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Aside from our cells, we humans are home to trillions of microbes– including bacteria, viruses, fungi and more– that live all over and inside us. They are absolutely critical for our survival and have evolved alongside us over time. These microbes (bacteria make up the bulk of them by far) influence our mood, weight, metabolism, digestion, immune function, and even disease expression. They also protect your health by displacing harmful bacteria and producing antimicrobial compounds. The bacteria in your gut ferment the foods you eat and produce beneficial compounds that keep you healthy; however, certain species of bad bacteria produce endotoxins that may cause disease or increase cancer risk.

Your gut microbes do more than help you digest your food. Gut bacteria balance and diversity can actually impact your weight. Human studies are showing that obesity correlates with fewer bacterial species in the gut. (source) Imbalances in gut bacteria and between certain bacterial strains can make an individual more susceptible to weight gain. Understanding how to improve gut health and manipulate your gut bacteria may hold the key in weight loss and lowering obesity.

Bacterial Diversity

We’re really only beginning to scratch the surface of the microbiome, which is the collection of genetic material of alllll those microbes, AKA the microbiota, that live in and on us. Yes, the bacterial and other microbes contain genetic material, which sounds pretty sci-fi, but also means we are only part human because we are also part bacteria.

Your microbiome is established in infancy, and you are inoculated with your mom’s flora in the birth canal. So you inherit your mother’s microbiome and her array of microbial species. A rise in C-section births is one theory behind our collective dwindling bacterial diversity. C-section babies don’t get this bacterial starter package, and that can have consequences like increased risk for autoimmune conditions, eczema, asthma, and digestive issues later in life. (source)

The more bacterial species we have in our digestive tracts, the healthier we are. Robust bacterial diversity keeps our immune system strong, improves digestion, and helps keep bad bacteria in check. Humans harbor around 300-500 bacterial species, but these numbers are dwindling because of factors that kill our beneficial bacteria, like antibiotics, medications, poor diet, alcohol, sugar, chemicals in our environment, and hormonal birth control.

Ideally we have about 85% “good” bacteria and 15% “bad.” We all have the bad guys, which have the potential to become pathogenic if they overtake the good guys. When the bad guys overtake the good, or when your bacterial species dwindle, you may experience unpleasant GI symptoms, inflammation, or even be at risk for IBS, IBD, or even diabetes and other inflammatory diseases.  (source)

How Your Gut Bacteria Influence Your Weight

Now for the part you’ve been waiting for: What does your gut bacteria have to do with your bodyweight? And more importantly, what can you do about it?

The four dominant bacterial phyla (divisions of the dominant bacteria) in the human gut are Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. Your Firmicutes to Bacteroidetes ratio influences your weight and metabolism the most, and these 2 make up the most dominant phyla (about 90%). Due to their negative influence on glucose and fat metabolism, firmicutes are considered “bad” gut microbes, and increased ratios of Firmicutes to Bacteroidetes species has been correlated with obesity and Type II diabetes. (source)

A recent study showed that the weight of mice could be changed by over 15 percent just by shifting their intestinal bacteria. Along with weight changes, certain bacteria can also influence and even predict heart disease, high blood pressure, Alzeheimer’s and other inflammatory diseases. (source) Of course mice aren’t human, but we assume the same or similar for our human guts.

Increased ratio of Firmicutes : Bacteroidetes was found in the gut microbiota of humans with obesity (source). Typically higher firmicutes and lower bacteriodetes means higher risk for obesity. SO if you wish to positively influence your weight via your gut bacteria, you want lower firmicutes and higher bacteriodetes.

