Salt and stomach cancer -- the data aren't terribly convincing

Warning!
A new cancer scare has been all over the British press -- the World Cancer Research Fund (WCRF) is recommending that because salt has been linked with stomach cancer, "traffic light" color-coded food labeling should be required of all processed foods, the most significant source of dietary salt.  The WCRF says that people should eat 6 grams of salt per day, or about a teaspoon; 75% of salt consumption is from processed food, only 25% added at the table.

Source: BBC

How does salt cause cancer?  The explanation that comes with these new warnings is that salt damages the stomach lining, which leads to disease -- as far as we can tell, this is based on experimental studies feeding rodents high salt diets.  Of course, this doesn't really explain it since cancers involve genetic mutations, whether inherited or somatic. 

Some papers, including some rodent studies, suggest it's an association with Helicobacter pylori, the very widespread bacterium that causes ulcers. Or, it's not added salt but salt-processed foods like meats and pickles. Whatever the mechanism, The Guardian reports that an estimated 14% of stomach cancers could be prevented if salt consumption were reduced (this estimate comes from the WCRF).  We blogged about salt and cancer a while ago (here; sorry, ugly table in that post), but now that the story is back, we thought we'd take a look at how definitive the data are.

But...
First, as we point out every time we blog about dietary factors and epidemiology, it's extremely difficult to get reliable data on any food consumption, and salt is no exception.  Data are almost all retrospective (taken after the fact) and either population based, with the correlation made between salt consumption and stomach cancer rates based on population-level data, or based on individual data from dietary recall (sometimes asking people to remember their diet a year ago or more) or food diaries, which are notoriously unreliable, or household food intake, which assume everyone in the household eats the same diet, and so on.  There's no good way to figure out how much salt people eat in the long run, nor is it clear how long before a tumor excess salt intake would be carcinogenic, nor for how long.  And, the few prospective studies of salt and cancer (following healthy people for years, and assessing their diet along the way) have had conflicting results.  And so on.

So, at the very best, the data are rough.  But don't just take our word for it. A 2011 paper in the British Journal of Cancer on lifestyle factors and cancer says this:  "Although it is currently not possible to pinpoint exactly what constituents of diet are protective against cancer, there is a consensus that diet is an important component of cancer risk."  Not terribly helpful.

The same paper states: 

The difficulties in estimating salt consumption in epidemiological studies probably contribute to the very heterogeneous findings; nevertheless, the consensus view, most recently expressed in the WCRF report (2007), is that salt intake (as well as sodium intake and salty and salted foods) is a probable cause of gastric cancer.  

The calculation of excess risk assumes a simple log-linear increase in the risk of gastric cancer with increasing salt intake. The evidence for this is somewhat equivocal: it is apparent for total salt use in cohort but not case–control studies, whereas for sodium intake it was also apparent in case–control studies; for salted and salty foods, the reverse was observed (dose–response relationship in case–control but not cohort studies; WCRF, 2007).

A book chapter on stomach cancer ("The Epidemiology of Stomach Cancer," in a 2009 book called Cancer Epidemiology) states

The best established risk factors for stomach cancer are Helicobacter pylori infection, the by far strongest established risk factor for distal stomach cancer, and male sex, a family history of stomach cancer, and smoking . While some factors related to diet and food preservation, such as high intake of salt-preserved foods and dietary nitrite or low intake of fruit and vegetables, are likely to increase the risk of stomach cancer, the quantitative impact of many dietary factors remains uncertain, partly due to limitations of exposure assessment and control for confounding factors [italics ours].

And, confounders again...
And, there's the problem of confounders, variables that may be relevant but aren't measured or are difficult to control for.  Every story we've seen about the salt and cancer link this week includes this quote:

Kate Mendoza, head of information at the [WCRF], said: "Stomach cancer is difficult to treat successfully because most cases are not caught until the disease is well-established.

"This places even greater emphasis on making lifestyle choices to prevent the disease occurring in the first place – such as cutting down on salt intake and eating more fruit and vegetables.

Cutting down on salt and eating more non-processed foods.  That's changing two factors, but it's hard to measure the effect.  Are fruits and vegetables actually protective?  Or is it that replacing the bad processed foods with neutral fresh foods decreases salt exposure, and thus cancer risk?  How does that get sorted out?

A book chapter on stomach cancer ("The Epidemiology of Stomach Cancer," in a 2009 book called Cancer Epidemiology) states

The best established risk factors for stomach cancer are Helicobacter pylori infection, the by far strongest established risk factor for distal stomach cancer, and male sex, a family history of stomach cancer, and smoking . While some factors related to diet and food preservation, such as high intake of salt-preserved foods and dietary nitrite or low intake of fruit and vegetables, are likely to increase the risk of stomach cancer, the quantitative impact of many dietary factors remains uncertain, partly due to limitations of exposure assessment and control for confounding factors [italics ours].

No one eats their daily 8.6 grams of salt alone.  High salt consumption generally implies high processed food consumption, and/or high nitrates consumption.  It's very hard to control for that -- that is, to determine that it's the salt and not any other component of the diet that might be associated with stomach cancer.  This is true of most studies of dietary components.  And, if the researcher just loads up rats with salt and measures its effects, that's an unnatural test of salt consumption and may have no correspondence with what's actually happening in human stomachs dealing with a normal diet.

We're not convinced
So we, at least, are not entirely convinced that salt alone is a significant cancer risk.  This, from the same British Journal of Cancer paper we cite above, doesn't help make the case:

The likely adverse effect on cancer risk in the UK is small, as the incidence of gastric cancer is low (gastric cancer ranks only 13th in terms of incidence in the UK, with incidence rates well below the European average (CRUK, 2011)). Average [salt] consumption in the UK is around 10 g per day, and had shown little change between 1986–7 and 2001 (Food Standards Agency, 2004)...There is no direct evidence from intervention studies of the benefit of reduced salt intake with respect to gastric cancer. In Japan, the national dietary policy has resulted in declines in dietary salt intake, and there has been an equivalent reduction in the incidence of gastric cancer (Tominaga and Kuroishi, 1997); however, there have been other changes in prevalence of gastric cancer risk factors – notably in prevalence of infection with Helicobacter pylori (Kobayashi et al, 2004) – and thus the part played by salt reduction is far from clear.

Note that the WCRF is recommending people consume 6g of salt daily, but consumption in the UK is now around 10 g per day -- and gastric cancer rates are low.  In fact, until the 1990's stomach cancer deaths were the leading cause of cancer death worldwide, but mortality had been falling for decades, and currently stomach cancer is "relatively rare" in North America and Northern and Western Europe, although it is still high in Eastern Europe, Russia and parts of Central and South America or East Asia (data from a 2009 paper in the International Journal of Cancer).

There is some thought that the year round availability of fresh fruits and vegetables is responsible for this, and the reduction in mortality may be due in some part to improvements in cancer detection and treatment may be as well. But, if salt is the strong risk factor the WCFR is suggesting it is, given that average salt consumption in the UK is almost twice the recommended amount, stomach cancer rates should not have been falling. 

So, a quick morning's review of the data on salt and stomach cancer leaves us unconvinced that the data are really solid on this.