Everyone has that friend with an interdimensional portal for a stomach. They eat whatever they want in massive quantities and never seem to gain a pound. They’re skinny, despite their best efforts to the contrary and our barely concealed envy. And everyone assumes that they’re healthy because they’re skinny.
But are they? Is skinny all it’s cracked up to be? Does it always equal healthy? Are these effortlessly skinny people more likely to be healthy than the person who struggles with their weight and has to watch what they eat?
There’s no easy answer, of course. This is the human body we’re talking about – that repository of confounding variables, shifting contexts, and paradoxical effects. Declarative, absolutist statements about it are almost guaranteed to be proven wrong. And this time is no different.
It turns out that while it’s generally healthier to be slimmer than fatter, skinniness doesn’t guarantee health. There are caveats (tons) and exceptions to the rule (that isn’t really even a rule).
As my first example, I submit the skinny-fat young-ish cool dad.
They look slim and lean in clothes and definitely sport a normal BMI, but underneath they’re doughy. I see it a lot in new, 30-ish dads. Guys with slim cut jeans, a band t-shirt or flannel depending on the weather, interesting facial hair, and a protruding gut. Fifteen years ago, they’d probably have a chain wallet. They’re not particularly active (since little Bronx was born, you can count on a Simpsons character’s hand the number of times they’ve taken the restored Bianchi out for a ride), can maybe bust out one or two pullups (or two or three chinups), and look healthy and fit enough in clothing. Lack of sleep paired with too many IPAs  and too little time to cook proper food or exercise are to blame.
A controversial term, skinny-fat. But here’s what I mean by it: normal or underweight BMI coupled with high body fat. It exists . Just like a common (and valid) complaint is that BMI overestimates the unhealthy overweight population by failing to account for people with a high lean body mass, BMI also overlooks the people with low body mass but high fat mass.
A sciencey way to describe this phenomenon is the metabolically unhealthy non-obese  (MUHNO). They’re also known as metabolically obese normal weight (MONW). A MUHNO has at least two of the following metabolic characteristics while retaining a normal weight BMI: triglycerides over 150 mg/dl, fasting glucose over 100 mg/dl (or diabetes), elevated C-reactive protein (marker of inflammation), elevated HOMA-IR (marker of insulin resistance), HDL under 40 mg/dl, systolic blood pressure over 130 and/or diastolic blood pressure over 85. MUHNOs may look healthy and skinny and “normal,” but their metabolic health puts them at a greater risk for several conditions:
- MUHNOs have an elevated risk of early atherosclerosis .
- They’re more likely to develop diabetes and cardiovascular disease .
- They have poorer glucose tolerance and secrete more insulin  in response to food.
- Their LDL particles are smaller and more easily oxidized .
MUHNOs tend to have more unhealthy visceral fat  around the organs than metabolically-healthy normal weight people, which probably explains the differences in cardiovascular risks  between the two groups. And this abdominal obesity is associated with a mortality risk from heart disease even higher  than other groups with different body fat patterns.
Next, take the hardcore methionine -counting CRONer, consistently restricting calories and key amino acids for a chance at eternal life.
He’s hunched over a keyboard, discussing supplement stacks, sipping Soylent, counting down the days until the Singularity hits and he’s free from the abomination that is the human body. He may very well live a long time – calorie restriction does show some promise – and he’s very thin, but is he healthy?
If he’s bone-thin and so frail he regularly loses fights with gentle breezes, no. If his idea of lean mass is sinew and ropy tendon, he’s in trouble. Skinny isn’t just skinny. It’s also scrawny. It describes a lack of fat and muscle. It means underweight. A skinny person offers a live lesson in skeletal anatomy without all that lean mass obscuring the view. Lean mass – solid muscle, strong dense bones – is important for health , as we know by now.
Lean mass doesn’t save you from everything, though. Consider the contest-ready bodybuilder.
Though he has more muscle mass than he knows what to do with, the contest-ready bodybuilder is in full metabolic shutdown . His heart rate has slowed to 27 beats a minute. His strength has diminished and never quite recovers, not even after 6 months. Testosterone  has plummeted. His total mood disturbance (a marker of mood; higher is worse) has increased from 6 to 43. Going from 14.8% BF to 4.8% BF for the contest and back to 14.6% during recovery takes a toll. 4.8% BF simply isn’t sustainable or healthy.
The pros know this to be true. They understand that walking around at competition levels of leanness is foolhardy, that competition leanness needs to be cyclical, not constant. But a lot of regular fitness enthusiasts working out and dieting down have the idea that 4-5% body fat is not just desirable, but healthy and optimal for everyday life. It’s not. It’s terribly unhealthy, as the case study linked to above shows.
In the elderly, skinniness can mean poor health.
Is skinniness indicating or causing illness in the elderly? Or both? It’s a tough relationship to parse, but it’s definitely a relationship:
- Underweight (and obese) elderly men have reduced respiratory function , though increased lean mass can attenuate the relationship. Respiratory disease is a major killer among the elderly.
- Following critical care, the underweight elderly are more likely to die  and, if they survive, display impaired functionality. Other studies confirm .
Both elderly men and women who are underweight have lower bone mineral density , albeit for different reasons. Low body weight is bad for men because it often means low lean mass. Low body weight is bad for women because it indicates less overall fat mass, and fat seems to help elderly women maintain bone density. Either way, being too skinny is bad for their health and can lead to falls and bone breaks.
For elderly people, becoming skinny may be bad and likely indicates worsening health or the development of an illness. Weight stable skinniness is probably better.
Finally, consider the female. Since I already wrote an entire post about the importance of body fat in women , I won’t get too deep. To a woman, extreme skinniness may be especially unhealthy. Women actually need more body fat than men to be healthy, and it goes in different places (butt, thigh, hips) that serve as storage facilities for baby brain construction material. You may not want to have a baby, but the ability to do so is a strong indicator of good health. There’s even something specific to women called the female athlete triad . Characterized by extremely low body fat and extreme energy deficits, the triad can result in amenorrhoea, osteoporosis, and infertility.
Can we make any blanket statements at all?
The right amount of leanness is healthy. Of course, the “right amount” depends on many variables, like sex, age, baseline health, as well as interpersonal variation. One person may be perfectly healthy at 8% body fat, while another might need 15% to be healthy. One woman may lose her period and develop osteoporosis at 14% body fat, while that same body fat percentage could be perfectly adequate for the next woman.
For all the statistics and correlations I’ve referenced in today’s post, they don’t represent the outliers in every group. If they don’t apply to you, they don’t apply to you. Don’t take offense and seek redress. Take pride in your individuality. Skinny old granny with a strong grip and stronger bones? Keep doing what you’re doing.
All else being equal and eliminating any underlying health issues unrelated to body weight, a slim person is probably healthier than an overweight or obese person, yeah. But nothing is really equal, is it? Life doesn’t work that way. It’s not neat and tidy.
If you take anything away today, I hope it’s the knowledge that skinny does not necessarily equal healthy. There is such a thing as “too skinny” – for everyone.