Is “low-carb” the new “low fat”?

Just as fat was the villian of the 80’s and 90’s, carbs seem to be on the chopping block in the nutrition world these days. While of course this is a nuanced, one-size-fits-none discussion, I will attempt to make a case for and against low(ish) carb diets so you can feel better about how to think about choosing your daily carbohydrates in a way that supports your health goals, whether you want to lose weight, improve your hormone health. 

For the purpose of this article, I am using these ballpark definitions (grams based on an 1800 calorie diet):

• Very low carbohydrate diet (ketogenic): about 5-10% of diet, between 20-50 grams daily
• Low carbohydrate diet: 25% or less of caloric intake, around 100 grams of carbs or less per day
• Moderate-low carbohydrate diet: around 25-35% of daily calories, about 100-150 g/day

 

Why low carbohydrate diets can be beneficial to your health:

 

Benefits for poor metabolic health and disease (a big one) – 

When considering an optimal diet for the management, reversal, or prevention of metabolic syndrome related diseases, like diabetes, heart disease, and fatty liver, lowering insulin and reducing carbohydrate/sugar intake is the goal. In these cases, a very low or low carbohydrate diet may be ideal. 

It’s important to understand how the metabolic dysfunction underlying these diseases develops if we are to understand how to prevent it: when you consume a carbohydrate of some sort, be it sugar from cake or starch from potatoes, the digestive system breaks all of them down into glucose molecules which is sent into the bloodstream to travel through the body to be sent into the cell which generates either ATP (energy), glycogen (readily available stored glucose), or fat (less readily available energy). The hormone secreted to move the glucose into the cells is called insulin. So, the more glucose that enters the bloodstream, the more insulin is secreted to move it where it needs to go. 

Over time, if glucose is consistently consumed in excess of what the body needs for energy, more and more is stored as fat – mainly under the skin and around organs. This increased fat drives both the insulin resistance (meaning it takes more insulin to clear blood glucose since it is less efficient) and decreased insulin production (the pancreas is less effective at producing insulin, potentially due to the fat accumulation around the organ) that we see associated with diabetes and dyslipidemia. This causes glucose to become backed up in the bloodstream, leading to high blood sugar that can damage the blood vessels (as with atherosclerosis) and cause nerve and organ damage if it goes unmanaged. In summary, excess glucose intake > insulin resistance and impaired insulin production > glucose backing up in the blood > damage to organs and blood vessels and weight gain.  

While the relationship isn’t completely understood, there seems to be a perpetuating effect between insulin resistance and weight gain – together, their effects are amplified. Research has shown that even with the same number of calories, a low-carbohydrate diet is more effective at improving markers associated with metabolic syndrome such as lipid profile, insulin resistance and blood pressure.  Thus, in the case of metabolic syndrome, since the body has become less efficient at processing carbohydrates, a very low or low carb diet can be extremely effective at helping to reduce stored fat and reverse insulin resistance to restore a healthy metabolism in those with diabetes, heart disease, and fatty liver. 

 

May aid in weight loss

In addition to the mechanisms above that go hand in hand with excess body weight, a similar point is that if you are producing insulin, you are not losing fat. They cannot happen simultaneously. While insulin is necessary to provide cells the energy they rely on, it’s not something you want being secreted continuously – frequent spikes of insulin in the body reduces the time that your body is burning energy reserves and fat for energy, making it difficult to lose weight.

This is a big reason that very low or low-carbohydrate diets are used for weight loss protocols, in addition to fasting. In addition, low carbohydrate diets tend to keep blood sugar more stable (more on this below) and therefore may reduce drastic spikes and drops that cause cravings and urge to eat more frequently.

Fats and proteins also tend to be more satiating, which may encourage eating less than higher carb diets. I would argue that the number of carbs is less significant compared to the importance of overall diet quality and diversity of plant foods, healthy fats and quality proteins that work together to support a healthy metabolism. 

Can help blood sugar and hormone balance

Consuming fewer carbs can help with overall blood sugar balance, reduce insulin spikes and subsequent blood sugar drops, and therefore cravings (as mentioned above), insulin sensitivity and hormone regulation. Blood sugar balance is crucial for healthy hormones, like cortisol and sex hormones. It works in a few ways:

• If blood sugar is imbalanced, it can increase cortisol and lead to increased hunger and weight gain.
• High insulin levels can decrease sex-hormone binding globulin and in turn increase the amount of estrogen circulating in your system, causing PMS, heavy periods, unwanted weight gain, mood swings, and insomnia. It can also cause more intense menopause symptoms, which no one wants!
• High insulin levels can increase female testosterone. Low carbohydrate diets have also been shown to benefit the hormone imbalance in conditions like PCOS where increase androgens (like testosterone) is an underlying cause. 

