We’re constantly told by health experts that we’re heading into the depths of an obesity crisis; the media is constantly slamming us with images of size-8 (thankfully not so much size 0 anymore) ‘perfection’.

But is it right? Is it fair? Is fat always bad and is thin always good?

No, as it turns out, to all of that.

Higher BMI; lower risk of dementia

New research, drawn mostly from the London School of Hygiene and Tropical Medicine, looked at medical records of nearly two million people for up to ten years and revealed those with a BMI of 25-29 (which is classed as overweight) were at an 18% lower risk of dementia than those within the healthy BMI range.

Even further up (and down) the scales – those who were obese (a BMI of 30 or more) had a 24% less likely to develop dementia – conversely, being underweight increased it by 39%.

It doesn’t always mean high BMI is healthy though…

Naturally, such a study isn’t an assertion that being fat is healthy.

“A BMI of over 30 increases the risk of cardiovascular disease, blood pressure, Type 2 diabetes and some cancers,” says registered dietician Catherine Collins.

She also points out the study isn’t foolproof: “there is some missing data”, and it doesn’t necessarily work out if it’s “correlation or causation” (ie: if being lower weight means higher risk of dementia, or if having dementia then means becoming more frail and thin).

But as a “preliminary result and as a proof of concept”, Collins accepts there is something behind the study. Which makes you wonder when else big might not mean bad.

Where is the cut off?

“There seems to a tipping point at a BMI of about 27 to 27.8,” says Collins, “where there’s a higher percentage risk category of heart disease, diabetes etc. Below that, there isn’t much increased risk.”

So the official verdict saying a BMI of 25 is overweight? Not necessarily a bad thing.

It’s not always about BMI

It’s about the health behind it.

Because, as Collins is also keen to point out, BMI as a whole is slightly flawed – a system established by an insurance company back in the 1930s, and a system that takes no account for muscle mass or the quality of diet.

You might, for example, have someone eating a lot of high-calorie but healthy foods - like olive oil, or whole grains - which means they might be a bit larger, but they’re actually much healthier than someone eating less of the wrong things (high salt, high sugar etc). Equally, BMI takes no account of exercise levels.

When else is ‘fat’ a good thing?

And then there’s the other instances when being overweight – in BMI terms – instead of in the ‘healthy range’ is also a good thing:

  • Lowers miscarriage Research has shown women with a low BMI before they become pregnant are 72% more likely to suffer a miscarriage in the first three months of pregnancy.
  • Lowers infertility Research from Aberdeen University suggested “a real connection between weight and sperm” – probably because oestrogen, which is as important for sperm production as testosterone (it’s all about the balance) is released by fat stores, therefore those with low fat stores produce less (or less healthy) sperm.
  • Lowers risk of depression Again, it’s about the men. A Swedish study reported thinner men are 12% more likely to commit suicide than those of normal weight - overweight men are 12% less likely.
  • Lowers risk of lung disease When women get older and are too slim, it’s thought they are more likely to suffer chronic lung problems (like bronchitis, pneumonia and asthma).