PM 2.5, Weight Gain, and Why You Feel Better in Italy
Article at a Glance
- Many people report tolerating food on vacation in places like Italy that they can’t handle in the US. This has little to do with changes in the food supply from country to country.
- Italy imports 65% of its wheat from Canada and the United States.
- Instead, the main driver of the change is likely attributable to changes in air quality.
- In an 8 year study of almost 4,000,000 US Veteran’s, a 10 µg/m³ higher annual average PM2.5 exposure was associated with an 8% higher risk of becoming obese (BMI ≥ 30) and a 7% higher risk of gaining ten or more pounds relative to baseline. Year over year, more polluted air was associated with greater BMI growth and greater weight growth.
- The weight gain impact began at levels below the “safe” EPA threshold of 12 µg/m³.
- PM 2.5 exposure drives inflammation across multiple organ systems, including parts of the brain responsible for signaling satiety.
This clip of Chamath Palihapitiya on Rogan describes a phenomenon many of us can relate to. It’s one of the core reasons we created the field of personalized environmental genomics at Gene Food with the launch of Gene Food Environment.
Everyone still assumes that the positive changes to health and mental outlook we feel when traveling are linked to the food. Our health and wellness conversations are reductionist to the extreme. “Oh, I heard the wheat might be different in Italy, therefore, yes, I can eat a pizza with no consequence in Europe, but have a terrible time in the States. Simple as that.”
Or is it?
Turns out that Italian wheat isn’t all it’s cracked up to be.
Italy imports most of its wheat
Italy doesn’t grow enough durum to supply its own pasta industry. They import ~1.1 billion kg from Canada per year, where pre-harvest glyphosate is standard. This isn’t to say that Italian wheat isn’t meaningfully better than American and Canadian sources, because it is. It’s milled differently, often has less additives, and is all around cleaner.
The issue is the cross contamination of American and Canadian wheat with Italian supplies. When you’re traveling in Italy, it’s just as likely you’re eating Canadian wheat as Italian. In many cases, the Italians blend their local wheat with higher protein Canadian wheat. The idea that when in Italy you are only eating Italian wheat, is just wrong.
Italy imports 65% of its wheat.
So what is the real differentiator that Rogan and Chamath miss in their conversation about the locations effect?
The air. The air in coastal Italian cities is pristine and dry. No major freeways, ozone, PM 2.5, benzene. The immune system quiets down and inflammation in the brain decreases. This in turn allows for the hypothalamus to properly recognize the satiety hormone, leptin and send the proper signals of satiety when we’ve eaten enough.
Don’t believe me?
Well, let’s have a look at “the science.”
PM 2.5 causes weight gain and obesity
The first study I want to highlight was conducted on almost 4,000,000 Veterans over 8 years. Titled “Ambient fine particulate matter air pollution and risk of weight gain and obesity in US Veterans,” the study used geocoded satellite estimates of PM 2.5 for the Vet’s homes and then mapped that against their clinician measured electronic records for weight and BMI.
You know what they found?
Veteran’s study
A 10 µg/m³ higher annual average PM2.5 exposure was associated with an 8% higher risk of becoming obese (BMI ≥ 30) and a 7% higher risk of gaining ten or more pounds relative to baseline. Year over year, more polluted air was associated with greater BMI growth and greater weight growth. The crazy part is that the weight gain impact began at levels below the “safe” EPA threshold of 12 µg/m³.
The researchers ran a negative control — they tested whether ambient sodium, an air constituent with no biological reason to cause obesity, was associated with weight gain. It wasn’t.
The next study looks at the mechanism, which as I mentioned is driven by the inflammatory nature of PM 2.5 and what it does to the brain and other organ systems.
Leptin signaling disrupted
In April 2023, researchers Lucio Della Guardia and Ling Wang published a comprehensive review in Obesity Reviews titled “Fine particulate matter induces adipose tissue expansion and weight gain: Pathophysiology.” Synthesizing animal experiments and human data, they mapped out exactly how chronic PM2.5 exposure drives weight gain.
- First, PM2.5 inflames the hypothalamus, the brain region that regulates appetite and energy balance, and creates leptin resistance meaning your brain stops responding to the hormone that signals satiety. Leptin is the hormone the body uses to signal its had enough to eat and PM 2.5 scrambles the signal so we don’t hear what it’s trying to tell us.
- Second, PM2.5 pushes brown adipose tissue (the kind of fat that burns calories to produce heat) to convert toward the white phenotype (the kind that stores calories). PM 2.5 exposure drives energy storing tissue creation in the body.
- Third, it suppresses thyroid hormone output, lowering your metabolic rate.
- Fourth, it impairs blood flow by reducing nitric oxide production (yikes).
The conclusion is striking: chronic exposure to fine particulate matter doesn’t just inflame your lungs or harm your heart.
It coordinately rewires the brain, the fat, the thyroid, and the muscles toward energy storage instead of energy expenditure.
Most of the strongest mechanism evidence comes from animal models, and the human evidence comes from epidemiology — the full causal chain in humans is still being mapped — but the sheer data size and result from the Veteran’s study is hard to ignore.
And to be frank, the evidence for PM 2.5 as the driver of the locations effect Rogan and Chamat are describing is 100X that of any evidence we have for the explanation centering around differences in the wheat supply (which are extremely oversold).
