Exercise and cancer- why it helps and how much is enough? (part 3)

Posted on May 9, 2016

(Last Updated On: May 11, 2016)

This is a four part series-

How much is enough? It is *not* all relative

Continuing with colorectal cancer, from this study,

“Evaluating the combined effect of moderate and vigorous physical activity on the risk of colon and rectal cancer showed that high levels of vigorous activity reduced the risk at nearly every level of moderate activity (table 6). Evaluation of high levels of moderate activity at low levels of vigorous activity did not show a similar risk reduction.

“Again, the strongest indicator of reduced risk was for vigorous physical activity performed at constant high levels over the past 20 years.”

This further clarifies the distinction. It’s not only whether you exercise, it’s how you exercise.

A prostate cancer study,

“In men with [prostate cancer], physical activity was associated with lower overall mortality and [prostate cancer] mortality. A modest amount of vigorous activity such as biking, tennis, jogging, or swimming for 3 hours a week may substantially improve [prostate cancer]-specific survival.”

When we tell people they should be exercising, and they have prostate cancer, are we being this specific with our advice? “You should be doing at least 3 hours a week of pretty vigorous activity. Examples of this type of activity include….”

Metabolic Equivalent of Task, MET, is how a lot of these studies assess physical activity. Examples include:

MET chart

These are MET / minute. If we jog for two hours, that’s 7 METs * 120 minutes = 840 MET minutes. 840 / 60 = 14 MET hours.

Look at this chart carefully:

Credit: Physical Activity and Survival After Colorectal Cancer Diagnosis

Credit: Physical Activity and Survival After Colorectal Cancer Diagnosis

For those essentially sedentary, below 3 MET hours per week, that’s our baseline. If you’re someone who does SIX TIMES more than the sedentary group, you get essentially no difference in outcome.

prostate cancer specific mortality graph high risk group

If you’re someone who does more than six times -the lowest dotted line- you get a huge benefit in outcome (in colorectal cancer in this scenario).

prostate cancer specific mortality graph low risk group

The risk of death after colon cancer diagnosis in the above,

  • 14.1% for less than 3 MET hours per week
  • 14.4% for those between 3 to 17.9 MET hours per week
  • 6.2% for those above 18 MET hours per week

If you get diagnosed with colon cancer, then start exercising, you can reduce your risk of death by more than half! But you need to be above a certain level of activity. How you feel is irrelevant. METs are not perception based outputs. They’re objective based outputs.

In Physical activity and cancer risk: dose-response and cancer, all sites and site-specific, a review of this topic, a demarcation is made between those who get activity requiring above 4.5 METs and those below. You need to get above to get the benefits.

For instance, from this paper on physical activity and lung cancer, incidence rate per 10,000 is the red circle:

lung cancer incidence rate based on activity

(The kj/week are for kilojoules, which are used for energy. For context, 1000 kilojoules = 239 calories and 5,880 kilojoules = 1400 calories.)

Notice those who got no activity below 4.5 METs had an incidence rate of 12.5 per 10,000. For those who got a lot of activity, but still below a 4.5 MET intensity, they burned over 1400 calories per week! (they just did it slowly), their incidence rate was 13.3 per 10,000. All that activity didn’t matter because they didn’t get above a certain objective intensity. 

lung cancer incidence rate based on activity threshold vigorous activity

Those who got no activity above 4.5 had an overall incidence rate of 16. However, those worked above 4.5 METs at something like at least 3 hours per week had an incidence rate of 7.2. No activity above 4.5 METs and you more than double your risk of lung cancer.

lung cancer incidence rate based on activity with circles

(Note we’re mixing relative risk and absolute risk. For instance, while the relative risk differences are large in the lung cancer example, the absolute is still quite low. However, the absolute difference is quite large in the colon cancer example.)

Look how many activities are below a 4.5 MET output:

MET chart with 4.5 box

We can’t go through every cancer in one post. What we can do is get some fairly solid thresholds we want to hit and or surpass. So far, it’s looks like we want at least get three hours or more of activity above 4.5 METs per week.

Let’s take a quick double check of this. There is a MET calculator from ExRx here. A speed of 3.8 miles per hour gives us 4.5 METs. For context, 4 to 4.5 miles per hour is when nearly everyone will start jogging rather than walking. Rather than worry about METs here, what we can say is you should get at least three hours per week of activity where you are at least right on the cusp of wanting to jog. Or walking at Manhattan speed.

