Hello everyone, it's been a bit since my last science related posting and I think it's time to get back at it! Today we are going to discuss an article published in the journal Nature: Scientific Reports titled "Association between fasting glucose and all-cause mortality according to sex and age: a prospective cohort study".

In this article the authors were looking at whether or not monitoring blood glucose levels in a fasted state (which begins around 12 hours after eating), is actually a good indicator of mortality (aka how long you are likely to live). Most of you (well probably all of you ) are familiar with the fact that higher blood glucose levels are symptomatic of diabetes, and that diabetes is associated with a variety of negative health effects. So the actual findings of this article may not be particularly surprising. However there is another reason why I want to discuss this article with you all. That pertains to how we should think about data when we see it. This article presents us with lots of nice data that we can use to hone our critical thinking and interpretive skills and in our modern internet connected and information filled world, strong analytical reasoning skills are becoming increasingly important.

Image Source

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Diabetes

Diabetes mellitus (or just diabetes) is a term given to a set of diseases (type 1 which people are born with, and type 2 which develops typically in adulthood) characterized by increased blood glucose concentrations. Unchecked diabetes can lead to a whole host of health issues including heart disease, kidney disease, and even damage to the central nervous system.

One way that diabetes is commonly diagnosed is through a physicians analysis of your fasting blood glucose concentration. A healthy blood glucose concentration is typically 70 - 100 milligrams glucose per deciliter of blood (coincidentally an average person will have around 5 liters of blood, which means in a fasted state all of that blood in total should only have dissolved in it around 3.5 - 5 g of glucose (or around the 10% of the amount that is in one 20 oz bottle of soda, yes yes I know soda is mostly fructose and sucrose... but the sucrose fraction is a disaccharide composed of one glucose and one fructose, so there's still around 25-35 g of glucose in there). Thanks to @cryspano for correcting my awful math.

Some Background Info On The Studies Methodology

In this article the authors divided people (12,455,361 people to be exact) up into eight separate classes based on their blood glucose levels:

<70, 70–79, 80–94, 95–99, 100–109, 110–125, ≥126 [126–179, ≥180 mg/dL]

and then explored the relationships between the mortality of the studies subjects and where they fell in these eight groups.

Some Interesting Data From The Article

One of the first things the authors discussed was just how blood glucose levels are not static as we age. On the contrary they increase with age, and also interestingly those age related changes are different for men and women.

In Figure 1A from the article (shown to the left) we can see the average fasting blood glucose concentrations for both men (green) and women (red) plotted with respect to their age. Women, interestingly, have significantly lower fasting blood glucose levels (on average) then men all the way until around 72 years of age. What you can take note of when looking at this data is that no matter the age, in general the average person falls within the "healthy" concentration we talked about above (70 - 100 mg/dL).

Some separate questions I have for you. 1.) Looking at the plot we see that the blood glucose levels of women appear to go a bit higher then men. Additionally the trend for women doesn't appear to plateau like it does for men. 2.) Would you expect that the overall blood glucose levels for women would continue to increase (for women) were data available for a cohort of people aged 90? I ask these questions because looking at the available data and making INFORMED assessments of what it is telling you is an important skill.

So lets try to answer those questions. 1.) The datapoints here are averages, thus they are the summation of a LOT of individual blood glucose levels. Thus they have a range associated with each of them, often times this range is represented as error bars that extend above and below the data point. As we are not presented with this range for either the men or women it is not possible given this plot to say that the blood glucose levels between the men and women are different beyond age 72. The argument could be supported that they are based upon the very large sample size included in this study, however my question pertains to analyzing this plot. 2.) The answer to this is maybe, but the trend is reliant on only one point 88 yrs, so no reasonable extrapolation should be made. The prior few data points look as though the glucose levels for women are also leveling off (like we see for men), however the point at 88 yrs appears to jump up for women. It is difficult to conclude that this trend is maintaining linearity based upon just this one data point.

Relationship Between Mortality And Blood Glucose Level?

Another aspect of their analysis was looking at the likelihood of death with respect to the blood glucose levels. On the plot to the right (Figure 2B) that likelihood is displayed as a "hazard-ratio." We see that the lowest hazard ratio is centered around 90 mg/dL glucose in the blood, lower amounts then that lead to an increased occurrence of death and higher values lead to a significantly higher risk of death. The trends look similar for both men and women.

If you have been reading my blogs then this "hazard-ratio" term is one you are likely familiar with (as we have talked about it before, so if you are familiar... great! You can skip this next box, for those that do not know, see the box below!)

So [hazard ratio](http://www.statisticshowto.com/hazard-ratio/) represents the increase in the chance that a particular group of people will (in this case die) compared to a different group. In the data sets below the various different glucose concentrations were compared with a healthy level (~90 mg/dL) you can see that the hazard ratio around that 90 mg/dL concentration is 1. If a given fasting blood glucose concentration resulted in someone being 50% more likely to die their hazard ratio would be 1.5. If they were 200% more likely to die then the hazard ratio would be 3 (as they would die at 300% the rate).

All Age Groups Show An Increased Chance Of Mortality Relative To Fasting Blood Glucose Concentrations

How about we just look at two extremes, some young people and some old people.

Source [1]: Figure 3

You can see that both age groups that I have picked out from the article show the trend of an increasing hazard ratio that we discussed above for the overall data. However here I have a question for you. Why is the hazard ratio increasing to such a large number for the young age group, but not for the older age group? Give your guesses in the comments :) You just might get some nice votes for sharing your opinions. Please begin your comment with: Young Vs. Old Hazard Ratio then explain why you think this discrepancy exists or what this means.

Conclusions/TL;DR

The article shows that blood glucose levels are in fact a good indicator of potential mortality. The data illustrates that blood glucose levels in the range of 80 - 94 mg/dL are the best for having the lowest risk of death from health complications.

I hope you found this article interesting, and at least thought provoking from a data interpretation standpoint.

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Sources

1. https://www.nature.com/articles/s41598-017-08498-6
2. http://www.nfs.uvm.edu/nfs-new/activities/tutorials/Integration.html
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4600176/
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4170131/
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4398900/
6. http://www.statisticshowto.com/hazard-ratio/

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