There’s a growing belief that our waist size is determined not so much about how much we eat, but what we eat. Is it really possible that ensuring regular consumption of certain foods can actually help us burn fat and keep fat off? The answer could lie with bacteria!
Every year there’s a new diet, sometimes it’s even crazier than the one before.
You’ll no doubt remember the 5:2 diet,a diet where you eat 500kcals for two days per week and then normal eating habits for five; the Atkins diet, the cabbage soup diet, even a kale and chewing gum diet!
And now, the gut diet – a diet that promotes the extensive populations of bacteria that live in our guts.
What are gut bacteria?
When we are using the expression gut, we are using a generalised term for the whole of the intestine that takes food from the stomach and passes it down through a long tubal network to the toilet!
The lower part of the gut, the large bowel or colon, contains trillions of bacteria.
In fact, for every single cell in your body, you will have ten bacteria, with highest concentrations in the lower gut (1).
Most of these bacteria are friendly and help us produce vitamins, protect the lining of the bowel and even interact with our immune system.
Some of the bacteria are not so friendly and can cause us to experience health issues; when we have more unfriendly bacteria, we develop a condition called dysbiosis.
Or some of the unfriendly bacteria can end up in the small gut, creating the condition small intestinal bacterial overgrowth (SIBO).
There are thousands of different species of bacteria that live in the gut (2), and everyone of us will have a unique microbiome. A set of genetic material from the bacteria that inhabit us.
What we eat determines the type of bacteria that will populate inside of us, most of which will thrive on undigested carbohydrates. Not a nice thought, but highly important to our health!
Types of Bacteria
The bacteria that take up residence in our gut can be divided into four distinct groups, although only two of these account for more than 90% of the entire population.
A lot of science concentrates on the ratio of these two main groups of bacteria and will be the main two that this article will discuss. So what do we know about these little fellas?
Firmicutes – this group of bacteria should account for less than half of all the populations of the top two types.
They prefer a fed state and so abound after we’ve eaten a meal.
If we have a diet high in fat or sugar, firmicutes will predominate in our gut as we’re giving them the environment they thrive in best.
Overweight people tend to have a higher ratio of firmicutes to bacteroidetes (3).
Bacteroidetes – this group of bacteria are pretty much the opposite to firmicutes.
They prefer a fasting state and will be the predominant bacteria when we’re not eating. They don’t thrive in a high fat, high sugar environment.
Thin people tend to have a higher ratio of bacteroidetes to firmicutes(3).
So…firmicutes bad, bacteroidetes good – right?
Not really, they both play important roles in our gut.
Firmicutes are great at breaking down complex carbohydrates and releasing important short chain fatty acids such as butyrate, but it is suggested that this additional carbohydrate metabolism adds to available energy.
One study suggested that a 20% increase in firmicutes and a reduction in bacteriodetes released an additional 150kcal per day (4), which we need to expend.
However, short-chain fatty acids are anti-inflammatory thus preventing a host of diseases, may have neurological protective properties, and promote good health in the cells that line the gut, and regulate gut hormones too (5,6).
We also know that overweight people who have had a gastric bypass operation see a reduction in firmicutes, but then have a long term impact on gut health (7).
Gender may also influence this ratio, some studies have found that women naturally have less bacteroidetes (8).
The science behind how gut bacteria affect our health is still relatively new, and studies are being published on an increasingly regular basis.
How does gut bacteria affect weight?
The effect of microbes on weight was first seen in mice.
When mice are raised in a sterile environment inside a lab, they don’t put on weight.
As soon as bacteria from the gut of an obese mouse is introduced to the gut of a sterile mouse, that mouse gains weight; not so if the bacteria are transplanted from a lean mouse (9).
This observation lead researchers to believe that certain bacteria that reside in our gut appear to have a direct correlation with our weight
New research suggests that one type of bacteriodes, called Prevotella, might actually hold the key.
A Danish study found that subjects who had a higher ratio of prevotella to bacteriodes were more likely to be successful in losing weight (10).
Prevotella tend to prefer a carbohydrate heavy diet compared to a high fat or high protein diet (ie, they like a plant-based rather than an animal-based diet)(11).
Therefore, vegetarians who don’t over-eat dairy or other high fat, high protein foods are likely to have more prevotella than their meat-loving cousins .
It appears, that having a higher bacteroides ratio overall is helpful in keeping us thin, but within that, the bacteroides needs to be dominated by Prevotella.