But let’s double check the time. If we say a person is 180 pounds, rough American average, and walks at 3.8 miles per hour for 180 minutes (three hours), that gives us a caloric output of 1100 calories. In the one study above, greater than 1400 calories is when the optimal benefit was found. It’s only one study, but if we walk for 3.5 hours instead of 3, we just about hit 1400 calories.

So let’s say our thresholds are an intensity of nearly jogging or more, for a total of 3.5 hours or more, every week. Or 30 minutes every day is another way to look at it.

Please again note 3.8 miles per hour is not a relative measure. We are not saying “do an activity hard enough you get out of breath.” We are saying “hit this speed for this long or else.”

First, this is different than heart health. We’ve found, given similar energy expenditure, walking gives similar results to running. Run for an hour vs walk for an hour, and running is clearly better. The running will expend -we’ll say for simplicity- double the energy. But run for an hour vs walk for two hours -similar energy expenditure- and walking is just as good.

That’s not what we’re seeing with cancer though. This is what’s meant by structuring our exercise parameters based on disease. Perhaps heart health is your primary concern. Well, you don’t need the intensity of exercise as someone who is mainly worried about prostate cancer. (In reality, most are worried about heart and cancer health. This is addressed at the end.) You need to worry about doing enough overall.

Second, this is critical because how many geriatrics can’t even walk this fast? Above a 3.8 miles per hour pace? How many are so out of shape they can’t exercise intensely enough to get the benefits? Because it’s not only whether you go for a walk, it’s how fast you walk. Just telling your geriatric client they need to start exercising is as meaningless as telling someone to start taking a drug. How much of the drug? When? Days per week? Side effects? Contraindications? Give them too little of the drug and you can’t say the drug is useless. The prescription was useless!

-> My experience training a lot of older people is once around or above ~55 years old, it is hard work to get people to be able to get above 3.8 miles per hour on the treadmill. Jogging tends to not be easy for people upwards of 60 years old and older. Decades of deconditioning become more apparent.


With something like resistance training, it’s a bit harder to give specific guidelines. In the study:

Can Resistance Training Contribute to the Aerobic Components of the Physical Activity Guidelines?

It was found resistance training gives a moderate to vigorous heart rate response for about 50% of the time. So if you’re training with the intention to get stronger, with ~2 minute rest breaks, for an hour, that could count as roughly 30 minutes of working at a 3.8 mile per hour intensity.

So many go to the gym and sit on a bike. So many cruise on the elliptical as if they’re watching a kite. Sit on machines and keep the weight at its most lean. So many think having kids means they’re active. That when they go on vacation, because they walk a lot, they’re doing enough to stay attractive. So many are too quick to think hitting the gym once a week is enough. That because they have a standing desk, they don’t need to huff and puff. That walking the dog means they’re healthier than a hog. While this thinking may help you feel functional, it is no doubt delusional.


Summarizing activity / practical implementation

  • Strive for 10,000 steps a day, regardless of speed.
    • This is a more general health goal, but also ideal for cardiovascular health.
  • At least three and half hours per week, move at an intensity on the cusp of you needing to jog, or greater.
    • These 3.5 hours of steps can be part of the 10,000 requirement.
    • Ideally, this is spread over at least two days per week, rather than done all on one day.
  • If strength training, it should be done enough to where a person is trying to get stronger i.e. lift more weight, for the entire body.
    • In the study I linked earlier on resistance training, it was found lower body training gave an aerobic heart rate response for 75% of a session, compared to only 46% for upper body training. Don’t skip the leg work!
    • If strength training is done but no faster walking / jogging / running is done, then we want the strength training to hit a total of at least seven hours per week.
      • Good luck hitting this. It’s probably best you get tuh walkin’.
    • (For those thinking I get my strength training in much faster than that, you’re probably not strength training then. That’s circuit training, which is ok. But the notion of “I get my exercise done in 30 minutes twice a week” is insufficient.)
  • Other than the obsessive exercise types / marathon people, when in doubt, don’t only do more, but do it more intensely. It’s really hard to do too much, and for 99.9% of us, the more we do, the harder we do it, the better our health.

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