Even if you’re lucky, and can eat what you want without gaining weight, should you? No! Because being skinny doesn’t automatically give you a clean bill of health.
25-30% of people with an ideal body mass are actually ‘skinny fat’. What this means is that thought they appear skinny on the outside they have fat deposited around vital organs on the inside.
Back in medical school we used to think of fat as an inert energy store – i.e. it just sat there and didn’t do much. But in fact fat actually sends out signals to the rest of our bodies, and visceral (central) fat is much more metabolically active than peripheral fat. Visceral central fat is associated with:
small, dense and more atherogenic LDL cholesterol particles
clotting risk factors
Some people’s genetics mean that the way they put down fat tissue is unhealthy – when they gain weight, rather than adding more fat cells their existing adipocytes just increase in volume and this causes their fat to secrete more unhealthy hormones like leptin and less healthy hormones like adiponectin. Even worse, adults who developed type 2 diabetes despite being normal weight had higher mortality than overweight adults with type 2 diabetes. 
There are many studies showing that central obesity has a much stronger association with cardiovascular and mortality outcomes than BMI. BMI as a predictor of disease is often explained by the concomitant risk factors of hypertension, hyperlipidemia, glucose intolerance/diabetes and the fact that there is a strong correlation between an increased BMI and central obesity for most individuals. But what about the people with a normal BMI but central obesity? A study looking at the clinical consequences of central obesity in individuals with normal weight found that people with normal weight central obesity had the worst long-term survival.  Men with a normal BMI and central obesity had twice the mortality risk of men who were overweight or obese according to BMI only. Women with normal weight central obesity also had a higher mortality risk than those who were obese according to BMI only
Central fat matters, even if your overall weight is normal. So how do you suspect if you are at risk of being a skinny fat person?
Signs of skinny fat syndrome include family history of type 2 diabetes, early onset of heart disease, or even having a little potbelly.
- Measure your waist to hip ratio.
- Get blood tests for fasting glucose and cholesterol even if you’re skinny!
- A high fasting glucose, or a cholesterol profile that shows raised triglycerides or low HDL suggests you’re at risk.
- More advanced tests to see if you are making unhealthy fat cells can be done by doing a fasting blood test for leptin (a bad hormone released by sick fat cells) and adiponectin (high levels are good). That’s important because people with low good hormones like adiponectin are up to 9 times as likely to develop type 2 diabetes and have a 2-fold increase in the prevalence of coronary artery disease .
A change in diet can normalise the biomarkers, reduce the bad leptin levels and increase the good adiponectin levels. Avoid artificial sweetners in soft drinks. Eat omega 3 rich foods. Do weight training. Ensure 7-8 hours of sleep per night. In one study oral magnesium supplementation improved the metabolic profile of skinny fat individuals.