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NHS Direct Health News

NHS Choices: Behind the headlines   + / -  
last updated: Wed, 02 Dec 2015 02:08:42 GMT

 Tue, 01 Dec 2015 11:10:00 GMT Do potatoes reduce stomach cancer risk?

"Eating lots of potatoes will reduce your risk of getting stomach cancer," according to enthusiastic media reports that seized on the UK's love affair with the spud.

The mouth-watering headline followed the publication of a large Chinese review into the link between diet and stomach cancer, which involved 76 studies and 6.3 million people across several countries.

However, the news reports were perhaps a little hasty in their conclusions – the study didn't find any specific link between eating potatoes and a lower risk of stomach cancer.

Stomach cancer is one of the most common cancers, accounting for almost 10% of cancer deaths. Research suggests some foods may help protect against stomach cancer, while others may increase the risk of getting it.

The media focus on potatoes seems to have come from the link researchers found between the cancer and white vegetables in general, such as potatoes, cabbage, onions and cauliflower.

The study found eating lots of different types of fruit, white vegetables and vitamin C was associated with a lower risk of stomach cancer.

A high intake of fruit was associated with a 7% reduction in stomach cancer. White vegetables were associated with a 33% lower risk. Meanwhile, a diet high in pickled vegetables, processed meats like sausages, salted foods and alcohol was associated with an increased risk.

Although it has several limitations, this large review will contribute to the growing body of evidence informing the dietary associations with stomach cancer.

However, it is not possible to give any firm conclusions based on this review alone. It is certainly not possible to say at this stage that eating potatoes will reduce your risk.

Where did the story come from?

The study was carried out by researchers from Zhejiang University in China, and was funded by the Chinese National Natural Science Foundation and the Natural Science Foundation of Zhejiang Province.

It was published in the peer-reviewed European Journal of Cancer.

The media seems to have gone with the slant that eating potatoes will decrease your risk of stomach cancer. 

But this was not a specific finding of this review, which actually found a reduced risk of stomach cancer was associated with a higher consumption of "white vegetables".

White vegetables include potatoes, as well as cabbage, cauliflower and onions. The review did not find any link at all when it looked specifically at potatoes. 

What kind of research was this?

This systematic review aimed to pool the results of published prospective cohort studies that examined whether individual dietary factors are associated with stomach cancer risk.

As the researchers say, stomach (gastric) cancer is the fourth most common cancer in men and the fifth most common cancer in women worldwide, and counts for just under 10% of the deaths from cancer.

Dietary factors are believed to play a role in stomach cancer risk. Many previous observational studies have looked into this, including the large European Prospective Investigation into Cancer and Nutrition (EPIC) study.

The researchers say these studies suggest that processed meat may slightly increase risk, while a higher consumption of fruit and veg may decrease risk.

A systematic review is the best way to identify all published research on a given question and summarise what this evidence suggests.

Singling out individual dietary factors associated with health outcomes is challenging, as other dietary and lifestyle factors play a part, and it can be difficult to remove their effects.

Usually, drawing firm conclusions about what causes a particular disease requires drawing together a wide range of different types of evidence.

What did the research involve?

The researchers reviewed several literature databases to identify prospective cohort (observational follow-up) studies published up to the end of June 2015.

Eligible studies had to have looked at any dietary exposure (food, drinks or nutrients) and examined the risk of stomach cancer as the outcome.

Relevant studies were assessed for quality and two researchers independently extracted data from the studies to reduce the risk of error.

In total, 76 studies met the inclusion criteria, all of which were rated as being of moderate to high quality. These studies had followed a total of 6,316,385 people for 11.4 years, on average, and identified 32,758 new cases of stomach cancer over this period.

Thirty-seven of the studies were conducted in Europe, 11 in the US, 21 in Japan, four in China and three in Korea. The diets they were examining varied widely, from alcohol and salted foods to green tea and ginseng. The researchers pooled studies looking at the same food or food type to give an overall result.

What were the basic results?

Looking at studies examining fruit and vegetables, the results of 22 studies were pooled in an analysis of total vegetable consumption. No link was found with stomach cancer.

