How much and how often?

Myth: "Exercising three times a week is enough."

There's a lot of conflicting advice about the amount of exercise you need to do to get results, ranging from 'anything is better than nothing' to 'over an hour a day, every day'.

There's a bit of truth in both extremes and everything in between, and it also depends on what you mean by 'get results'. For example:

• If you go for a 30- to 40-minute brisk walk, you can temporarily reduce blood sugar, triglycerides and blood pressure levels. So in that sense, just one session can be beneficial.
  
  
• People who've lost a great deal of weight (20kg or more) seem to require 60 to 90 minutes of moderate to vigorous exercise every day to maintain that weight loss.

Sport and Recreation New Zealand (SPARC) and Ministry of Health guidelines recommend 30 minutes of moderate exercise most days. The 30 minutes can comprise several sessions of at least 10 minutes each.

For greater health and fitness benefits, consider three or four sessions of vigorous exercise each week for at least 30 minutes each time, in addition to the moderate exercise listed above. Exercise should be continuous, and hard enough to make you puff - where it's difficult to talk in full sentences without taking a breath. Your heart rate should be about 70-85 percent of its maximum (which is 220 beats per minute, minus your age).

Finally, for good all-round physical health, you might consider including two or three resistance or strength training (weightlifting) sessions per week, as well as a flexibility and balance program (such as yoga or Pilates, or even just some gentle stretching), in your exercise routine.

Bottom line:

Anything is better than nothing, but for good health exercise moderately for at least 30 minutes a day on five or more days a week.

Myth: "I just ate a 1200 kilojoule chocolate bar. Walking burns 1200 kilojoules per hour, so if I walk for about an hour, I'll burn off the chocolate."

If you look up most exercise and kilojoule tables, they'll tell you that a 68kg person walking for one hour at a moderate speed of 5 to 6km/h burns between 1000 and 1250 kilojoules.

They don't usually remind you that by sitting around doing nothing or pottering about, you'd also burn some kilojoules - maybe up to 580.

In other words, they give 'gross' energy expenditure per unit of time, not 'net' expenditure (the amount above and beyond what you'd normally expend).

Assuming the chocolate is surplus to your daily energy needs and you want to burn off an extra 1200 kilojoules, you'll need to do 1200 'net' kilojoules of exercise.

Researchers have worked out that the net kilojoules burned walking in this situation would be about 750 kilojoules per hour.

This means more than 1.5 hours of walking to burn off the chocolate. And eating it took you ... how long?

Bottom line:

Calculating energy expenditure based on gross rather than net kilojoule burn overestimates the contribution of exercise to total daily expenditure.