Formal pairs suit Writing Task 2. Neutral ones work anywhere. Informal ones belong in Speaking.
1.a sedentary lifestyle
formala way of living that involves a lot of sitting and little exercise
Office work has led many adults into a sedentary lifestyle.
Lead and adopt both collocate with lifestyle.
2.a balanced diet
neutralfood that contains the right mix of nutrients
A balanced diet reduces the risk of heart disease far more cheaply than medicine.
Balanced ends in -ed.
Avoid: a balance diet → a balanced diet
3.take regular exercise
neutralexercise often as part of a routine
People who take regular exercise tend to sleep better and feel less stressed.
Take exercise is British; do exercise and get exercise are also correct.
Avoid: make exercise → take regular exercise
4.rising obesity rates
formalan increasing share of people who are seriously overweight
Rising obesity rates are placing great pressure on health services.
Obesity is uncountable, so rates or levels carry the plural.
5.chronic illness
formalan illness that lasts a long time or keeps coming back
Diabetes is a chronic illness that can often be prevented by diet.
Chronic disease and chronic pain follow the same pattern.
6.life expectancy
neutralthe average number of years a person is expected to live
Life expectancy has risen sharply over the last century.
Rise, increase and fall collocate with it.
Avoid: life expectation → life expectancy
7.preventive healthcare
formalmedical care that stops illness before it starts
Governments save money in the long run by investing in preventive healthcare.
Preventative is also correct but longer.
8.mental well-being
formalthe state of being emotionally and psychologically healthy
Time spent outdoors has a positive effect on mental well-being.
Mental health is the more common, neutral alternative.
9.place a strain on
formalcause difficulty by making heavy demands on something
An ageing population will place a strain on hospitals and care homes.
Put a strain on is equally correct.
10.a public health campaign
formalan organised effort to change people's health habits
A well-designed public health campaign can change habits that laws alone cannot.
Launch and run are the natural verbs.
11.curb the spread of disease
formallimit how far an illness moves through a population
Vaccination is the most effective way to curb the spread of disease.
Curb and control both work; stop is less formal.
12.undergo treatment
formalreceive medical care, especially over a period of time
Patients who undergo treatment early have a much better chance of recovery.
Undergo surgery and undergo tests follow the same pattern.
13.make a full recovery
neutralbecome completely healthy again after an illness or injury
Most patients make a full recovery within a few weeks.
Make collocates with recovery.
Avoid: do a full recovery → make a full recovery
14.side effects
neutralunwanted effects of a medicine or treatment
Every new drug must be tested for side effects before it is approved.
Usually plural. Cause and experience collocate with it.
15.junk food
informalfood that is high in fat, sugar or salt and low in nutrients
Banning junk food adverts during children's programmes would be a sensible first step.
Fine in Speaking and acceptable in Writing; processed food sounds more formal.
16.be under a lot of stress
informalfeel strong pressure or worry
I was under a lot of stress during my final exams, so I started running.
A natural Speaking phrase. In Writing, use experience high levels of stress.