Does fat 'disappear' into thin air?
“It really is thin air! Living in a high-altitude city nearly halves your risk of being obese,” the Mail Online reports.
A study looking at the US armed forces found that people in high altitude areas such as Colorado were less likely to go from overweight to obese weight categories than colleagues posted at lower altitudes.
The study has a number of inherent limitations in that it cannot prove direct cause and effect as other factors may be involved, such as diet and exercise.
Even if a direct relationship is proven, it is difficult to see what immediate practical applications it could have. Unless you have the resources to camp on top of Ben Nevis or have access to a high-altitude training oxygen chamber, there is not much you can do about the altitude you live at.
The study does raise the interesting question of whether a low oxygen environment might be linked to appetite suppression; which has been suggested by previous studies in mice.
And if so, could this lead to new treatments?
Current recommendations for the treatment of obesity remain unchanged.
Where did the story come from?
The study was carried out by researchers from a number of US Universities and the US Air Force. The authors’ fees were paid by the Armed Forces Health Surveillance Center. Those affiliated with the funding organisation contributed to study design, data collection and analysis, decision to publish, and preparation of the manuscript.
The study was published in the peer-reviewed medical journal PLOS One. It was published as an open access article meaning it is free to access online for all.
The Mail Online's reporting of the story was generally accurate. However, the site focused on a single possible biological explanation that low oxygen levels may increase levels of a hormone called leptin, which reduces hunger.
Though, there are many alternative explanations, for example, availability of healthy vs. unhealthy foods in high and low altitude cities that were not explained.
The reporting did not make it clear that it is difficult to pin point lack of oxygen as the cause of the difference in this study alone.
What kind of research was this?
This was a cross sectional study looking back at data collected on US armed forces personnel. It aimed to investigate whether being posted at different altitudes affected how likely a person was to increase in weight from the “overweight” category (body mass index [BMI] ≥25 <30kg/m2) to the “obese” category (BMI ≥30kg/m2). That is, whether or not high altitude residence confers benefit in humans.
The researchers pointed to studies in mice that indicated exposure to low levels of oxygen (hypoxia), may increase levels of a hormone called leptin. This may then reduce appetite, leading to weight loss or at least, less weight gain.
This gives a biological rationale for how exposure to hypoxia in humans might prevent weight gain, and it is this possible phenomenon the researchers wanted to investigate in their current study.
A cross sectional study such as this cannot prove causation (in this case that that hypoxia prevents weight gain). It can only point to a possible association. This association could be due to the effect of leptin as described, or it could be due to a range of other factors such as diet and physical activity.
Other study types would be needed to prove or disprove any direct cause and effect between reduced oxygen levels and weight gain prevention.
While it would be incredibly impractical, an ideal study design would be a randomised control trial (RCT), where participants of similar baseline characteristics were randomised to live at either a low or high altitude environment.
What did the research involve?
The study gathered information on hospital medical encounters for overweight military service members in the U.S. Army or Air Force from January 2006 to December 2012 who were stationed in and around the US.
Next they looked at the history of where they were stationed, noting the altitude levels of the postings.
They then looked to see if there was any link between the person increasing in weight from the overweight category to the heavier obese category and the altitude of their past postings.
All postings were in the US rather than overseas. Everyone at the start of the observation period (2006) had to have been in the military for at least two years, be overweight (but not obese) and have no prior diagnosis of obesity while in military service.
The researchers were interested in progression from overweight to obese, rather than a healthy weight category to an unhealthy weight category (overweight or obese).
The analysis made adjustments for average levels of smoking in each area (linked to weight gain); individual measurements were not available.
It also factored in demographic information such as:
- age
- self-reported race/ethnicity
- sex
- branch of military service
- time in military service
- occupation category
- baseline BMI
- home address
No assessment of physical activity or diet appeared to be factored into the analysis.
What were the basic results?
There were 98,009 individuals included in the analysis, contributing an average (median) of 3.2 years of information. The median length at each posting of a different altitude was 1.2 years.
The main finding was that military personnel had a lower relative risk of being diagnosed as obese if stationed at high altitude (classed as more than 1.96km above sea level) compared to lower altitude (less than 0.98km above sea level).
The relative risk [RR] was 41% lower in the higher altitude group compared with the lower altitude group (Hazard ratio [HR] 0.59, 95% confidence interval [CI] 0.54 to 0.65).
This factored in variation in enlistment BMI, branch of service, time in service, occupation, sex, race/ethnicity, age, and housing allowance.
Several additional analyses were carried out to test the robustness of the findings. All suggested those posted at high altitudes were less likely to gain weight and become obese, but they varied in the precise relative risk estimate.
For example, one sensitivity analysis factored in the civilian obesity rates in the same area as the military posting. They found military and civilian obesity rates were strongly linked.
This analysis found the relative risk of obesity was 17% lower in the higher altitude group compared with the lower altitude group (HR 0.83, 95% CI 0.73 to 0.95). This was a big reduction on the 41% reported above.
How did the researchers interpret the results?
The researchers concluded that, “high altitude residence predicts lower rates of new obesity diagnoses among overweight service members in the U.S. Army and Air Force. Future studies should assign exposure using randomization, clarify the mechanism(s) of this relationship, and assess the net balance of harms and benefits of high altitude on obesity prevention.”
Conclusion
The results indicate that overweight US military personnel posted at higher altitudes were less likely to progress from overweight to obese weight categories than colleagues posted in lower altitude locations.
A plausible biological explanation was put forward suggesting that lack of oxygen at altitude may reduce appetite and food consumption due to increases in release of the hormone leptin. However, this theory was not tested or proven in this study.
Also, the food and drink intake of the military personnel was not recorded to confirm their appetites were suppressed and they ate less.
In addition to this, the study was a cross sectional design meaning it cannot prove lack of oxygen was causing the differences. Other factors, such as differences in diet and physical activity levels, not measured in this study, could account for all or some of the results observed.
A further limitation is the use of BMI as a measure of body fatness. BMI only assesses weight as a proportion of height. Those who put on muscle weight rather than fat could also move from the overweight category to the obese category, which would distort the results.
This is a particular issue amongst military personnel who are plausibly more likely to have increased muscle bulk because of their occupation and training.
It is not clear whether the study took account of this in its categorisation of obesity.
This study also provides only the relative risk figures for changing from the overweight to obesity category depending on altitude of posting. We know nothing of the absolute numbers.
We also don’t know what the average BMI of the overweight people changed to after their time at their high or low altitude posting – we’re only told the risk of them becoming obese.
Overall, it would be useful to know what number of people were in these categories and how much their BMI was changing by.
This isn’t the first time altitude and obesity has made the news, those with a long memory might recall a study with similar results made a splash last year.
Interestingly, this study did account for possible differences in physical activity and still found a link.
The study does raise the question of whether a low oxygen environment might be linked to body weight through leptin mediated appetite suppression. However, as the researchers mention in their publication, little robust research has been carried out in humans to test this theory. This still seems to be the case as this study alone does not provide robust proof either way.
Even if it was proven that high altitude directly leads to weight loss, this may have limited impact on tackling the global obesity problem. Though if there is a link between leptin levels and appetite then this could potentially lead to novel treatments. Previous attempts at using appetite suppressants to tackle obesity have proved to be unsuccessful as they often turned out to be addictive, and in some cases, caused heart damage. (The Guardian has a good overview of the history of appetite suppressants).
For an overweight or obese person, eating a healthy balanced diet high in fruit and vegetables and low in saturated fats and sugars, and taking regular exercise in line with current recommendations, are likely to be better considerations than moving to a place of high altitude.
