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WHO, CDC and such all recommend the same thing which is 75 min a week of vigorous exercice minimum + 2 weight-lifting sessions.

In the UK the NHS recommends the same, I suppose we find the same in most Western countries as well.

But what is the science behind this?

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    You linked to a WHO site that contains all the documentation and research that led to the recommendation. Have you read that? – Alec Apr 24 '20 at 18:09
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Synthesis of syntheses

The Global Recommendations on Physical Activity for Health (PDF) is an attempt by the World Health Organization to synthesize the large body of scientific evidence across countries that has to do with "links between the frequency, duration, intensity, type and total amount of physical activity needed for the prevention of [noncommunicable diseases]" (Executive Summary, page 7).

One could even describe the WHO recommendations as a synthesis of syntheses, as its evidence collection phase was based on existing large-scale literature reviews, themselves based on both meta-analyses and more direct studies. To get an idea of its scope, here's the rational for selecting one of those five reviews:

This publication was a result of the search of the Medline literature - covering the period of January 1, 1995 -November 2007 - 14,472 abstracts were triaged, and of these, 1,598 papers were reviewed. The review included: cohort studies, case control studies, randomized control trials, non randomized control trial, meta analysis, observational studies, prospective studies and cross sectional studies. All cause mortality, cardiorespiratory health, metabolic health, musculo-skeletal health, functional health, cancer, mental health and adverse events. The populations studied were children and youth, adults and older adults. This is an extensive, global, high quality and up to date review which covers the outcomes of interest.

(Appendix 1, page 42)

The fractal-like evidence behind a specific recommendation

One can follow the thread of a specific recommendation almost as far as one likes, to verify the work of the WHO and all the intermediate experts. This is the basis of public, reproducible scientific inquiry. For instance, recommendation one (page 26) is:

Adults aged 18–64 should do at least 150 minutes of moderate-intensity aerobic physical activity throughout the week or do at least 75 minutes of vigorous-intensity aerobic physical activity throughout the week or an equivalent combination of moderate- and vigorous-intensity activity.

We can trace this with the help of Appendix 2: DETAILED REFERENCE OF LITERATURE, which has a table on page 47: "Evidence used for the age group: 18 - 64 years old". For each aspect of the recommendation, it cites the source and in most cases section and page on which one can find the supporting research.

For instance, the recommendation quoted above is given for enhanced metabolic health benefits. How do we know the type and frequency of exercise in the recommendation provide those benefits? Looking it up in the table, we can find evidence in “The Health Benefits of Physical Activity in Developing Countries” (published 2005) in sections 4.1.3, 4.1.7, 4.2.3, 4.2.6, and 5; more precisely, on pages 24, 29, 30-33, 39, 40, and 41-43. This is in addition to supporting evidence in Warburton et al 2007 (only for 20-55 year olds) and Warburton et al 2009, and supporting evidence (page numbers omitted) in the 2008 CDC Literature Review.

Someone interested in drilling down to the original source would look in the References section. Number 13 is the full citation for the sections and page numbers above:

Bauman A, Lewicka M, Schöppe S. The Health Benefits of Physical Activity in Developing Countries. Geneva, World Health Organization, 2005

The full text of this source does not appear to be online, which is disappointing. (I hope it's not a wide-reaching conspiracy of the global elite.) But the other major source, Physical Activity Guidelines Advisory Committee Report (PDF), is. If we look up just one of six specific references in that document, for instance G3-9 - G3-29, we find twenty pages (221-241 in the PDF), explaining individual studies in detail. Excerpts follow; emphasis will be mine. (T1D/T2D mean type 1 and type 2 diabetes; RCT is a randomized controlled trial; CVD is cardiovascular disease.)

The range we're looking at covers the following questions:

  1. Does physical activity have a role in preventing and treating type 2 diabetes?
  2. Does physical activity have a role in reducing macrovascular risks in type 2 diabetes?
  3. Does physical activity have benefits for type 1 diabetes?
  4. Does physical activity have a role in preventing and treating diabetic microvascular complications?
  5. Does physical activity and exercise have a role in preventing and treating gestational diabetes?

The section for Question 2 starts with the conclusion:

Increased levels of physical activity are associated with significantly decreased risks of developing T2D. Most of the studies addressing T2D prevention have focused on vigorous activity, but a number have included walking at moderate intensity, which has proven efficacious as well. Importantly, two randomized controlled trials (RCTs) and results of observational studies provide empiric evidence to support 150 minutes per week of moderate intensity physical activity for T2D prevention. Several studies have shown that 30 minutes per day of moderate intensity exercise 5 days per week are effective in preventing T2D. Available data do not enable minimal recommendations, although some of the large observational studies show that any amount of increased physical activity is associated with T2D prevention. Recommendations are valid for both men and women. Data are insufficient to clearly show that the benefits are uniform across all ethnicities and racial groups but no data support a lack of benefit and available data do support the benefit in these groups.

Seven pages follow just on this question, summarizing many studies. To take just one example, part of the reason for the recommendation is the role of cardiorespiratory fitness in type 2 diabetes. They spend two paragraphs, citing ten studies, one sentence thereof being:

It has been well established that a single bout of moderate exercise has a profound effect on glucose metabolism that may last up to about 18 hours (88).

This sends us to the reference list on G3-38 (PDF page 250), which lists:

  1. Devlin JT, Hirshman M, Horton ED, Horton ES. Enhanced peripheral and splanchnic insulin sensitivity in NIDDM men after single bout of exercise. Diabetes 1987 Apr;36(4):434-9.

The abstract for this study is available online, but the full text seems to be behind paywalls. If we were doing serious inquiry, we'd read it, as well as at least some of its references, and some of the basic and topic-specific texts in that field, which would perhaps allow us to form an opinion on its findings.

To summarize: that's one study (of 10) related to a single question (of five), from one review source (of ~three), providing supporting evidence for one part (of two) for one aspect (of five) of the recommendation.

Zooming back out

That's what the WHO recommendations are based on: an entire subfield of study consisting of many individual studies like this one, which together form an aggregate on which we form some single broad hypothesis or sense of where the literature suggests the truth is for one narrow topic, which in turn is part of an even broader aggregate of linked hypotheses and understandings that form a likely answer to some question, which is one question of many that contribute to our overall task of trying to figure out what is broadly true on some aspect of fitness, which itself forms yet another part of a yet broader fitness recommendation that is suitably simple and reliable to be told to hundreds of millions of people.

One suspects that if tallied up, the total supporting basis for the recommendations numbers in the thousands of sources, because it is in essence the practical distillation of an entire branch of science.

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