Nutrition: Principles Before Position

By Dr Sam Manger

Part 1: Diet is a tool and way of life, not a belief system

As to the methods, there may be a million and then some, but principles are few. The person who grasps principles can successfully select their own methods. The person who tries methods, ignoring principles, is sure to have trouble.

– Harrington Emerson

In a 21st century realm, home to chronic diseases, wellness advocates, Instagram influencers and genuine, well-meaning health professionals, nutrition is the oversized – and ever-growing – dragon lurking within. In the midst of the diet war battlelines being drawn, it remains possible to separate the signal from the noise and find truth within. In order to better explore the details of nutritional therapy, elucidating the guiding principles that underpin it is both expedient and necessary.

What is evidence-based medicine?

Evidence-based medicine (EBM) is defined as a systematic approach to clinical problem solving by the integration of the best research evidence with real-world clinical expertise and patient values (1). This integration is commonly referred to as the ‘EMB triad’ (below).

As in all fields of medicine – lifestyle medicine included – EBM is an important principle of medical nutrition therapy (MNT) recommendations. At all times, researchers and clinicians should be guided by the best available evidence. In the interests of honesty and transparency, however, we must acknowledge that evidence in nutrition has its limits, for several reasons:

  • Evidence may be derived from observational and epidemiological data, which, though useful, can be marred by various biases, including the healthy user bias, recall bias and selection bias;
  • Prospective trials of satisfactory quality are limited; it thus follows that crystal-clear recommendations are also limited;
  • The ‘PICO’ (population, intervention, control, outcome) of the existing trials may not match the patients being treated.

Where these limitations exist – as well as where they do not – we must also consider our individual clinical expertise, generated from real-world experience and, of equal importance, our patients’ values, expectations and bio-psycho-socio-cultural and spiritual needs.

Guidelines for all vs. tailored recommendations for one

Sufficient and consistent evidence exists to inform general nutritional recommendations for health. There is, however, no ‘one-size-fits-all’ dietary approach for every individual person or specific disorder, so let’s review how these general recommendations can be translated to each individual’s unique needs and circumstances.

Please note: the following section does NOT thoroughly explore every fine detail and specific hypothetical situation, but instead gives examples of the nuance that may be required when using MNT with a patient to improve their health and wellbeing.

For the wider population, the Australian Dietary Guidelines provide general healthy eating advice and are a good starting point for people wanting to improve their eating habits. They include five main evidence-based guidelines:

  1. To achieve and maintain a healthy weight, be physically active and choose appropriate amounts of nutritious food and drinks to meet your energy needs;
  2. Drink plenty of water and enjoy a wide variety of nutritious foods from the five food groups (vegetables, fruit, grains/cereals, meats/alternatives and dairy/alternatives) every day;
  3. Limit your intake of foods containing saturated fat (replace with mono/polyunsaturated fats), alcohol, added salt and added sugars (e.g. soft drinks, chips, cakes, biscuits, pastries, lollies etc.);
  4. Encourage, support and promote breastfeeding;
  5. Care for your food, ensuring that you prepare, cook and store it safely.

Or, to quote Michael Pollan: Eat (real) food, not too much, mostly plants. Pollan’s words underscore the common ground that does exist between multiple MNTs, including the Mediterranean, vegan/vegetarian, low carbohydrate, ketogenic and paleo diets, and can allow for the inclusion of nutrient dense animal products. No studies currently exist on more recent diet trends, such as the ‘carnivore diet’, at the time of writing; any reported benefits are purely anecdotal and must be viewed with significant caution and objective scepticism. In general, avoiding foods wrapped in foil or plastic and eating as close to the source as possible is a good rule of thumb.

Translating guidelines into clinical practice requires an understanding of both the available evidence and the unique individual in front of you. Every human being is unique not only in their genetic/biological make-up, but also in their spiritual, cultural, social and psychological make-up:

  • A person may choose to not eat animal products for ethical or religious reasons (spiritual);
  • A person/group may live or have lived in such a manner, or have access to a specific set of local foods, that encourages certain dietary choices both in the present or past (cultural);
  • A person may have too many competing social demands (e.g. work, family, children) to follow strict MNT, so a compromised MNT plan must be tailored and supported (social).

With respect to psychological variables, the interaction between beliefs, perceptions and conditioned responses to food (e.g. the placebo and nocebo effects) is highly complex and our understanding of this is still in its infancy. One striking example of the relevance of psychological variables can be found in a 2019 paper by Turnwald and colleagues, published in Nature Human Behaviour. The authors genotyped 107 individuals and identified genetic factors which may predispose to obesity. The subjects were then randomly assigned a test result of either ‘high-risk of obesity’ or ‘protected from obesity’, irrespective of their actual test result, which was hidden from them. The subjects then engaged in a task to test their physiological satiety (GLP-1 levels) and perceived fullness. The results showed that merely being told (false) genetic risk information altered the individuals’ physiological satiety and perceived fullness, sometimes to a level greater than the effects associated with actual genetic risk (2).

