Obesity does not really explain order of magnitude fewer cases, and obesity is much less of a factor than age for mortality. I can't find the numbers now, but it is better to be younger and quite fat than much older and fit. Japan does not lack people above 85 years where the fatality rate is double digit if you catch the disease.
The only reasonable answer I have seen so far about Japan is that we don't really know. It is certainly true that many people don't wear masks properly in Japan (e.g. every time I get out, even just to combini, I see people not wearing masks on the nose). But is it worth than not wearing one ? I am not sure we really know. I've seen conflicting info there.
Well, problems with obesity might also correlate with older, wealthier populations but they also looked at that:
> The figures are affected by the age structure
of national populations and a country’s relative wealth and reporting capacity, but our findings appear to be independent of these contributory factors
It's got links to a lot of other studies (admittedly, I'm yet to read those) but the summaries they give seem to really place the importance on the size of the waistline.
No one is disputing that age is a factor. From the very first line of the foreward:
> As we show in this report, increased bodyweight is the second greatest predictor of hospitalisation and a high risk of death for people suffering from COVID-19. Only old age rates as a higher risk factor.
But if you're really going to compare against age then you'd need to correct for obesity or you're simply including it and possibly compounding any risk.
> the risks of hospitalization in 85+ years is more than 10x the risk in 30-39 year old range.
Is the difference 10x in Japan, too?
The point is (or was) to discern a difference between why Japan has lower cases and/or mortality vs Europe and America. All three have plenty of old people, Japan has the greatest percentage. There is also a marked difference between their average waistlines too.
Japan should have the highest cases and/or mortality of the three and yet it has the lowest. I'm not a betting man and "masks are the reason", without a shred of evidence to back it, isn't going to change that habit, especially given the competition:
> Is this link between COVID-19 and overweight found across the globe?
> Yes.
> A large number of studies and systematic reviews have found increased risk of severe symptoms from COVID-19 associated with excess bodyweight. The table here gives some examples (pg 14)
And the table really does give plenty of examples. From the US:
> People with obesity more than twice as likely to need hospitalisation and more than six times as likely to need mechanically assisted breathing and more than six times as likely to die following development of COVID-19. (pg 14)
From the UK:
> People with overweight 67% more likely to need intensive care, and people with obesity three times as likely to need intensive care, following development of COVID-19. (pg 14)
…among many others. It continues:
> Taking data from over 160 countries, we find linear correlations between a country’s COVID-19 mortality rates and their estimated degree of overweight or obesity prevalence (pg 15)
and here's the kicker:
> The diagram shows high mortality rates *only* in countries where overweight prevalence exceeds around 50% of the adult population. (pg 15)
Emphasis mine. As to directly comparing Japan and the US and UK, they've added a helpful index:
- COVID-19 deaths per 100,000 population (01/01/2021)
- Adult overweight BMI >25kg/m2 (2016)
- Adult obesity BMI >30kg/m2 (2016)
- Population age over 65 years (%) (2020)
- Per capita Gross Domestic Product $US (2019)
- Healthy life expectancy years (2019)
Insufficient physical activity % adults (2016)
- Japan 2.60, 27.2, 4.3, 28.4, 74.1, 35.5
- UK 110.73, 63.7, 27.8, 18.7, 70.1, 35.9
- US 105.68, 67.9, 36.2, 16.6, 66.1, 40.0
As to mask wearing, well, if survey results of real world usage are to be believed, then it really is no contest, as the charts here show:
I have heard air humidity cited as one relevant factor, which was also recently brought up as one of the reasons masks may have a protective effect (humidity from breathing collects in there). Something about keeping membranes being healthier in warm, humid air than in cold, dry air?
The only reasonable answer I have seen so far about Japan is that we don't really know. It is certainly true that many people don't wear masks properly in Japan (e.g. every time I get out, even just to combini, I see people not wearing masks on the nose). But is it worth than not wearing one ? I am not sure we really know. I've seen conflicting info there.