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Nutritional myths that need to die

As a personal trainer, you will encounter many clients who have been told, and will strongly defend, many nutritional myths.  However, these nutritional myths may actually stop your clients from achieving their goals so it is important that you can shed some light on what the current literature says. Below, I have identified the 4 most common nutritional myths that personal trainers encounter and what the research actually says:


Myth #1 - Gluten is bad for you and affects weight loss

Gluten has received a lot of bad publicity in recent years and many clients may have eliminated gluten from their diet in fear of negative health consequences or may even believe that by cutting out gluten they will lose weight. In truth, when someone cuts gluten out of their diet they tend to reduce their caloric intake which subsequently results in weight loss but this is independent of gluten. This is evident from the research that found when celiac patients removed gluten from their diet, they actually increased their body fat1. From a health perspective, when clients are not celiac, gluten will have no negative effect on performance, gastrointestinal symptoms, well-being or inflammatory markers2. It’s also important to remember that if you have a recently diagnosed celiac client who cuts out gluten, it may lead to weight gain due to an increased absorption of calories as a result of improvements in their gastrointestinal health.


Myth #2 - High protein diets will harm your kidneys

This is probably the most common myth heard by all personal trainers, particularly when you ask your clients to increase their protein in order to build muscle or increase satiety. This myths arises from the fact that kidney filters protein or amino acids from the blood, so the general population often fear that by increasing their protein intake, they will put additional pressure on the kidney as it tries to filter the extra amino acids. However, there is extensive research into high protein diets and the effects they have on the kidneys. In healthy individuals with no existing kidney issues, high protein diets have been shown to have no effect on kidney function3 and when high protein diets result in weight loss, it may even reduce the risk of renal dysfunction4.

Myth #3 - Organic foods are better for health

It’s logical to think that foods with reduced levels of pesticides and processing may be healthier but is it worth the significantly higher price for organic foods? It is true that organic foods do appear to have lower levels of pesticides than their non-organic counterparts. However, it’s important to note that non-organic foods have levels of pesticides that are deemed safe for consumption5.  When it comes to the effect organic foods have on nutrient levels within the blood and subsequently health, there appears to be no significant difference between organic and non-organic foods6. Some clients may prefer to pay the extra money for organic food but you can rest assured that your clients who eat non-organic foods will be no worse off!

Myth #4 - Fasted cardio will increase fat burning

Again, this is probably one of the most popular myths proposed by clients. The theory is that when you train in a fasted state, you can easily tap into fat stores. Technically speaking, this is actually true. If a client trains fasted they will increase fat oxidation during that session. However, we now know that if you burn more fat during a training session, you will burn less in the hours after the session. Likewise, if you burn more carbohydrate during a session, you will then burn less carbohydrate in the hours proceeding that session. What does this mean? Calories are king and the substate utilisation during a session is irrelevant for weight loss7

When it comes to body composition, calories are the most important factor to consider. Once calories are considered and set appropriately, it’s then a matter of client preference as to whether they eat gluten or organic foods but making them aware that their decision will have no effect on health or performance will empower them to make a more informed choice. High protein diets are important regardless of whether a client’s goal is weight loss or building muscle and will not have an impact on kidney function provided they are healthy. For many, training fasted in the morning is convenient but again this will provide no additional benefits and should be a matter of choice and lifestyle as opposed to manipulating eating habits to try elicit further fat loss!

Note: This article is supporting content to the Optimum Nutrition for Health and Performance course and is for educational purposes only. It does not reflect the opinion of Glanbia Performance Nutrition, nor is it intended for product marketing purposes.


  1. Capristo, Esmeralda, Giovanni Addolorato, Geltrude Mingrone, Andrea De Gaetano, Aldo V. Greco, Pietro A. Tataranni, and Giovanni Gasbarrini. "Changes in body composition, substrate oxidation, and resting metabolic rate in adult celiac disease patients after a 1-y gluten-free diet treatment." The American journal of clinical nutrition 72, no. 1 (2000): 76-8
  2. Lis, D.M., Stellingwerff, T., Shing, C.M., Ahuja, K.D. and Fell, J.W., 2015. Exploring the popularity, experiences, and beliefs surrounding gluten-free diets in nonceliac athletes. International journal of sport nutrition and exercise metabolism, 25(1), pp.37-45.
  3. Friedman, A.N., 2004. High-protein diets: potential effects on the kidney in renal health and disease. American Journal of kidney diseases, 44(6), pp.950-962
  4. Tay, J., Thompson, C.H., Luscombe-Marsh, N.D., Noakes, M., Buckley, J.D., Wittert, G.A. and Brinkworth, G.D., 2015. Long-term effects of a very low carbohydrate compared with a high carbohydrate diet on renal function in individuals with type 2 diabetes: a randomized trial. Medicine, 94(47).
  5. Dangour, A.D., Lock, K., Hayter, A., Aikenhead, A., Allen, E. and Uauy, R., 2010. Nutrition-related health effects of organic foods: a systematic review. The American journal of clinical nutrition, 92(1), pp.203-210
  6. Smith-Spangler, C., Brandeau, M.L., Hunter, G.E., Bavinger, J.C., Pearson, M., Eschbach, P.J., Sundaram, V., Liu, H., Schirmer, P., Stave, C. and Olkin, I., 2012. Are organic foods safer or healthier than conventional alternatives?: a systematic review. Annals of internal medicine, 157(5), pp.348-366
  7. Schoenfeld, B.J., Aragon, A.A., Wilborn, C.D., Krieger, J.W. and Sonmez, G.T., 2014. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1), pp.1-