Meanwhile, 30 studies of total fruit consumption found a higher intake of fruit was associated with a 7% reduction in stomach cancer (relative risk [RR] 0.93, 95% confidence interval [CI] 0.89 to 0.98).

There was no association with stomach cancer for many of the specific fruits and vegetables examined. However, there were significant links with a few:

  • white vegetables were associated with a 33% decrease in risk (RR 0.67, 95% CI 0.47 to 0.95; data came from six studies)
  • pickled vegetables were associated with an 18% increase in risk (RR 1.18, 95% CI 1.02 to 1.36; data came from 20 studies)
  • tomatoes were associated with an 11% increase in risk (RR 1.11, 95% CI 1.01 to 1.22; data came from five studies)
  • spinach was associated with a 21% increase in risk (RR 1.21, 95% CI 1.01 to 1.46; data came from five studies)

Interestingly enough, despite the media focus on potatoes, no significant link was found between potatoes and stomach cancer (RR 0.93, 95% CI 0.82 to 1.06; seven studies).

Looking at other food types, significantly increased risk was found with:

  • processed meat (13 studies)
  • salted fish (11 studies)
  • high-salt foods (12 studies)
  • salt (8 studies)
  • alcohol (24 studies)
  • beer (13 studies)
  • liquor (12 studies)

A few studies looked at the effects of specific nutrients. The one significant result to come out of these analyses was that vitamin C was found to reduce risk by 11% in a pooled analysis of five studies (RR 0.89, 95% CI 0.85 to 0.93).

Where there was data available to examine the quantities of a specific food or drink needed to have an effect, the researchers found significant links for:

  • total fruit – 5% reduced risk per additional 100g a day
  • citrus fruit – 3% reduced risk per additional 30g a day

There weren't, however, significant dose links with white vegetables or vitamin C. If a factor dose directly affects risk, researchers would expect to see what they call a dose response – meaning the more fruit you eat, for example, the more your risk changes.

There were also significant dose links for:

  • salted fish and high-salt foods (4% and 10% increase for each item per week, respectively)
  • salt (12% increase per 5g a day)
  • alcohol (5% increase per 10g a day)

How did the researchers interpret the results?

The researchers concluded that, "This study provides comprehensive and strong evidence that there are a number of protective and risk factors for gastric cancer in diet." 

They say their findings "may have significant public health implications with regard to prevention of gastric cancer, and provide insights into future cohort studies and the design of related clinical trials."


This systematic review has gathered and summarised the results of prospective cohort studies published to date that have examined links between specific dietary items and the risk of stomach cancer.

The review has many strengths, including the large number of studies that have been reviewed and quality assessed, the large sample size, and extensive analyses carried out by individual food type.

But there are various points to note when interpreting these results. The media attention focused on potatoes and how we should eat these to reduce the risk of stomach cancer – this was not a finding of this study. It presumably comes from the decreased risk found with white vegetables.

However, exactly what white vegetables this included was not specified. Studies that specifically examined potatoes found no link with stomach cancer.

Also, researchers weren't able to say how many white vegetables should be eaten to have a preventative effect. The researchers would expect to find this sort of link if a specific food is affecting the risk of stomach cancer.

Eating lots of fruit and vitamin C were also associated with a lower risk. But, again, while researchers could say each 100g increase in fruit a day was associated with a decreased risk, no dose response was found with vitamin C.

Given the varied results for fruit and vegetables – overall and by specific type – it is difficult to give specific advice, other than that the findings generally support advice to eat a healthy, balanced diet with plenty of fruit and vegetables.

While all the studies were rated to be of moderate to high quality, they varied widely in the population assessed, follow-up time and the main food item being examined.

There are many unknowns that could impact on the strength of the evidence found by the individual studies. This includes the methods of assessing dietary intake and over what period this was examined, how cancer outcomes were assessed, or whether researchers adjusted for other health and lifestyle factors that may influence the results.

For example, smoking is an established risk factor for stomach cancer. Smoking – or not smoking – may be associated with other "healthy" or "unhealthy" dietary habits.

Generally, a diet high in fruit – and possibly certain vegetables – has for some time been recognised to potentially decrease the risk of stomach cancer. 