But how and why does your firmicutes to bacteriodetes ratio affect your weight? Well, your gut bacteria digest your food for you. The reason gut bacteria affect our weight is because they regulate how much fat and how many calories we absorb from our food. Higher numbers of Firmicutes bacteria and lower Bacteroidetes causes one to absorb more calories from the foods that you eat. That means you could be eating the exact same diet as your neighbor, but because you have different levels of gut bacteria, you could actually be absorbing more calories from your diet because your gut bacteria are not using/fermenting/digesting your food in the same way as your neighbor with better gut diversity. Pretty wild, right? (source)

Why Our Microbiome is Changing

Our collective microbiota is changing, and not in a good way. Our gut bacteria look drastically different from our ancestors who thrived on a diverse, whole foods diet. They weren’t eating Cheerios and Cheetos. Here in the U.S., those of us living in urban areas have the poorest bacterial diversity, which we now know is a risk factor for disease and obesity. Why are our beneficial bacterial species dying?

  • overuse of antibiotics, which decimates good bacteria in the gut
  • our overly hygienic lifestyles that include antibiotic cleansers and products are killing beneficial bacteria
  • processed food diets that do not properly feed our gut bugs
  • too much sugar, booze, and meds that kill our good bacteria
  • environmental pollutants, harsh cleaning products, and chemical bodycare products
  • inflammation, which causes bacterial species to dwindle, from poor diet and stress
  • stress is a real killer, and chronic stress increases cortisol that changes the terrain of the gut

These factors can poorly impact the gut bacteria you need for a healthy metabolism. The good news is that research has shown that within two to four days of switching to a more microbiome friendly diet, your gut microbiome can change! By changing your diet and lifestyle habits, you can invite new bacterial species to your gut party, and they can positively impact your health, weight, inflammation, and disease risk.

Best & Worst Foods for Your Gut Bacteria

Your goal is to focus on foods that nourish your existing gut bacteria, and you want to introduce new species into your gut. You want to lower Firmicutes and raise Bacteriodetes if you want to lose weight. It’s not as easy as taking a probiotic, unfortunately, which does help while you take it, but the probiotics do not actually stick around and colonize the gut. Probiotic supplements can support a weight loss plan though! I recommend this one.

You can change your diet to improve bacterial diversity. Here are some of the best foods you can eat to nourish your gut (also make sure you read this post).

  • Anti-inflammatory diet. Read how to do that here. Think Mediterranean.
  • Ditch vegetable oils, seed oils, and hydrogenated fats. You want to consume the healthier fats like olive oil, coconut oil, cultured butter, and avocado. Higher fats that cause inflammation increase firmicutes. (source)
  • Eat beans! Beans are among the very best foods to raise your Bacteroidetes. In fact, if you don’t digest beans well, you are probably low in Bacteroidetes. You can slowly introduce beans into your diet and increase over time to encourage more Bacterodietes. (source) Try my kale white bean sausage stew for a delicious way to get some beans! And try my lentil chard stew too. Lentils have nearly 20 grams of fiber per cup.
  • Increase prebiotic foods like alliums, radish, artichoke, asparagus, banana, resistant starch. Add this fiber blend with resistant starch to smoothies. The starch helps support optimal blood sugar and insulin levels, appetite control, cardiovascular health, and healthy weight!
  • Other gut superfoods include pomegranate, green tea, fermented foods, even dark chocolate! I recommend this green tea. Fermented foods like raw kraut and probiotic-rich beverages help introduce new species into your gut. Try this coconut water kefir.
  • You’ll often see whole grains recommended, but grains don’t work for everyone. Gluten free oats, quinoa, and wild rice are what I most often recommend, and I eat them sparingly myself.

Foods to avoid include the following:

  • processed junk that contains ingredients you cannot pronounce
  • too much sugar (more than 25 grams of added sugar daily), which stimulates growth of bad bacteria
  • artificial sweeteners are shown to reduce beneficial gut bacteria (source)
  • vegetable oils
  • excessive alcohol. The exception might be small amounts of red wine that is rich in polyphenols. Emphasis on the small.
  • soda
  • juice (very high in sugar)

In your home/lifestyle habits:


If you’re on a weight loss program, consider your gut! You can also do testing to get an excellent picture of what’s going on in there. That also lets you know how to better design your diet. You can order the GI MAP (what I use and recommend) here.  It will tell you your ratio of firmicutes to bacteroidetes. The GI MAP will also let you know about bad bacteria overgrowth and inflammation.

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