Your gut is arguably the most important part of your body and the linchpin to long-term health and weight management. Don’t get me wrong, we wouldn’t survive for even a second without our other vital organs, but in terms of your health, everything starts in the gut. The relationship between gut health and weight gain can get a little complicated. Thankfully, you don’t need to know how everything operates, but you do need to know what you can do to keep your body’s gut healthy and happy.  

I always think it’s important to understand the why

  • WHY does an unhealthy gut lead to weight gain?
  • WHY does an imbalance of gut bacteria increase inflammation?
  • WHY are my hormones affected by good and bad gut bacteria?

I’ll answer all of those questions. But first, let’s dive into the basics about gut health.

Gut Health and Weight Gain

Your gut is more than just your stomach; it’s your entire digestive tract. From the moment you put anything into your mouth, digestive enzymes in your saliva begin the process of breaking food down. 

Weight gain and fat storage can happen for a lot of different reasons. If you eat too much, even of foods that are considered good for you, you can gain weight. If you eat too much sugar, you can definitely gain weight. If you don’t get enough exercise, you lose muscle, your metabolism is affected, and you gain weight. 

But how does an unhealthy gut lead to weight gain? Well, here is some enlightening information that will hopefully motivate you to take stock of what is happening in your gut.

Why Does an Unhealthy Gut Lead to Weight Gain?

In many cases, if your gut is mildly or moderately unhealthy, you may not even know it. A lot of people tolerate regular bloating, gassiness, and constipation as part of their normal constitution. Just because something happens all the time doesn't make it normal - even when symptoms are tolerable.

Unless an unhealthy gut is addressed, it’s highly likely that things will only get worse. 

Bacteria, Gut Health and Weight Gain

When my oldest daughters were toddlers, I had them start taking a probiotic. They always asked why they needed to take a probiotic. I explained it in this simple way: probiotics are like soldiers made up of good bacteria that fight the enemy, or bad bacteria. As long as the good guys are winning, your body is much more likely to stay healthy.

The research around bacteria has evolved a lot over the past 20 years. We used to think that bacteria “colonized” in our gut, building homes and living forever. We now know that bacteria “vacations,” spending a few weeks before packing up and moving out. 

The fluctuations in the bacteria within your body make up your microbiome - or all the microbiota that live inside your tissues and even on the surface of your skin. 

What you eat and drink plays a tremendous role in the health of your microbiota. Unfortunately, our diet culture has contributed to this greatly with fad diets, such as keto and very high protein diets, damaging this delicate balance of flora. 

Good vs Bad Bacteria 

There are literally tens of thousands of species of bacteria. Of all the species, there are two overarching phyla I want you to be aware of so you can make better decisions when it comes to eating better…and hopefully losing a few unhealthy pounds of body fat along the way. 


Firmicutes are a type of bacteria that live in all of our guts. Generally speaking, it’s better to have less of this phylum of bacteria in our body than more. Firmicutes impact sugar and fat metabolism. People with greater amounts of Firmicutes are more overweight, have more body fat and are more likely to develop metabolic conditions, including diabetes. 

Eating foods that increase the creation of Firmicutes, in the end, increases markers of low grade inflammation. 


Bacteroidetes are a phylum of bacteria that also makes a home in our guts. Think of Bacteroidetes as the helper soldiers. It’s better to have more of this phylum of bacteria in our bodies. People with a greater ratio of Bacteroidetes are leaner, healthier and have better sugar and fat metabolism. 

Eating foods that increase Bacteroidetes downregulate, or diminish inflammatory markers, making it much easier to manage weight and have better digestion and feel more energized. 

Firmicutes and Bacteroidetes: The Foods

People who consume foods that are higher in fat and higher in refined sugars have considerably more Firmicutes. It is also possible to increase the ratio of Firmicutes by not eating enough vegetables or fruits. 

High fat and high sugar foods, in terms of gut health, affects your body’s ability to use energy, forcing your body to store the calories you eat from sugary or high fat foods. 

To be totally clear, it is entirely possible to eat a low fat diet and still have high (bad) Firmicutes. In fact, if you’re eating a mostly processed, higher-sugar lower-fat diet, you’ll do this. Remember, even if something doesn’t have any added sugar, but is a refined carb (i.e. a plain white bagel), it will convert to sugar in your body quickly. 

On the flip side, you can also raise Firmicutes by eating a high-fat diet, especially one that’s lower in fiber. 

So are the people who have a higher ratio of Bacteroidetes-to-Firmicutes? They’re people who eat a mostly plant-based diet that’s loaded with fiber, lower in fat, and lower in refined sugar.  

It’s really that simple. There’s no pixie dust or magic wand that’s going to help your body balance this important ratio. You have to eat the right foods. 

Why Does Gut Bacteria Affect Inflammation?

Gut bacteria isn’t always the bad guy. Remember there are 30,000+ species of bacteria, and a lot of those species are really good for you! 

But the bad bacteria can really turn your gut and your body upside down and inside out. 

Your gut is lined with something called a mucosal lining. It’s a semi-permeable wall that lets the good stuff out (i.e. vitamins in the foods you eat), and keeps the bad stuff in, (like toxins) so they stay in your digestive tract to be eliminated later on. 

When you have too much bad bacteria in your gut, it starts gnawing away at the lining of your stomach and intestines creating a hole. This hole might be microscopic, but it’s large enough to let bad stuff out. When bad stuff leaks out of this protective lining, inflammation sets it. 

This is also known as leaky gut, an all-too-common problem that can result in fatigue, constipation, weight gain, bloating, headaches and skin conditions. 

A leaky gut can be healed by eating more nutritious foods, avoiding high sugar-food, and avoiding processed high fat foods. Leaky gut can sound like a lot of things. If you have a question about your gut health, talk to your doctor. 

Why are my hormones affected by good and bad gut bacteria?

The bacteria in your gut can create their own hormones, which have a direct impact on how we think and feel. The bacteria in our gut also helps to modulate other important hormones, like estrogen. This is just part of the reason why the foods you eat play a role in helping you feel happier, or even depressed. 

A majority of your body’s neurotransmitters, like serotonin, are created in your gut. The foods you eat play a role in keeping the production and transmission of these important messengers in check. 

Bottom line: What you eat has the potential to create good and bad bacteria. The good bacteria have the potential to make you look and feel great. The bad bacteria do the opposite, making you feel lousy.

Think of your body as a flower. If you were to plant a flower seed in soil that’s rich with nutrients and full of good bacteria, you can expect that a beautiful flower will grow. If you were to plant a flower seed in a mound of dirt, that’s depleted of nutrients and full of bad bacteria, the flower may not flourish at all. 

The more you take care of your body by giving it the nutrition it needs, the better you’re going to feel. And if weight loss is your goal, the faster you’ll see progress.

How do I get my gut health in a better place so I can more easily manage weight gain? 

You have to do the work and stay committed to a healthier way of eating, avoiding the foods that are causing the bad bacteria (and other problems associated with bad bacteria) to flourish. 

This may all sound very cliche, but eat more vegetables and eat less sugary foods. Eat modest amounts of healthier fats, avoid unhealthy processed vegetables oils, like corn, canola and soybean.