March 31, 2021

expert reaction to study looking at diets high in heat-treated foods and kidney disease in rats

A study published in Science Advances looks at consumption of processed foods and risk of microvascular diseases in rats.

Prof Tom Sanders, Professor Emeritus of Nutrition and Dietetics, King’s College London, said:

“This paper reports a detailed mechanistic study in rats fed for a duration of 24 weeks a semi-synthetic rat chow that was either fresh (control) or subjected to heat (160 C for one hour – about the same treatment used in baking cereal products).  The study found evidence of damage to the kidney glomerular cells and related changes in gene expression in animals fed the heated chow.  The authors suggest the effects could be attributed to the presence of advance glycation products in the heated diet derived from reactions with carbohydrates in the diet.  However, the heat treatment would also cause oxidation of polyunsaturated fatty acids and sterols to potentially toxic compounds as well as changes amino acids and micronutrients.  The dietary design of the study is thus flawed.  Furthermore, it would be misleading to assume this study applies to all heat-treated or processed foods because only one food (rat chow) was used.  We cannot conclude from this study that all “ultra-processed” foods might be the cause of chronic kidney disease.

“High blood pressure and poorly controlled type 2 diabetes are well known causes of chronic renal disease.  It can also be caused by chronic exposure to toxic minerals such as cadmium and arsenic.  The relevance of this study to human nutrition is highly dubious and provides no evidence for people to avoid baked carbohydrate containing foods (such as bread and biscuits).”

Dr Duane Mellor, Registered Dietitian and Senior Teaching Fellow, Aston Medical School, Aston University, said:

“This is an interesting study that provides evidence of how processed foods may lead to chronic disease.  It is important to note that this study was undertaken in rats, not humans.  This could mean that although rats may regularly consume cooked food as waste in our cities, they only do this when the opportunity arises and this is not a diet rats are likely to have evolved and possibly adapted to, unlike as has been suggested for humans.  The link between highly processed foods and disease have been seen as an association in humans before and this study helps shine a light on how this might happen.  It is also important to remember that some processing of foods has been invaluable to human health, allowing for the safe food supply for growing cities to be possible and helping to reduce risk of food borne illness such as diarrhoea.

“Perhaps the most important message from this study is that resistant starch and fibre can help to reverse the risk caused by the heat treated rat food.  This perhaps represents how we eat as people, including minimally processed foods, such as vegetables, pulses, nuts, seeds and wholegrains as part of meals alongside smaller and less frequent amounts of processed foods is a way to minimise any potential negative effects.  So, although this study helps to show how heat treated or processed foods may lead to ill health, it also helps to demonstrate the value of including sources of fibre such as vegetables, pulses, nuts, seeds and grains as encouraged as part of a healthy diet.

“One thing to consider when looking at rodent studies, is that what is fed to the animals is not what we might think is food.  Instead they use chow.  In both the heat treated (processed) and non-heat treated chow in this study, they used a chow called SF-AIN93G which is 40% wheat starch, 20% casein (extracted from milk), 13% dextrinised starch (processed or part digested starch), 10% sucrose (table sugar) and 7% canola oil, which might easily be considered to be quite highly processed.  So, although useful in triggering kidney disease in rats, it can be hardly seen as being representative of a normal healthy and varied diet.

“In addition, many studies in humans looking at associations between diet and disease look at several hundred and often many thousands of people.  In rats studies like this, which look at a large number of markers to explore how a disease might occur only look at 5 to 10 genetically very similar animals.  This is in part as humans tend to vary more than these animals which are often from the same litter, and the idea of the study is to explore the mechanism of a disease.  As this study looks at disease mechanism, the statistics should therefore be interpreted with caution as, these methods are designed to reject or accept a hypothesis, ideally testing one predetermined hypothesis as a primary outcome.  In studies like this, a wider range of tests are undertaken generating a large number of statistical outputs with impressive p-values.  When this happens it can increase the chances of type 1 statistical error, where the hypothesis is incorrectly accepted.”

‘Processed foods drive intestinal barrier permeability and microvascular diseases’ by Matthew Snelson et al. was published in Science Advances at 19:00 UK time on Wednesday 31 March 2021.

DOI: 10.1126/sciadv.abe4841

Declared interests

Prof Tom Sanders: “Member of Science Committee British Nutrition Foundation, Honorary Director of Nutrition HEART UK.”

Dr Duane Mellor: “No conflicts of interest.”