A brief digression on biological variables and personalisation

With respect to biological personalisation, there is an extensive array of variables – many of which are not well understood – that can influence an individual’s progress and outcomes. These range from the patients’ genetics, epigenetics and microbiome, to other lifestyle factors including exercise, stress and sleep. Below are just a few examples:

  • Blood glucose responses to identical meals are highly variable between individuals; their response can be predicted based on the individual’s blood parameters, dietary habits, anthropometrics, physical activity levels and gut microbiome (3).
  • Our circadian rhythms (these exist in many – if not all – cells and organs in the body as well as the brain) influence metabolism. Clinical trials have demonstrated that obese women consuming a greater proportion of calories earlier in the day lost more weight and had improvements in blood sugar, insulin resistance and blood triglycerides than women eating a similar diet who ate more calories later in the day (4,5).
  • Some people with diabetes may respond well, often to a point of ‘remission’, to a lower carbohydrate diet that includes whole foods, above-ground vegetables and nutrient dense protein sources (animal or plant), whereas some respond well to a Mediterranean or vegetarian diet (6).
  • Sleep habits, exercise, stress and microbiome may play a more significant role than genetics in individual responses to diet (7).
  • Different dietary approaches, such as low fat or low carbohydrate, may have equal efficacy in weight loss, and different genotypes are not always associated with different outcomes (8).
  • Systematic reviews have found that genes associated with obesity do NOT have significant predictive value in response to diet types (9).
  • There are factors other than the specific macronutrient content of foods which determine health outcomes: mortality reduction has been shown to be similar between low-fat and low-carbohydrate diets, provided the diet was based on whole foods. On the other hand, both low-fat and low-carbohydrate diets based on processed foods showed increased mortality (10). Put simply: the quality of the food we eat matters; the evidence that ultra-processed food is associated with poor health outcomes is fairly consistent (11).

If we are to take a couple of key messages from these findings (aside from the fact that it’s all very complicated), they are:

  • Genes matter, but so does lifestyle. Given that we can control just one of these, lifestyle factors should be the focus.
  • Whether you are a vegan, a paleo, or Mediterranean advocate – or just a regular person who wants to improve their health – eat real food wherever possible and avoid processed food.

It is clear that the emerging field of personalised MNT is just that: emerging. Specific recommendations are thus difficult to make. While there is a lot of common ground between healthy diets, and substantial evidence exists to inform various MNT recommendations for specific conditions, dogmatically adhering to ‘one choice for everyone’ will inevitably further contribute to the bitter diatribe that exists today. Instead, the EBM triad reminds us to use the evidence we have, combined with our clinical expertise and the experience and preferences of the patient.

Given these barriers and opportunities, a skilled clinician must therefore create a personalised treatment plan that is based on a strong and empathetic therapist-patient relationship, coupled with health coaching, support and motivational interviewing principles to optimise the patients’ nutrition, long-term adherence and, ultimately, health outcomes.

Part 2: Is ‘lifestyle’ really a choice? The socio-cultural-political-corporate determinants and the myth that ‘personal responsibility’ is all that matters

If you are interested in further pursuing some of the principles outlined above, several of my podcasts cover these topics in-depth:

#105 The Doctor’s Kitchen and Culinary Medicine with Dr Rupy Aujla, GP

Apple Podcasts | Spotify | Show Notes

#91 Nutrition, pathophysiology of DMT2, the environment & ending the diet wars

Apple Podcasts | SpotifyShow Notes

#61 The truth about food with Dr David Katz

Apple Podcasts | SpotifyShow Notes

#115 Fasting as Medicine with Professor Valter Longo

Apple Podcasts | Spotify | Show Notes


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  2. Turnwald BP, Goyer JP, Boles DZ, Silder A, Delp SL, Crum AJ. Learning one’s genetic risk changes physiology independent of actual genetic risk. Nat Hum Behav. 2019;3(1):48-56.
  3. Zeevi D, Korem T, Zmora N, Israeli D, Rothschild D, Weinberger A, et al. Personalized Nutrition by Prediction of Glycemic Responses. Cell. 2015;163(5):1079-94.
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  5. Garaulet M, Gómez-Abellán P, Alburquerque-Béjar JJ, Lee YC, Ordovás JM, Scheer FAJL. Timing of food intake predicts weight loss effectiveness. International Journal of Obesity. 2013;37(4):604-11.
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  7. [Internet]. Bethesda (MD): National Library of Medicine (US). 2007 Jun 27 – . Identifier NCT03983733, PREDICT 2: Personalized Responses to Dietary Composition Trial 2; 2019 Jun 12 [cites 2020 May 7]; [5 pages]. Available from:
  8. Gardner CD, Trepanowski JF, Del Gobbo LC, Hauser ME, Rigdon J, Ioannidis JPA, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. 2018;319(7):667-79.
  9. Livingstone KM, Celis-Morales C, Papandonatos GD, Erar B, Florez JC, Jablonski KA, et al. FTO genotype and weight loss: systematic review and meta-analysis of 9563 individual participant data from eight randomised controlled trials. BMJ. 2016;354:i4707.
  10. Shan Z, Guo Y, Hu FB, Liu L, Qi Q. Association of Low-Carbohydrate and Low-Fat Diets With Mortality Among US Adults. JAMA Internal Medicine. 2020;180(4):513-23.
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