Does Your Gut Bacteria Affect Weight Loss? Simplifying The Science


[Last updated 8th November, 2018]

Researchers have learnt so much about our gut bacteria in the last decade.

The potential effects they have on health is quite extraordinary.

Some suspect they may have a strong influence on metabolic diseases, including obesity.

This article looks at how gut bacteria may affect weight, as well as what you can do about it.

What Are Gut Bacteria?

What are gut bacteria?Gut bacteria refers to the community of micro-organisms that permanently reside inside our intestinal tract (1).

These bacteria are also commonly referred to as gut microflora, gut microbiota, or the gut microbiome.

Studies over the past decade have begun to reveal just how influential these bacteria are on our immune function, metabolism, nutrient absorption, and risk of numerous metabolic diseases.

In fact, the gut microbiota is often considered a hidden or extra “organ” due to the way they can positively or negatively influence our health (3, 4).


Summary: Your gut bacteria is a community of micro-organisms that live in your intestines. They can positively or negatively influence many aspects of health.

Can Gut Bacteria Affect Weight Loss?

Can Gut Bacteria Affect Weight Loss?If you consume more calories than you burn, you will gain weight.

While this is fundamentally true of a positive energy balance, gut bacteria transplant studies indicate it’s not nearly as simple as “calories in calories out”.

The class or type of bacteria in your gut also appears to influence energy balance to some degree.

Studies on rodents found that transplanting the gut bacteria of obese mice into lean mice (fecal transplants) caused the lean mice to gain fat cells rapidly (5).

Since then researchers have found striking differences between the gut bacteria of lean and obese individuals (6, 7, 8).

Faecal sample analyses indicate that relative proportions of Firmicutes and Bacteroidetes – both “classes” of bacteria in the gut – can influence energy balance to some degree (5, 9, 10).

Specifically, human studies found that the ratio of Bacteroidetes to Firmicutes was decreased in obese individuals, as was overall diversity of gut bacteria (11, 12).

In other words, more Firmicutes and fewer Bacteroidetes is not ideal.

mice bacteroidetes and firmicutes
Comparing lean and obese mice


Researchers hypothesize that this “obese microbiota” may enhance signals that trigger the amount of energy we harvest from food. This in turn increases the amount of calories absorbed, and therefore weight gain (3, 12).

Summary: Early research suggests the types and proportions of bacteria in our gut may influence likelihood of weight loss or weight gain. This may be due to its influence on mechanisms that affect energy storage and energy balance.

Probiotics and Weight Loss

Can probiotics affect weight loss?Probiotics are bacteria that we eat specifically for health benefits.

They enter the digestive tract to alter and improve the current makeup of our gut bacterial community.

It appears they can influence digestive health, and now researchers are now looking to see if regular probiotic supplementation can influence weight. So far only a handful of human clinical trials have been published, but findings do support the idea that gut bacteria affects weight loss.

From the weight of evidence currently available, Lactobacillus gasseri appears to be the probiotic strain that can best assist weight loss in humans.

Note that some strains of bacteria appear to “protect” from gaining more fat, while others are linked to weight gain (13, 14, 15).

Summary: Some clinical studies have found that certain probiotic strains can influence weight gain. This supports the idea that our gut bacteria environment influences weight management.

Diet Recommendations to Improve Gut Health

Diet Recommendations to Improve Gut HealthIt’s unclear what specific strains of bacteria we need more or less exposure to for improved gut health.

The same goes for promoting weight loss.

Unfortunately, this means specific dietary recommendations are limited.

What we do know is that consuming more probiotic-rich food, as well as nourishing our existing gut bacteria, are fundamental for overall health.

Fermented Foods

Fermented foods naturally contain lots of beneficial bacteria and should become a regular addition to your diet.

Think of them as a kind of natural probiotic supplement that “top up” the bacteria in your gut.

Fermented foods are actually very common in our diet, but healthier options include quark, plain yogurt or kefir, sauerkraut, and other non-pasteurized pickled vegetables.


