The First law is not going to be violated, of course, but the confusion is not just due to open or closed systems in my view. The confusion is caused because chemists or physicists think about energy in a different way to nutritionists. The energy in an organic substance (heat of combustion or Gibbs free energy) can be determined by combustion in a bomb calorimeter. If you were to do that for say, 5 grams dry weight of wheat chaff and 5 grams dry weight of sucrose, you would probably get a similar number.

However, from a nutritional perspective the energy difference between these two substances is likely to be very high because sucrose is completely bioavailable (able to be digested and used), whereas wheat chaff is not. So for the calorific value of foods there is a fudge-factor (pun intended) applied to the combustion energy to take into consideration digestion efficiency of the food in question. This is usually based on the Atwater system, which is an empirically-derived set of rules about the bioavailable energy of foods. There are several weaknesses with the Atwater system, not the least of which is that carbohydrates are treated more or less equivalently. This is inaccurate because sucrose is 100% bioavailable and converted to energy whereas carbohydrate-based dietary fibre is not.

An additional confounding issue is that both Gibbs free energy and "food energy" is discussed using the same units; calories or kJ (worse still - a "food calorie" is actually 1000 energy calories, or a kcal).

So from the perspective of the chemist or physicist, the calorific value of a food (its Gibbs free energy) is quite different from the nutritionist's view of the calorific value of the same food. Arguments abound on this topic due to this misunderstanding. As Socrates said, "the beginning of wisdom is a definition of terms".

Fay K wrote: "There is still much debate about how (the First Law of Thermodynamics) works with respect to human biology as the First Law is meant to apply to a 'closed system' and it is arguable whether the human body could be considered 'closed'."

My response: The First Law applies in closed and open systems. It applies to human metabolism (which is an example of an open system) equally as to any other process in the universe that involves transformation of energy from one form to another.

Fay K also wrote: "I am aghast that anyone could resolutely suggest this type of macro nutrient ratio (high protein/ very low carb) is any better or worse than others. There is so much mixed evidence between whether low carb/mod protein/high fat, high carb/mod protein/low fat, low carb/high protein/low fat etc. are more effective on any health marker than others. So I would not suggest any of these diets are necessarily 'inappropriate' given the evidence thus far."

My reply: My point was that his suggested intake of carbohydrate is lower than the minimum recommended by the National Health and Medical Research Council for good health. We may not know the appropriate macronutrient ratio for optimal health for any individual, but the current science-based evidence strongly suggests that we can dismiss Venice A Fulton's extreme suggestion with respect to carbohydrate intake.

Chris Borthwick wrote: "... fad diet books being held to academic standards? Waste of everybody's time."

My reply: I didn't hold Six Weeks to OMG to 'academic standards'. Rather I criticised the almost complete lack of understanding of science displayed by the author, which resulted in a best-selling book that has the potential to cause much harm to many vulnerable people.

Chris Borthwick: "The only illumination comes from the review's own mad overreach. Take, for example, "the recommendations of the American College of Sports Medicine, which include the advice that “Adults should train each major muscle group two or three days each week using a variety of exercises and equipment.” Well, they would say that, wouldn't they? And the fact that it's utterly inconceivable that a majority of the population could or would do anything like that leaves them in the enviable position of saying that their panacea has never been tried and thus can't be falsified. Is Chris seriously suggesting that the way to health is to carry out in full the recommendations of every American College of Remarkably New Professions? There aren't enough hours in the day."

My reply: Do you believe it would be inappropriate for a doctor to advise his patients to quit smoking because he/she knows that many will not follow that advice? If so, I strongly disagree with you. How about advising people not to exceed the speed limit or drive while under the influence of alcohol? Many people continue to speed and to drive while alcohol-affected, but I still believe that education campaigns about safe driving are appropriate. Similarly, many people ignore the dietary guidelines, but I believe it would reprehensible for the NHMRC to not provide the most up-to-date, science-based advice on diet that will, if followed, improve health and well-being. And there are enough hours in the day for the levels of physical activity recommended by the ACSM--I know this because I, and many other people I know, follow those guidelines.

Chris Borthwick wrote: "You haven't checked whether there's any factual basis in what such books say. You've checked whether there's any factual basis in what one book says. If you did one article for every diet book you'd be doing one a day, and if you feel this review covers all diet books you certainly didn't say so. You're picking one weed in a field and feeling that you've made a meaningful contribution. Not so. Next time, get a bundle of twenty and try and generalise."

My reply: You appear to be responding to a comment by Reema Ratten. Reema did not write the article, I did. I have not made the claim you accuse me of making (that I have somehow reviewed all fad-diet books), and neither did Reema, by the way. I have reviewed one fad-diet book and found it to be severely lacking in any scientific credibility. I also found it to be potentially dangerous.

Chris Borthwick (quoting Reema Ratten): "Also, most of us can probably manage 30 minutes of exercise 3 times a week ..."

Chris Borthwick (responding to Reema Ratten): "Perhaps, but that's grotesquely less than what you said in the article, which was an apparent endorsement of the statement that 'Adults should train each major muscle group two or three days each week using a variety of exercises and equipment.' Are you really suggesting that the two statements are equivalent? If not, which one should we take to heart?"

My reply: Again you are confusing me (the author) with someone else (Reema Ratten) who has written a comment on my article. My suggestion is that you read the guidelines on physical activity published by the ACSM and consider following them (as I wrote in my article).

Chris Borthwick commented: “Why is the ACSM different (to other colleges)? Why are its prescriptions better than the competition?”

My response: When it comes to recommendations on physical activity for health, there is virtually no competition to the ACSM. It is the world’s peak body in this regard. The ACSM annual conference is attended by thousands of physiologists from around the world who are there to hear about the latest research results in exercise physiology for performance and health, sports nutrition and sports medicine. The flagship journal of the ACSM, Medicine & Science in Sports & Exercise, is arguably the leading exercise physiology journal (with the Journal of Applied Physiology as the only genuine competitor for that title). The recommendations published by the ACSM constitute state-of-the-science guidelines.

I recommend the NHMRC Australian Dietary Guidelines as providing the most appropriate guide to nutrition for Australians, while for physical activity I recommend the guidelines of the ACSM.

Chris Borthwick also described the ACSM as “an association of people who profit from having people undergo complicated programs of exercise ...”.

My response: The ACSM is a non-profit organisation. As described above, its recommendations are based on the latest science-based evidence, and constitute the best advice on physical activity for health that can be recommended.

Chris Borthwick: “... any analysis that takes for granted that we have a duty to follow the guidelines of the ACSM should surely take the trouble to say why the ACSM ... is a good guide to whether we should perform complicated exercise programs.”

My response: I have never claimed that anyone has ‘a duty to follow the guidelines of the ACSM’. Rather, they are the best guidelines available, so I recommend them.