For a great deal of us, staying in shape is a constant struggle. We can try to eat healthily and go to the gym as often as possible, but - every now and then - we may end up dipping below/sneaking above our target weight. This isn't necessarily a terrible thing, as all of us will experience fluctuations in weight depending on what's going on in our lives at any particular moment in time.

However, any long-lasting deviances from a "healthy" weight over the long-term could actually end up costing a person years of their life.

This information comes from a new study of almost two million people in the UK (the largest ever study of its kind), which found that having a BMI (body mass index) either above or below "normal" lowers life expectancy by an average of 3.5 to 4.5 years.

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The study, which was published in the Lancet Diabetes and Endocrinology journal, found this from their research:

"Associations between BMI and mortality were stronger at younger ages than at older ages, and the BMI associated with lowest mortality risk was higher in older individuals than in younger individuals. Compared with individuals of healthy weight (BMI 18·5–24·9 kg/m²), life expectancy from age 40 years was 4·2 years shorter in obese (BMI ≥30·0 kg/m²) men and 3·5 years shorter in obese women, and 4·3 years shorter in underweight (BMI <18·5 kg/m²) men and 4·5 years shorter in underweight women."

In simple terms: those who were underweight or obese had shorter life expectancies after the age of 40 than their peers who were at a healthy BMI.

However, being a healthy weight does not necessarily mean that people are at the lowest risk of disease.

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According to report author Dr Krishnan Bhaskaran, "For most causes of death we found that there was an 'optimal' BMI level, with risk of death increasing both below and above that level."

He continued:

"At BMIs below 21, we observed more deaths from most causes, compared with the optimum BMI levels. However, this might partly reflect the fact that low body weight can be a marker of underlying ill-health.

"For most causes of death, the bigger the weight difference, the bigger the association we observed with mortality risk.

"So a weight difference of half a stone would make a relatively small (but real) difference; we could detect these small effects because this was a very large study."

In other words: sometimes the cause of a low BMI is a pre-existing health condition, so the cause and effect are reversed. Overall, however, the bigger the deviation from a healthy BMI, the higher the risk of an earlier death.

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Essentially, then, the best advice is the same as it always has been: keep your BMI within the healthy range as best you can, and maintain a sensible diet/exercise routine by eating well and staying active. There are obviously some exceptions to the rule (such as people who have underlying health conditions), but, if you're just an average Joe looking for some tips on improving your lifestyle, this is the way to go.

New research reveals being too fat or too thin 'can cost four years of life'