May favorably change the gut microbiome

Some researchers have found that a very low carb diet (ketogenic) may shift the ratio of firmicutes and bacteroidetes in a way that favors weight loss – suggesting that some of the positive results of weight loss on the ketogenic diet are due to metabolic changes as a result of an altered microbiome, which we know can have a significant effect on body composition by influencing things like energy absorption from food. 

 

*Disclaimer: of course, carbs aren’t the only factors to consider in debating the optimal diet. Research has shown that low carb diets are worse than moderate carbohydrate diets if it is animal protein heavy and does not consist of enough plant foods. To reap the benefits of going low-carb, ensure you are getting plenty of nonstarchy vegetables and other plant foods.* 

 

Why low carb diets may not be good for your health:

 

Very low/low carb diets can often be hypocaloric diets (aka “crash diets”):

Since low carb diets restrict a major source of energy in a traditional Western diet, there’s a chance that it could lead to inadequate energy intake. While eating in a caloric deficit short term can have health benefits like reducing inflammation, aiding in detoxification, improve insulin sensitivity, slow aging, etc., chronic hypo-caloric diets may not be the best for your health. Some concerns are that it can lead to a loss of lean muscle mass, compensatory hormone fluctuations, decreased metabolic rate, can cause stress and alter thyroid and/or adrenal function, and negatively impact microbial diversity. Let’s explore. 

 

May negatively affect hormone function (sex, thyroid and adrenal):

Eating a very-low or low carb diet, especially in women more sensitive to energy balance, may be a form of stress on the body and cause increase in cortisol production. This is especially significant in women who are already living a life of “burnout” aka not enough sleep, high demands from work and/or family, doing intense forms of working out all the while trying to maintain a particular physique with a very low carb diet. Additionally, when the body is not receiving enough energy, a few things happen: the body may dial down the production of sex hormones (think about it this way – before refrigerators and grocery stores, inadequate nutrition often meant a famine in the land, aka not a good time for the body to reproduce) and cause something called the “pregnenolone steal.” Since pregnenolone is a building block for both sex hormones and cortisol, increased cortisol production results in less available for other hormones to be created. Finally, low carbohydrate diets can be contraindicated in hypothyroidism because glucose is needed to convert inactive thyroid hormone into the active form.  

 

*If you have symptoms or lab values associated with any of these imbalances, consider your carbohydrate intake and work with a practitioner to determine if it is optimal for your specific needs. 

 

May not be sustainable and can lead to bingeing 

Many studies have shown that restricting food intake can prompt a strong physiological response known in the scientific literature as “hyperphagia” which means overeating. Reducing carbs too much may cause both a physiological response AND a psychological response, because to many people, carbs = comfort. If you’re restricting carbs too much, this could lead to binge behavior and set you up for some unhealthy patterns and loss of touch with your hunger and fullness cues. 

 

Reduced microbial diversity and phytonutrient intake

On the other side of the same coin mentioned above, it’s not yet clear if the microbiome shifts due to a very low carb diet are beneficial or not. Bifidobacteria has been shown decreased in low-carb diets, which is generally known as a beneficial strain for human health. Short chain fatty acid production is also decreased in low-carb diets, which is vital for colon health and have anti-inflammatory/anti-disease action in the body. This is likely due from the low intake of resistant starch found in higher carb foods like potatoes, beans, and rice. Furthermore, if we’re just worried about number carbs and not sources, we could end up reducing plant foods and thereby reduce microbial diversity and intake of phytonutrients found in colorful fruits and vegetables.

 

My take on “how many carbs should I be eating” as a functional nutritionist

After looking at both sides, I tend to side with a more moderate-low carbohydrate diet (around 25 – 35% of calorie intake) generally speaking, while reserving the right to alter it based on the individual. Just like throwing out fat in the 90’s had major health ramifications, we can’t do the same with carbs. We’ve come to realize that it’s more about the types of fats and carbs and the context of the rest of the diet and lifestyle that really matters.

For clients with metabolic diseases, a very low carb diet may be the best option, while my female “burnout” clients may benefit greatly from upping their carbohydrate and overall energy intake. For the general population, an ideal approach may be to reduce intake of refined grains and high sugar fruits, while being liberal with non-starchy vegetables, low sugar fruits and consuming legumes and starches in moderation in a way that forms resistant starch to feed the microbiome (by cooking and cooling first before consuming or reheating). Also, prioritize blood sugar balance by choosing appropriate carbohydrate portions at each meal, prioritizing low to moderate GI carbs, and always pair with protein, fat and/or fiber. I preach this to all my clients because it’s such an important piece of healthy hormones and metabolism! Finally, short periods of hypocaloric diets and fasting can be an important part of a nutrition therapy regimen as part of an individualized protocol. 

 

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