High Animal Protein, Fat & Low Fibre

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Table of Contents
The Western Diet
Carbohydrates vs Fat
High Fibre vs High Protein
Protein Source
How Much Protein?
Impact of Specific Fats
Further Reading And References

The Western Diet

The western diet tends to be high in animal protein, high in animal fat and low in fibre. This diet leads to:

a decrease in total bacteria in the gut
a decrease in diversity of bacteria
a lower abundance of beneficial bacteria and increase in the abundance of pathogenic bacteria in the intestine through the consumption of refined oils and other elements of the Western diet including red meat, refined sugar, salt, and artificial sweeteners (15)
a reduction in beneficial bacteria such as Bifidobacterium, Eubacterium and Lactobacillus
an increase in bad bacteria such as Bacteroides and Enterobacteria
an increase in cancer-promoting nitrosamines; for example, high protein consumption may produce toxic by-products related to diseases such as colorectal cancer
an increase in inflammation in the gut; long term inflammation can lead to higher risk of cardiovascular diseases, non-alcoholic fatty liver disease (NAFLD), cancer, dementia and potentially type 2 diabetes and obesity (12). Western diet contains high omega-6 polyunsaturated fatty acids (PUFAs) and low omega-3 PUFAs, which induces pro-inflammatory mediators and suppress anti-inflammatory mediators (15)
the utilisation of intestinal mucus as gut bacteria’s main source of energy related to compromised gut barrier integrity
a compromised gut barrier through the high consumption of saturated fats impacting the microbiome, affecting the expressions of tight junction proteins and barrier-forming proteins in the gut, in addition to increasing bile acids which induce gut permeability (15)
reduction in the manufacture of health promoting short chain fatty acids (SCFAs) in the gut, which can occur from a diet that is low in fibre, which also suppresses the production of mucus, antimicrobial proteins, and immune system regulatory T cells (15)
aid the release of toxic bacterial metabolites (lipopolysaccharides) into the bloodstream which results in systemic inflammation due to the compromised gut barrier. Such inflammation can increase the risk of various non-communicable chronic diseases (15)
increases the production of trimethylamine N-oxide (TMAO), which elevates the risk of cardiovascular disease, diabetes, and obesity (15)
increases the risk of IBS and depression (15)

The lack of feed for the microbiome in western diets contributes to the negative effects listed above. Complex carbohydrates have the greatest influence over the microbiome, but not all complex carbohydrate fibres feed bacteria in the gut. Fibres that cannot be digested by humans but can be digested by gut microbes could be called “microbiota-accessible carbohydrate” (MAC). The western diet is considered a low MAC diet. Complex carbohydrate, plant-based diets are high in MACs.

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Carbohydrates vs Fat

A study, published in the “British Medical Journal” was among the first to investigate different proportions of dietary fat in humans. It investigated the dietary transition from the traditional diet of our ancestors, which was low in fat and high in carbohydrate, to a developed country modern diet of higher proportion of fat and a smaller proportion of carbohydrates. The more traditional diet (low fat, high carbohydrate) may result in a healthier microbiome and increases in protective chemicals from beneficial bacteria. The modern (high fat, low carbohydrate) diet can lead to unfavourable markers in the blood for inflammation. Obesity and cardiovascular disease have increased in line with the shift away from the more traditional diet. (3)

High Fibre vs High Protein

Proteins are broken down by enzymes present in the mouth, stomach and small intestine, into smaller components called amino acids. Unused amino acids are a source of energy for specific intestinal bacteria. Bacteria in the large intestine ferment amino acids resulting in a by-product called metabolites. Metabolites influence our metabolism, immune system and nervous system.

The western diet is typified by high protein and low fibre dietary sources. Harmful metabolites are promoted as a result of high protein consumption. Conversely, there is protective effect from inflammation and a reduction in the number of harmful metabolites created by bacteria from high fibre consumption. Not all bacterial protein fermentation leads to these harmful effects. The protein tryptophan produces indole, which is an essential amino acid, which aids the nervous system by having beneficial effects on neural development and providing protection against the autoimmune disease multiple sclerosis.

Protein consumption aside, consuming enough fibre increases the health of your microbiome resulting in the most overall health benefits.

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Protein Source

Depending on a person’s gut microbiota profile and gut microbial gene richness (relating to diversity – the total number of species per volume), animal based proteins may influence insulin sensitivity and diabetes. In these instances, certain bacterial groups and gut microbiota-derived metabolite, called trimethylamine N-oxide (TMAO), are increased by the animal protein. TMAO has been related to the development of atherosclerosis.