Foods that disrupt gut bacteria
There are a few things we consume that could have a detrimental affect on our gut bacteria, that could in turn impact upon our ability to lose weight:
- artificial sweeteners are common in diet drinks (such as diet cola and low calorie flavoured water), fat free yoghurts
- processed food: by this I mean foods high in salt and sugar such as ready meals, tinned soups, baked beans, pizza
- high fat, high sugar confectionary products like pastries, cakes, biscuits, sweets, donuts, buns
- alcohol, with the possible exception of red wine. Red wine is high in polyphenols, and is thought to offsets the harm alcohol does to our bacteria (assuming you don’t over-indulge)
- not consuming enough plant-based dietary fibre (ie, a good range of fruit and vegetables)
Prebiotics vs. Probiotics
The bacteria that live in our gut are predominantly anaerobes (13), which means that they thrive in an oxygen-depleted environment (unlike their hosts!).
Probiotics are largely bacteria that produce lactic acid by fermenting sugar, and are the same bacteria that turns milk into yoghurt, kefir or cheese; cabbage into sauerkraut and kimchi; and grains into beer or spirits.
Stores are now full of different types of probiotics, many in the form of yoghurts or fermented milk drinks such as kefir or Yakult, although they are also widely available as a tablet form.
Studies showing how effective these are at changing the gut environment don’t always agree, but luckily for us, someone conducted a systematic review which was published this year.
A systematic review pools together the results of many studies by using very strict criteria that decides which studies are included.
Changing the diet to include more prebiotic foods might have a better chance at improving the gut and metabolism long term, but the science is too new to really say one way or the other.
Either way, if you choose to use probiotic supplements, then it would be wise to look at your diet too, and include foods that help encourage the right bacteria to thrive. These are largely plants, ie fruits, vegetables and legumes (16).
Specific foods that are beneficial to gut bacteria:
There are three food groups that are particularly beneficial to gut bacteria:
- Foods high in polyphenols
- Foods high in a type of sugar called polysaccarides. A classic example is inulin, found in garlic, onion, leeks, Jerusalem artichokes, and asparagus.
- Foods high in probiotics, such as kefir, yoghurt, blue cheeses, sauerkraut, kimchi, kombucha, and tempeh.
Examples of foods high in polyphenols are:
- Apples appear to increase the population of bifidobacteria (17) and restore the firmicutes/bacteriodetes ratio (18, 19), with polyphenol and prebiotic effect having a potential postive effect on cardiovascular health (20)
- Chocolate may increase both bifidobacteria and lactobacilli (21). Foods high in polyphenols are great for bacteria, as polyphenols aren’t broken down until they reach the gut, when the bacteria do the job for us.
- Olive oil is rich in polyphenols – the gut bacteria love anything the rest of our gut doesn’t breakdown.
- Tart cherry (Montmorency) juice contains unique polyphenol compounds that may promote a beneficial gut bacterial population (22)
Whilst not a food, we can’t ignore exercise.
Studies show that people who engage in physical activity have a richer diversity of bacteria in their gut than those who do not.
The biggest problem with probiotics bought as supplements is keeping the bacteria alive until they reach the gut.
This issue has many stages: manufacturing and preparing the probiotic product; storing and transporting the product to the retailer; storing the product in the shop; transporting the product from the shop to our homes, and then how we store and use them.
This whole process can cause many, if not most, of the beneficial bacteria to die. So by the time we buy a product and ready to consume it, a lot of these vulnerable and delicate species may no longer be active.
The two biggest threats to probiotic supplements are heat and humidity.
Another threat may also be the manufacturer choosing to combine multiple strains that compete with each other, so one strain could kill off another.
Although, you’d hope that the manufacturer would do their research first before designing a probiotic product! This is particularly problematic for “probiotic” yoghurts.
Freeze-drying appears to be the best method of preserving bacteria, and then keeping them at temperatures at around 4c (25).
Many manufacturers have found ways to preserve bacteria at room temperatures, but once the product has left the manufacturing plant, can we be sure it is kept at optimal temperatures?
So, it is recommended to buy a product that was manufactured relatively close to home, is stored in the retailer’s refrigeration system, and not kept on the shelves under bright store lights.
Some species don’t require refrigeration nor do probiotic yeasts such as S. boulardii, but most of the ones you’ll be interested in taking (such as the Lactobacillus and Bifidobacterium) do.
Buying probiotics that are in blister packs rather than bottles or jars is recommended too, since each individual capsule will be protected from moisture before you’re ready to take it.
Don’t pop them out of their blister until you’re ready to take them (so don’t transfer them to pill boxes).
Alternatively, powdered forms that are individually packaged in dose size sealed packets are a good option.
Keep your probiotics in the fridge. Whether the manufacturer tells you to or not – it’s safer that way.
But don’t expect them to resolve your issues, manufactured probiotics quickly disappear from your system the moment you stop taking them. You’re better off changing your diet.
Seb is a writer and blogger of food and nutrition. He holds a bachelors and a masters degree in nutrition science, and has studied sports and exercise nutrition at postgraduate level. He specialises in plant-based nutrition and believes passionately that we can all live with a little less meat.