The World Cancer Research Fund published a similar review in 2007, concluding there was evidence to suggest that eating more fruit, non-starchy vegetables, and allium vegetables such as onions probably reduced stomach cancer risk, while salted and salty foods probably increased risk. At that point, there was not enough evidence to assess the effects of potatoes, vitamin C, or alcohol on stomach cancer risk.

This large study will contribute to the body of evidence informing the dietary associations with stomach cancer. However, it is not possible to give any firm conclusions based on this review alone. It certainly should not be advised that eating potatoes will decrease your risk of stomach cancer. 

Links To The Headlines

Potatoes can help cut cancer risk. The Daily Telegraph, November 28 2015

Potatoes reduce risk of stomach cancer. The Independent, November 28 2015

Want to reduce your risk of developing cancer? Eat more POTATOES, say experts. Daily Express, November 29 2015

Links To Science

Xuexian F, Jiayu W, Xuyan H, et al. Landscape of dietary factors associated with risk of gastric cancer: A systematic review and dose-response meta-analysis of prospective cohort studies. European Journal of Cancer. Published online November 14, 2015

 Mon, 30 Nov 2015 11:00:00 GMT Sugar and water 'as good as a sports drink', says study

Adding a spoonful of table sugar to a glass of water could be just as good as – or better than – a sports drink, several media outlets have reported. The news comes from a study that compared whether a group of long-distance cyclists performed better when they had a glucose or sucrose mix drink.

Fourteen experienced male cyclists were randomly given a drink of sucrose or glucose stirred into water before and during a three-hour cycling stint.

Both drinks maintained the body's glucose stores, which are broken down to provide energy during physical activity if there's not enough glucose available in the bloodstream. However, British researchers found the cyclists performed better on the sucrose drink.

Many sports drinks designed to provide energy during exercise use sucrose or mixes of glucose and fructose – but many still rely on glucose alone. Sucrose is made up of glucose and fructose, whereas glucose is available in a form ready to be used by the body.  

The researchers suggest glucose-only drinks could produce gut discomfort, and sucrose-based alternatives, or simply sugar in water, could make exercise easier.

While the findings are interesting, this is a small study involving just 14 male endurance cyclists. The results can't inform us of the effects in women, less experienced exercisers, or people performing different types of exercise. Even for male cyclists, a much larger sample may give different results.

This study does inform us about how the body may use sucrose and glucose differently during exercise, but limited firm conclusions can be drawn about the best form of nutrition before, during or after exercise based on its results alone.  

Where did the story come from?

The study was carried out by researchers from the University of Bath, Northumbria University, Newcastle University, and Maastricht University.

It was funded by Sugar Nutrition UK and Suikerstichting Nederland, and was published in the peer-reviewed American Journal of Physiology – Endocrinology and Metabolism.

The news reporting is generally representative of the study's main findings, but would benefit from acknowledging that this research has limited implications because it used such a small, select sample group.

What kind of research was this?

This small randomised crossover trial aimed to compare the effects of glucose and sucrose (table sugar) drinks on the body during endurance exercise. A crossover trial means the participants acted as their own controls, drinking both drinks on two separate occasions.

Carbohydrate – which includes sugar – and fat are the main energy sources used during moderate-intensity endurance exercises. The carbohydrate source comes from glucose in the blood, which is continuously being topped up from the liver by the breakdown of glycogen, the stored form of glucose.

The body's glycogen stores therefore become depleted during exercise, unless carbohydrate is taken in the form of food or drink to provide a fresh source of glucose.

The researchers aimed to better understand the effect that drinking different types of sugary drinks has on the depletion of glycogen stores during exercise.

What did the research involve?

This study involved cyclists who performed endurance exercise while drinking either glucose or sucrose drinks. Researchers compared the cyclists' glycogen stores before and after exercise.

14 healthy endurance cyclists (all male) were involved in the study. They were randomised to either a glucose or sucrose (granulated sugar) drink before an exercise test. One to two weeks later they performed a repeat test after drinking the other drink.

On each occasion, participants arrived at the test centre after fasting for 12 hours and having avoided strenuous exercise for the previous 24 hours. The cyclists' last meal was standardised by the researchers, so they all had the same energy intake.

