By Michelle Roberts
BBC News health reporter
Weight is a national obsession. On the one hand we are in the grip of an obesity epidemic and experts are warning us against getting fat.
Climbing the stairs should not be a problem for most
On the flip side we are seeing a backlash against the super skinny.
So how can you tell if you are healthy?
Indeed a naturally slim girl who is blessed with a metabolism that stops her getting fat can be healthy and a size zero - UK size 4.
But others starve themselves to reach a goal weight when instead they would be far better off carrying a few excess pounds.
Weight is not a reliable indicator.
Doctors use a measurement called body mass index or BMI - a calculation of kilograms/metres squared - to judge a person's size against weight.
As a rule of thumb, a BMI between 18 and 25 is healthy.
But someone who is very fit and muscular could have a BMI greater than 25 which would suggest, incorrectly, that they were overweight.
Conversely, someone who has a petite frame might fall below 18 but still be healthy.
The average person should be able to:
Walk a mile in 15 minutes
Carry two bags of shopping from the supermarket to the car
Climb the stairs in a house without getting breathless
And BMI is a useless indicator in people under 16. Instead, weight for height ratio should be used to factor in the young person's age and potential for growth.
Alarm bells would ring, for example, if a child was gaining weight too quickly for their age or if they were not gaining enough weight or growing.
Body shape can also help make the picture a bit clearer. Doctors are increasingly looking at waist measurements too because they know that "apple-shape" people who carry excess weight around their middle can be at risk of obesity-related conditions such as heart disease.
Then there's what lies beneath the skin - all of the vital organs do the jobs essential to keep us alive.
These rely on the right nutrition and exercise.
Guidelines say we should eat five portions of fruit and veg a day and get 30 minutes of moderate exercise at least five times a week for good health.
And there are a host of signs and measures that can indicate if we are doing enough.
Lisa Miles, nutrition scientist at the British Nutrition Foundation, said it was sometimes possible to tell if someone was malnourished just by looking at them, even if they were a 'normal' size and weight.
"For example loss in skin colour, dull and dry hair, redness or swelling of mouth or lips, problems with gums or eyes, dryness of skin and brittle/ridged nails can indicate malnourishment."
But lab tests are needed to get a full picture of a person's nutritional status, she said.
'NORMAL' ADULT VITALS
Blood pressure below 140/90 mmHg each time it is taken
A resting pulse of around 70 beats per minute
A respiratory rate of around 16-20 breaths per minute
When it comes to physique, fitness levels vary from person to person and depend on many factors, including age and sex.
But there are crude ways to tell if a person has a good general level of fitness, says John Brewer, director of the Lucozade Sports Science Academy.
"Being able to walk a mile in 15 minutes is an indicator of a reasonable level of fitness.
"You should be able to carry a couple of shopping bags from the supermarket to the car, climb the stairs in your house without getting puffed and cope with occasional, small bursts of unexpected activity."
Doctors can also look at a person's vital signs, such as pulse and blood pressure, and carry out fitness tests looking at things like lung function and capacity.
But Mr Brewer cautioned: "Looking at a one-off snap shot can be difficult. You need to look at changes over time and know what is typical for that patient.
"A general healthy lifestyle is absolutely crucial. If you do exercise, being overweight is far less of a problem. I would much rather someone be overweight yet exercise than someone be underweight and do no exercise at all."
Then there is healthy and unhealthy behaviour to consider.
Susan Ringwood, chief executive of UK eating disorders charity "beat", said: "You can't always tell by a person's size, weight and shape if they are unhealthy.
"You can be seriously affected by an eating disorder and still be overweight.
Eating can become an obsession
"Most people diet at some point. Diets do not cause eating disorders. But all eating disorders start with a diet."
She said there were warning signs to look out for.
"Someone who is really obsessed with food and has rituals around it. It might be restricting types of food."
She said there were screening questions to help spot when an innocent diet might have escalated into an eating disorder: Are you worried about your food? Have you ever made yourself sick because you were concerned about being too full?
And there are physical signs.
"A girl's periods might stop. She might have downy hair on her face and body in response to a low core temperature when the body isn't getting enough energy in."
There are medical tests that can aid diagnosis.
"There may be signs of heart failure or heart rhythm problems due to the poor nourishment," Ms Ringwood said.
But she added: "It's a psychological assessment that is really needed."