In order to nourish existing bacteria, you must regularly eat prebiotic foods (not to be confused with probiotics).

Prebiotics are a form of carbohydrate (mostly fiber) that humans can’t digest. It acts as “food” for the beneficial bacteria in your gut to grow and thrive.

Foods rich in prebiotic fiber include:

  • Oats
  • Bananas
  • Berries
  • Beans and legumes.
  • Onions
  • Garlic
  • Asparagus

As though we needed additional reasons to eat more legumes and berries!

Limit Junk Foods

Limit junk foodsNot only are junk foods high in calories, but high sugar foods appear to promote the growth of potentially harmful bacterial species (16, 17).

Feeding the wrong bacteria enables them to colonize and grow more rapidly, without as many beneficial bacteria to prevent them from thriving (18, 19, 20).

The growth of this harmful bacteria may indirectly influence many aspects of health, including weight gain. Individuals who eat a high calorie diet appear to have a poorer ratio of Bacteroidetes to Firmicutes, which is associated with absorbing more calories (21).

Summary: To maintain a diverse gut bacteria that promotes health, ensure your diet includes lots of prebiotic foods and fermented foods, and limit junk foods.


Current evidence suggests the balance and diversity of our gut bacteria can influence how easily an individual gains or loses weight.

Until we learn more, the best way to nurture a healthy gut bacteria is by eating a diverse diet rich in prebiotic foods and fermented foods. If this is not possible, probiotic supplementation may be a good option.

It’s also a good idea to limit junk foods, but you knew that already.

Researchers suspect our gut bacteria may have a strong influence on how easily we gain and lose weight. This article explores what you can do about it.

About Joe Leech, Dietitian (MSc Nutrition & Dietetics)

Joe Leech is a university-qualified dietitian from Australia.

He graduated with a Bachelor's degree in exercise science, followed by a Master's degree in Nutrition and Dietetics in 2011.

Learn more about him on the About page

14 responses to “Does Your Gut Bacteria Affect Weight Loss? Simplifying The Science”

  1. If an article is going to be respected for its content, editing for proper grammar would be helpful. The word is learned, not learnt and have begun, not have began.

      • I think ur info is very helpful and useful and easy to understand how you have written it out and explained everything and then summed it up to simple understandings that way people who may not have had a very good education or just have trouble reading and or with a learning disability such asDick’s dyslexiaand ADHD can stay focused and know exactly what you’re talking about and be able to implement it into their daily life so I would pay no mind to that lady I think you are very helpful

  2. I’m new to your site. I’m finding it very informative.
    I just wanted to make a comment about spelling. I am an Australian and my son went to school here where is was acceptable to spell a word the way it sounded whether I thought it was wrong or not.
    I feel sad for that lady, all she’s doing is showing her ignorance.

  3. Hi Joe, I have heard different views on whether probiotics can survive the acidity in the stomach or not. What’s your view on this? Great article, thanks! Helen

  4. Thank you for the reassuring information. I’m thinking if I’m eating a True Paleo diet 100% (no diary (except occasional grass fed butter) organic grass fed animal flesh, variety of sustainably (not farmed) caught seafood and poultry that are out in trees and grass taller than themselves all day eating bugs and fed grains non GMO no soy etc organic veggies only good fats no chemically washed industrial oils. Apply continually (daily) Intermittent fasting rules which make my body happy eating once a day, when I’m hungry, generally after 18-24 Hrs. I will fire up not only my mitachondria with additional 30 mins daily walking, swimming and getting sunshine on my body as close to midday as possible regularly to increase my vitamin D levels naturally and couple times a week do get puffed out doing short sprints, practice mindfulness my guess is all will come right NATURALLY without taking AMPK supplements. I make this assumption based on the 60kg’s I’ve lost. What do you think…

  5. Thanks for all the information. You write: “In order to nourish existing bacteria, you must regularly eat prebiotic foods, among them onions garlic and asparagus”. However, in a Fodmap list I found these to be in the High F. list. So are they beneficial for IB-C or should be avoided?

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