The carbohydrate test drinks were made up of 108g of either glucose or sucrose mixed with 750ml of water to give a 7% carbohydrate solution. Participants were given 600ml of the drink (86.4g carbohydrate) immediately before exercise, with a further 15ml (21.6g carbohydrate) given every 15 minutes during exercise.

The exercise involved a five-minute warm-up at 100 watts, after which power was increased up to 50% of the individual's peak power output (established during preliminary tests) for the remaining three hours.

A special imaging technique called magnetic resonance spectroscopy (MRS) was used to examine the breakdown of glycogen in liver and muscle tissue before and after exercise.

The researchers took blood samples to look at glucose and lactate levels, as well as expired breath samples to look at oxygen and carbon dioxide levels. They also questioned the participants about abdominal discomfort and how tired they felt during exercise.

Four of the participants also attended on another occasion to perform a control exercise test, where they drank only water.

What were the basic results?

Liver glycogen stores did not decrease significantly after the exercise tests, and did not differ between the two drinks. Muscle glycogen stores did significantly decrease after the tests, but again were not significantly different between the two drinks. Comparatively, both liver and muscle stores declined when only water was consumed during exercise.

Carbohydrate use was estimated by a calculation looking at the difference between the carbon dioxide expired and the oxygen used up during exercise. This was significantly greater with sucrose than glucose, suggesting the sucrose drink was being better used to provide energy.  

Participants also reported their perceived exertion increased to a lesser extent during exercise when they had sucrose compared with glucose. Gut discomfort was also less with the sucrose drink.

How did the researchers interpret the results?

The researchers concluded that, "Both glucose and sucrose ingestion prevent liver glycogen depletion during prolonged endurance exercise".

They say sucrose ingestion does not preserve liver glycogen concentrations any better than glucose, but sucrose does increase whole-body carbohydrate utilisation compared with glucose.


This study aimed to see whether having a sugary drink available during endurance exercise preserves the body's glucose stores in the form of glycogen. They also wanted to see whether there was any difference between sucrose or glucose in terms of performance.

As may be expected, the researchers found drinking both sucrose and glucose drinks during exercise provided an energy source, thereby preserving the body's glycogen stores.

However, the body seemed to make better use of the carbohydrates to provide energy when it was given in the form of sucrose rather than glucose, and participants felt they were getting less exhausted.

The findings suggest both sucrose and glucose are a good energy source during exercise, though plain sugar (sucrose) in water had the slight edge in this study.

These tests involved just 14 male endurance cyclists, which is an important limitation of this study. This means we should take care before applying the results to all groups – for example, women, less experienced sportspeople, or people performing different types of exercise. Even for male cyclists, a much larger sample could have given different results.

There are also many different aspects related to sports nutrition that could be examined, such as the effects of eating food and drink containing different nutrient sources an hour or two before exercise, or the effects of eating after exercise in replenishing energy stores.

Overall, this study informs us about how the body may use sucrose and glucose differently during exercise, but limited firm conclusions can be drawn about the best form of sustenance before, during or after exercise.

Links To The Headlines

Energy drinks run out of fizz as scientists discover spoonful of sugar in water has same effect. The Daily Telegraph, November 27 2015

Sugary water better for performance than some sports drinks – study. The Guardian, November 28 2015

Forget sports drinks - a spoonful of sugar is the secret to athletic success. Daily Mail, November 28 2015

Links To Science

Gonzalez JT, Fuchs CJ, Smith FE et al. Ingestion of Glucose or Sucrose Prevents Liver but not Muscle Glycogen Depletion During Prolonged Endurance-type Exercise in Trained Cyclists. American Journal of Physiology – Endocrinology and Metabolism. Published online October 20 2015

 Fri, 27 Nov 2015 12:50:00 GMT High-strength 'skunk' cannabis linked to brain changes

"Scientists warn smoking 'skunk' cannabis wrecks brains," The Sun reports, somewhat simplistically. A small study found some users of the high-strength skunk strain of cannabis had changes in nerve fibres in a specific part of the brain.

Researchers used MRI scanners to scan the brains of 99 adults – some with psychosis, some without – looking for any links between small changes in their brain structure and their cannabis habits.

The researchers looked specifically at the effect on the fine structure of the corpus callosum. This is a band of nerve fibres joining the left and right sides of the brain and is thought to help different parts of the brain "communicate" with each other.

