A new study published in the British Medical Journal (BMJ) has demonstrated a dose-response effect between physical activity and 5 diseases- ischemic heart disease, ischemic stroke, diabetes, breast cancer and colon cancer.
Metabolic Equivalent of Task (MET): It is a physiological measure expressing the energy cost (or calories) of physical activities.
One MET is the energy equivalent expended by an individual while seated at rest.
While exercising, the MET equivalent is the energy expended compared to rest so MET values indicate the intensity. An activity with a MET value of 5 means you are expending 5 times the energy (number of calories) than you would at rest.
MET minute: The MET (of a task) multiplied by the duration (in minutes) the said task was performed for.
MET of Walking (7 km/ hour) = 5
If someone walked at that MET for 60 minutes, the person would have
5 METs x 60 minutes = 300 MET minutes.
If the same activity was performed for 7 days a week, we could calculate MET minutes per week:
300 MET minutes per day x 7 days per week = 2100 MET minutes / week
The WHO recommends a minimum of 600 MET minutes / week for adults.
The article (published on 9 August 2016 in the British Medical Journal [BMJ]) makes use of METs to determine risk of 5 diseases- diabetes, breast cancer, colon cancer, ischemic heart disease, ischemic stroke.
The investigators wanted to determine if there is a dose-response relationship between physical activity and risk of disease. (In simple terms, they wanted to see if the risk of disease decreased (or increased) with increase in physical activity (measured in MET minutes).)
They performed a systematic review and meta-analysis, and calculated dose-response between physical activity and disease risk.
The computations involved comparing the risk of disease among adequately physically active (600 MET minutes/ week or more) against insufficiently physically active (<600 MET minutes/ week).
1. When compared to those who are insufficiently physically active, those with 600 MET minutes/ week had a 2% lower risk of diabetes.
2. Compared to those with <600 MET minutes/ week, those with 3000-4000 MET minutes/ week had an additional 19% lower risk than 1. above
3. The same amount of increase yielded much smaller returns at higher levels of activity: an increase of total activity from 9000 to 12000 MET minutes/week reduced the risk of diabetes by only 0.6%.
Compared with insufficiently active individuals (total activity <600 MET minutes/week), the risk reduction for those in the highly active category (≥8000 MET minutes/ week) was
- 14% (relative risk 0.863, 95% uncertainty interval 0.829 to 0.900) for breast cancer;
- 21% (0.789, 0.735 to 0.850) for colon cancer;
- 28% (0.722, 0.678 to 0.768) for diabetes;
- 25% (0.754, 0.704 to 0.809) for ischemic heart disease; and
- 26% (0.736, 0.659 to 0.811) for ischemic stroke.
The use of METs is not without problems- METs are not suitable to capture the true nature of exercise performed. Although a classification system was proposed in 1993, the classification of activities based on METs has been troublesome, with marked variations noticed in scientific articles using the same system!
The calculation of METs is for individuals of a specific weight, but individuals with the same weight could experience the same activity at different METs merely on account of the difference in fitness levels.
Moreover, it cannot tell us whether risk reductions would be different with short duration intense physical activity or longer duration light physical activity.
According to the study, the minimum recommended physical activity levels (600 MET minutes/ week) are unlikely to yield substantial reduction in risk of diabetes, ischemic heart disease, ischemic stroke, colon cancer or breast cancer.
The most gains are to be had if the physical activity level is increased to the 3000-4000 MET minutes/ week range (3600 MET minutes/ week to be precise). This corresponds to 25 minutes of walking each day, or 15 minutes of climbing stairs, etc.
Link to the article in the BMJ (open access) [PDF]:
Link to article explaining METs:
Link to Compendium of Physical Activities (imperial system used):