however increased.
Effect on microbiome
The researchers expected to see a change in the gut microbiome after either intervention, but there was none. They concluded that the gut microbiome was “resilient” to the bread intervention.
…the overall microbiome signature of subjects remains unique and conserved throughout the study
Each individual has a unique microbiome “signature” and despite changes in the diet, their signature never deviated too much from before the change in diet.
However, there was a difference between the type of bread…this part was a bit confusing, but white bread increased numbers of Eubacterium ventriosum as well as species in the Anaerostipes genus. Both these types of bacteria produce butyrate, a short-chain fatty acid that is good for our colons. Butyrate is a subject of controversy however, as its chemopreventive properties depend on dose, type of fat in the diet, and other variables.
Variability in the glycemic response
There was significant variability in the postprandial glycemic response (PPGR); some people’s blood sugar went up more after white bread and others’ went up more after sourdough bread. The PPGR is an independent predictor of developing diabetes, so the researchers were particularly interested if they could predict your glucose response based on your microbiome, and even have an app now.
As a reference, we note that the published glycemic index for similar bread types was 70 and 54 for white bread and sourdough bread, respectively (Foster-Powell et al., 2002). As the glycemic index inherently ignores inter-individual differences, it would result in wrong classification for the ten subjects who had lower glycemic responses to white bread than to sourdough bread (Figure 4A).
They don’t elaborate on this further but they produced an algorithm that accurately predicted an individual’s response to bread based on their microbiome. This is the information that’s in their app so they probably don’t want to give away their secrets.
White bread didn’t cause inflammation, or increase cholesterol levels, or seem to adversely affect the health of these twenty participants, whether it was consumed during the first one-week intervention period, or during the second one. In fact, two taxa of bacteria saw significant increases, and these taxa are butyrate producers, suggesting that white bread may have improved colon health to an extent; although, the microbiome was largely unaffected by either type of bread.
What the researchers focused most on was that there were marked inter-individual differences in the PPGR. Some subjects saw their blood glucose go up more after sourdough bread and others saw it go up more after white bread. I would think there are other factors outside the microbiome that could affect this too, but the microbiome was the focus here and their model accurately predicted how people responded.
The results of this study are enhancing our understanding of personalized nutrition through science. I’m a huge fan of intuitive eating, but I certainly am also interested in understanding how my stool could influence my food choices. I’d love to try this out and then see how my own intuitive eating methods validate the recommendations from algorithms such as the ones these researchers devised.
In conclusion, health is a two-way street. The healthiness of a food depends on your gut, your digestion, and your unique needs. Until more is known about this, it is prudent to use your intuition and common sense to check in with your body to understand how a food is affecting you, rather than assuming that a single food is inherently healthy or unhealthy. This is something I talk about a lot in my upcoming book; our ideas of what foods are healthy are really just estimates, and are often wildly inaccurate. Although white bread does seem to cause me to gain weight, I may try some out later this week, in light of these findings, as I have been sticking to sourdough whenever I do buy bread (not often), and I can’t say I’ve been enjoying it too much, although I really want to knowing that it’s in theory supposed to be healthier and is prepared in a more traditional way than most bread currently on the market.