They found users of skunk – as well as those who used any type of cannabis on a daily basis – had different structural changes in the corpus callosum, compared with those who smoked less or lower-strength strains.

What this study doesn't tell us is whether these structural changes do any harm or cause any negative mental health effects – which is why The Sun's headline is too strong. The study simply didn't look at this.

The effects of cannabis use – both in the short and longer term – are not firmly established. But cannabis is known to be one of many substances that can trigger a psychotic episode. Read more about psychosis.

The study adds new knowledge about the potential effect of cannabis smoking on the brain, which other researchers can build on. But this was exploratory research and cannot provide any concrete cause and effect conclusions.   

Where did the story come from?

The study was carried out by researchers from King's College London and the Sapienza University of Rome.

It was funded by a King's College London Translational Research Grant, the National Institute for Health Research (NIHR) Mental Health Biomedical Research Centre at the South London and Maudsley NHS Foundation Trust, and King's College London.

The study was published in the peer-reviewed Psychological Medicine on an open-access basis and can be read online for free.

Generally, the UK media covered the story accurately, but some of the headline writers overstepped the mark. The Sun's headline, "Scientists warn smoking 'skunk' cannabis wrecks brains", and the Daily Mail's, "Proof strong cannabis does harm your brain", were not based on any evidence.

This type of study cannot prove cause and effect, only suggest a possible link, so "proof" is too strong a term. Also, the study didn't look at how the small changes in the brain associated with skunk affected thoughts or other brain functioning, so it was not fair to say skunk "wrecks" the brain.

This study wasn't designed to look at the effect of skunk on mental health illnesses, only small changes in brain structure, so it tells us little about the link between cannabis use and the development of a mental health illness. 

What kind of research was this?

This cross-sectional study looked for differences in the structure of a specific area of the brain called the corpus callosum in people with psychosis and those without.

It also looked at how this was linked with their reported cannabis use. The researchers were most interested in the effect of cannabis potency and how regularly cannabis was used.

The research team says high-strength cannabis (skunk) has been associated with a greater risk and the earlier onset of psychosis – the experience of hallucinations or delusions, a characteristic feature of the mental health condition schizophrenia.

However, the possible effect of cannabis potency on brain structure has never been explored. The researchers set out to investigate this by studying the fine structure of the corpus callosum, a band of nerve fibres joining the left and right sides of the brain.

This type of study can't prove cannabis causes changes in brain structure or any associated mental health illness. A long-term cohort study would be needed for this – a randomised controlled trial wouldn't be appropriate for ethical and, in the UK, legal reasons. But this type of study can point to possible or probable links for further investigation, a useful exercise to guide the next round of studies. 

What did the research involve?

A group of 56 people with psychosis (37 cannabis users) and 43 people without psychosis (22 cannabis users) had their brains scanned. The scans were used to look for possible links between their cannabis habits and any differences in the structure of the corpus callosum area of their brains.

Those with psychosis had been medically diagnosed with first episode psychosis, which simply means someone who has experienced psychosis for the first time. Most of those with psychosis were taking antipsychotic medication (53 of 56), just three were not.

The brain scans used an MRI imaging technique – diffusion tensor imaging tractography – that maps how different parts of the brain are linked to each other and how easily information is transferred between both sides. This technique measures the efficiency by which signals in the brain travel (diffusivity), where low diffusivity scores indicate a healthy functioning brain and high diffusivity may indicate some form of damage. 

The team looked at four common diffusion tensor imaging measures:

  • fractional anisotropy (FA)
  • mean diffusivity (MD)
  • axial diffusivity (AD)
  • radial diffusivity (RD)

FA is a sensitive way of picking up small brain structural changes and is relatively generic. MD, AD, and RD give more specific indications of where changes happen.

All participants filled in an illicit drugs questionnaire that included their cannabis smoking habits, when they first started, the strength they used, and how often they used it.

The statistical analysis took account of the following confounders:

  • sociodemographic factors
  • age
  • gender
  • ethnicity
  • some lifestyle factors, such as alcohol intake  

What were the basic results?