In contrast, plant based proteins impact cardiovascular disease risk less negatively, by a shift in bacterial metabolites with increased levels of the health promoting SCFAs, i.e., acetate, propionate, and butyrate.

Note that chronically low protein diets reduce nitrogen and energy sources for gut bacteria resulting in a lack of essential amino acids which the body associates with nutritional stress. In these cases supplementation with cellulose and inulin fibres allows gut microbiota processing of amino acids by using cellulose as an alternative carbon source.

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How Much Protein?

2g per kg of body weight of protein per day is safe for healthy adults long term. (7)

3.5g per kg of body weight of protein per day is the tolerable upper limit for those adults well adapted to higher amounts of protein. (7)

Greater than 2g per kg of body weight of protein per day for adults, which would be considered chronically high intake, can result in digestive, renal (kidney related), and vascular abnormalities and should be avoided. (7) In a study high protein diets caused alterations in renal health status. (9) Excessive protein consumption has also been associated with liver disease. (10) A mother has called on the health industry to regulate sales of protein powder and supplements after her daughter died in 2017 due to a protein-heavy diet. (11)

Media, health professionals and popular diet books discuss high protein intake to assist with weight control and muscle growth with a lack of scientific data and safety considerations behind these aims. The gastrointestinal tract can only absorb amino acids from proteins at a specific rate (1.3g to 10g per hour) and the liver will have a particular capacity to process proteins and produce urea for excretion of excess nitrogen. 0.8g · kg⁻¹ · d⁻¹ accepted level is purely for structural requirements and does not take into the use of protein for energy metabolism. High protein diets advocate around 200 to 400 g/d, which can equate to levels of approximately 5 g · kg⁻¹ · d⁻¹. This could exceed the liver’s capacity to convert excess nitrogen to urea. Danger arises when protein is greater than 35% of total energy intake. This can lead to hyperaminoacidemia (excess amino acids in the bloodstream), hyperammonemia (raised levels of ammonia, a nitrogen-containing compound), hyperinsulinemia (high insulin level) nausea, diarrhoea, and even death (the “rabbit starvation syndrome”). It is important to take into account three measures of protein intake in combination including absolute intake (g/d), intake related to body weight (g · kg⁻¹ · d⁻¹) and intake as a fraction of total energy (percent energy). A suggested maximum protein intake based on bodily needs, weight control evidence, and avoiding protein toxicity would be approximately: 25% of energy requirements at approximately 2 to 2.5 g · kg⁻¹ · d⁻¹, corresponding to 176 g protein per day for an 80 kg individual on a 12,000kJ/d diet. This is well below the theoretical maximum safe intake range for an 80 kg person (285 to 365 g/d). (8)

Impact of Specific Fats

A study looked at the impact of milk fat vs olive oil vs corn oil vs low fat consumption on the microbiome. Interestingly the low fat diet was not favourable, but health benefits of fats seems to be related to specific types of fat as each type of fat has a different impact on the microbiome:

A low fat diet resulted in a lower amount of good bacteria and a higher amount of negative (bad) bacteria in the gut
The corn oil diet, which is rich in omega-6 fatty acids, resulted in disease promoting increases in inflammation, oxidative stress and intestinal barrier dysfunction. When challenged with a pathogen (Citrobacter rodentium) the corn oil diet promoted exacerbated immune-driven damage
A diet with olive oil led to significantly lower facultative anaerobic bacteria (bacteria that can live with or without oxygen) compared to the milk fat and low-fat diet groups. Facultative anaerobes are related to common pathogens (bad bacteria) such as Streptococcus species and the Enterobacteriaceae (e.g., Escherichia coli) which can cause disease (13). However, olive oil, rich in monounsaturated fatty acids (MUFAs) also led to lower microbiome diversity. Despite this, when challenged with a pathogen (Citrobacter rodentium) the olive oil diet was protective against it. This suggests that microbial diversity alone may not be a predictor for better health
Although the milk fat diet, rich in saturated fatty acids (SFA), increased inflammation that could lead to intestinal damage, there was a compensatory protective response by the production of sirtuins and short chain fatty acids (SCFAs). Sirtuins are health promoting by playing a key role during cell response to a variety of stresses, such as oxidative or genotoxic stress and are crucial for cell metabolism (14)

The study highlights the fact that not all high-fat diets are detrimental to the microbiome and whether helpful or healthful depends on the type of fat consumed.

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