In the past decades, some studies have shed light on the importance of diet in reducing the risk of cancer. More and more evidence is accumulating that gut microbiota could be involved in this relationship between food and disease. A senior investigator at the National Cancer Institute (USA), Rashmi Sinha studies the link between what we eat, our health and the gut microbiota; and she does so from an epidemiological point of view, recruiting thousands of people and following them for decades. She is interested in understanding the long-term factors that may end up affecting our risk of developing metabolic diseases, and also cancer.

Last summer Sinha attended the Barcelona Bdebate Conference (June 30th to July 1st) on the human gut microbiome, held at the Cosmocaixa Science Museum. There we talked to her to find out more about the epidemiological studies she is carrying out in this area.


Is there a link between nutrition, microbiota and the risk of developing cancer?

We know that there is association between diet and cancer, but what is not very clear is how the microbiota is involved in that. At this point, in the case of liver and colon cancer[s], for instance, there are case-control studies, which are done when somebody already has cancer and then you look at the difference between [cancer] cases and people who are controls. The problem is when people already have cancer, things have changed: their diets have changed, their microbiota has changed, so you can’t tell: was it the diet that caused the disease or the microbiota [that] caused the disease? Or is it the disease that has caused the change to occur? At this point, until we do prospective cohort studies, we can’t tell that. So yes, we think it might be involved, but we don’t know for a fact.


In the case of metabolic diseases, the link with gut microbiota seems to be clearer.

Microbiota is quite likely to be involved with obesity and diabetes. The problem with studying cancer is that cancer takes such a long time and you need such large numbers, while metabolic diseases, you can see it in a shorter time frame. But again, we don’t have cohort studies, only associations: diabetes is related to this microbiota profile or obesity is related to that microbiota profile, but you don’t know if the microbiota caused this thing to happen or whether obesity changed that. We would need to follow people when they were healthy for five, ten, fifteen years, until they get the disease. Only then you can look back and say, “Oh, it was this that caused the disease.” So until we are able to do that kind of [study], we are not going to be able to understand the cause and effect.


You are now involved in setting up large cohort studies to investigate the relationship between diet and cancer.

What I’m looking at right now is meat and cancer, and coffee and cancer. Those are the two main areas I am studying. But then I also look at trying to set up large cohort studies, large populations, like 100,000, 200,000, 300,000 people, to collect their microbiota and then follow them for the next 10, 15, 20 years and to make the kind of studies that we need to be able to answer the questions that you are asking me.


We will need to repeat the interview in 10 years, then.

Yes [laughing]. Talk to you then! There are some things we already know. For instance, it’s likely that there are different diets related to different kinds of bacteria. We can’t say which ones and we can’t say, “those bacteria cause cancer.” We can look at the association between meat and bacteria, but not, for the moment, between bacteria and cancer.

I only do human studies. We get a large group of people–maybe, ask them to answer questions about diet, their health, smoking, alcohol, physical activity, their height and weight. We get their blood measures, [their] urine, their faeces, and then we just wait until there are some of them who develop different diseases and then we can go back and look at all the different factors that [explain] why these people got cancer versus the other people that didn’t get cancer.


And what about coffee? Are there studies showing coffee may be beneficial for our gut microbiota?

I don’t know if the coffee is good for the microbiota but I do know the microbiota converts many of the coffee compounds into something that is thought to be good for people. I don’t know if the microbiota changes, but I know the microbiota metabolises the compound[s] in coffee that are thought to be protective, for instance, for liver cancer and endometrial cancer. But, again, we don’t know how the microbiota fits in there.