There were some interesting results, not all of which were picked up in the media reports. For example, those diagnosed with psychosis were more likely to have used cannabis at some stage in the past compared with those without psychosis.

But there were no differences between people with and without psychosis in terms of how long they had used cannabis, how old they were when they first used the drug, the type of cannabis used, how often it was used, and the strength.

Three of the four measures of corpus callosum function were no different in people with psychosis compared with those without (MD, RD, AD). FA was found to be different, but was borderline statistically significant, meaning there is a reasonable probability the result is down to chance – specifically, a 1 in 25 chance, p=0.04.

As the corpus callosum structure wasn't that different between those with and without psychosis, the researchers pooled the groups to study the effect of cannabis on the brain. Overall, they found the corpus callosum structure was negatively affected in people using high-potency cannabis, compared with those using a lower-potency strain or not using cannabis at all, across MD, AD and RD diffusion measures, but not the more generic FA.

These alterations were similar in users with and without psychosis. A similar mixed pattern was found for frequency of use, with daily users having the most changes compared with occasional or never users. No link was found between those first using cannabis before the age of 15 and those starting after in terms of changes in the corpus callosum structure. 

How did the researchers interpret the results?

The researchers concluded: "Frequent use of high-potency cannabis is associated with disturbed callosal microstructural organisation in individuals with and without psychosis.

"Since high-potency preparations are now replacing traditional herbal drugs in many European countries, raising awareness about the risks of high-potency cannabis is crucial." 


This research studied the brains of 99 people – some with psychosis and some without – looking for any links between small changes in their brain structure and their cannabis habits. The researchers looked specifically at the effect on the fine structure of the corpus callosum, a band of nerve fibres joining the left and right sides of the brain.

They found the corpus callosum wasn't very different in those with or without psychosis. But smoking high-strength cannabis (skunk) and using any type of cannabis daily was linked to structural changes in the corpus callosum, compared with those who smoked less or lower-strength cannabis.

What this study doesn't tell us is whether these structural changes do any harm or cause any negative mental health effects. The study simply didn't look at this, a subtlety much of the news reporting failed to recognise.

The study also can't tell us whether cannabis use is the direct cause of these observed differences, or whether other factors could be having an influence. Cohort studies following people over time, examining their cannabis use and carrying out follow-up brain scans, would be beneficial to look at this.

The researchers made the best of what they had in terms of collecting a sample of almost 100 people and analysing the results appropriately. 

However, as with all research, this study has its limitations. For example, 100 people isn't enough if you are splitting people into many groups, such as those with and without psychosis and different levels of cannabis use.

Some of the group numbers start to become quite small, which increases the chances you won't have enough people to find statistically significant differences, even if they exist. It can also throw up some unusual findings that wouldn't be the case in a larger group. This study carries these risks.

Similarly, the findings themselves weren't entirely consistent. There is a mix of significant and non-significant findings for the four measures taken (FA, MD, RD and AD). This lack of consistency muddies the picture somewhat and reduces our confidence in the findings a little.

The effects of cannabis use – both in the short and long term – are not firmly established. This study adds new knowledge about the potential effect of cannabis smoking on the brain that other researchers can build on. But it was exploratory research and so cannot provide concrete cause and effect conclusions. 

Cannabis is a class B drug that is illegal to possess (up to five years in prison) or supply (up to 14 years in prison). And while it may not trigger mental health problems in everyone, it may make pre-existing symptoms such as depression and paranoia more severe. If you think you may be using cannabis to cope with mental health problems, contact your GP for advice.

Links To The Headlines

Scientists warn smoking 'skunk' cannabis wrecks brains. The Sun, November 27 2015

Smoking high-strength cannabis may damage nerve fibres in brain. The Guardian, November 27 2015

Skunk 'causes damage to vital nerve fibres'. The Daily Telegraph, November 27 2015

Proof strong cannabis DOES harm your brain: Regularly smoking 'skunk' found to damage area that carries signals - and can lead to mental illness. Daily Mail, November 27 2015

Smoking skunk may damage brain junction. The Times, November 27 2015

Links To Science

Rigucci S, Marques TR, Di Forti M, et al. Effect of high-potency cannabis on corpus callosum microstructure. Psychological Medicine. Published online November 27 2015


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