Is white bread just as healthy as brown?

"Sliced white bread is 'just as healthy as brown', shock findings reveal," The Sun reports.

A small study looking at the effects of eating different types of bread – white versus brown sourdough – found no significant differences.

But the researchers also reported responses varied from person to person, depending on their gut bacteria.

The study measured 20 health markers, but was mainly focused on increased blood sugar levels after eating (glycaemic control).

Researchers found no overall differences in glycaemic control when people ate white bread compared with wholemeal sourdough bread.

But when they looked at people's individual responses to bread, they found some responded better to white bread, while others responded better to wholemeal sourdough bread.

The researchers said the response could be predicted by the types of bacteria living in the gut.

The question of whether wholemeal or white bread is healthier isn't settled by this study, which only lasted two weeks and involved just 20 people.

Wholemeal bread is a good source of fibre – a diet higher in fibre reduces the risk of bowel cancer, helps digestion, and may help people feel fuller, avoiding weight gain.

Where did the story come from?

The study was carried out by researchers from the Weizmann Institute of Science in Israel. We don't know who funded it.

Two of the researchers declared a conflict of interest as they are paid consultants for a company that promotes personalised nutrition based on gut biome analysis.

The study was published in the peer-reviewed journal, Cell Metabolism.

The Sun reported "shock results" that "white bread is just as healthy as a brown loaf", a claim echoed by the Daily Mail – although this isn't really what the research found.

But both newspapers did quote nutrition experts, who pointed out that a week-long trial in just 20 people doesn't provide a conclusive result.

The Guardian rightly highlighted the fact that there were different results for different people. None of the papers mentioned the researchers' conflict of interest.

What kind of research was this?

This was a small randomised cross-over trial of two types of bread, eaten by healthy volunteers for one week each.

One week may not be long enough to show meaningful results.

What did the research involve?

Researchers recruited 20 healthy people. They were provided with either white bread or wholemeal sourdough bread, and instructed to eat at least a certain amount each day for a week, but no other wheat products.

They had a two-week break after seven days, then switched to the other type of bread for one week.

The participants were tested on a range of health markers before, during and after the study.

Researchers looked at whether the markers were different when the people were eating one type of bread compared with the other.

They took blood tests to measure triglycerides (fats), LDL and HDL cholesterol ("bad" and "good" cholesterol), and levels of liver enzymes (ALT, AST, GGT, LDH), iron, calcium, creatinine, urea, thyroid stimulating hormone, and C-reactive protein (a measure of inflammation).

They measured blood pressure, weight and metabolic rate, and tested people's stools for bacteria. 

Blood glucose levels were measured in the 15 minutes after waking up, and blood glucose response to a glucose test (the body's response to consuming glucose syrup) was also tested.

The researchers did a post-hoc analysis to see whether people's results were different after eating any type of bread compared with their usual diet before the study, and whether their gut bacteria (measured from stool samples) were different.

They also tested people individually on their response to eating white or brown sourdough bread, and looked at whether their gut bacteria could predict how they would respond.

What were the basic results?

Researchers found no significant differences between results in any of the clinical parameters measured, including blood glucose (glycaemic) response after seven days of eating white bread or wholemeal sourdough bread.

Glycaemic response increased after a week of eating white bread for some people, while for others it decreased. The same was true for wholemeal sourdough bread.

The researchers found two types of bacteria were more common after people had eaten white bread for a week, but the clinical significance of this isn't clear. They showed that for most people, eating bread of any type had little effect on gut bacteria.

The researchers say there was more "interpersonal variability" between glycaemic response to the type of bread than you would expect by normal distribution.

They go on to say analysis of glycaemic response according to six measures of gut bacteria could "predict" individual people's response to each type of bread.

How did the researchers interpret the results?

The researchers concluded that, "Understanding the interpersonal variation in the effect of bread, one of the most consumed staple foods, would allow the personalisation of bread-related nutritional recommendations and optimisation of food choices worldwide."

They say their study "underlines the importance of personalisation in dietary recommendations", and suggests that "universal dietary recommendations may have limited efficacy". 

Conclusion

Studies that suggest "everything you thought you knew about healthy eating is wrong" create great headlines, but rarely stand up to much analysis.

There are many reasons why you might choose wholemeal bread over white bread, and results from a week-long study in 20 people aren't going to change all of those.

The main measure of interest in the study is glycaemic control, a measure of how quickly the body can process glucose consumed in the diet.

Poor glycaemic control is seen as a risk factor for type 2 diabetes, where the body can't process glucose properly, leading to high blood sugar, which can damage blood vessels.

The study showed no overall difference over the course of a week in people's ability to process glucose, assessed by which type of bread they ate.

It may be that the study was too short to show a change. But there are other reasons to eat wholemeal bread, including the benefits of eating plenty of fibre, which can help digestion and has been linked to a lower risk of bowel cancer.

What the study did seem to find was that different people react differently to different foods, which isn't a big surprise.

It's interesting that this seems to be linked to the bacteria that live in your gut. This might be of interest to dietitians and doctors treating people with diabetes or poor glycaemic control.

But there's no need to worry about getting tests of your gut bacteria or swapping from your preferred type of bread if you have normal blood glucose.

While conflicts of interest are common in scientific research, it's worth keeping in mind that two of the researchers involved in this study work for a company that offers to "balance your blood sugar with personalised nutrition", offering dietary advice based on the results of stool tests.

Find out more about how wholegrain foods are an important part of a healthy diet.