The ketogenic diet is effective for short-term weight loss and has legitimate medical uses, particularly for epilepsy. But its long-term safety profile is mixed. Staying in ketosis for a year or more raises specific concerns for your heart, kidneys, thyroid, and gut, and most people don’t stick with it anyway. About two-thirds of people following a ketogenic diet are still adherent at two years, but by three years that number drops to just 38%.
What Happens to Your Cholesterol
This is the biggest red flag in long-term keto research. A case series published in the National Library of Medicine found that patients following a ketogenic diet for an average of 12.3 months saw their LDL cholesterol jump by an average of 187 mg/dL, a 245% increase. The average LDL in those patients reached 316 mg/dL, with some individuals hitting over 800. For context, most cardiologists want LDL below 100 mg/dL in high-risk patients.
Apolipoprotein B, a marker many researchers consider a better predictor of heart disease risk than LDL alone, averaged 191 mg/dL in the keto group. Levels above 130 are generally considered elevated. Not everyone on keto experiences these dramatic spikes, but the subset who do (sometimes called “lean mass hyper-responders”) can see alarming numbers. The American Heart Association scored the ketogenic diet just 31 out of 100 for heart health, placing it in the lowest tier of ten popular diets evaluated. The AHA noted that keto is high in fat without limiting saturated fat and has not been shown to outperform less restrictive diets for long-term weight loss.
Kidney Stone Risk
Kidney stones affect roughly 5.9% of people on ketogenic diets, compared to a fraction of a percent in the general population. That’s a meaningful increase. The risk comes from two mechanisms working together: your body produces less citrate (a natural stone inhibitor) because of the mild chronic acidosis that ketosis creates, and calcium excretion through urine increases. Studies tracking patients on keto for epilepsy found elevated calcium-to-creatinine ratios in anywhere from 8% to over 26% of participants, depending on the study.
The acidosis itself deserves attention. When your body runs on ketones, blood pH shifts slightly more acidic. Your kidneys compensate by pulling minerals from bone to buffer that acid, which is part of why calcium shows up in higher concentrations in urine. This isn’t dangerous over a few months, but over years it creates a persistent environment where uric acid and calcium stones are more likely to form.
Thyroid Function Shifts
Ketogenic diets consistently lower levels of T3, the active thyroid hormone your cells actually use. In a controlled crossover trial, T3 dropped significantly during the keto phase (from baseline to 4.1 pmol/L) while remaining unchanged during a high-carb phase. At the same time, T4 (the inactive precursor) rose significantly. TSH, the hormone your brain uses to signal the thyroid, stayed the same.
What this means practically: your thyroid gland is working fine, but your body is converting less of its output into the active form. Lower T3 can slow your metabolism, make you feel colder, contribute to fatigue, and potentially affect mood. Some researchers interpret this as a beneficial metabolic adaptation (your body becoming more efficient), but it’s an open question whether sustained low T3 has consequences over many years.
Gut Bacteria Take a Hit
Cutting carbohydrates to keto levels starves specific populations of beneficial gut bacteria. The most consistent finding across multiple studies is a significant and persistent reduction in Bifidobacterium, a genus that relies exclusively on carbohydrates for energy. Five studies found this decline was highly significant, and it didn’t bounce back even after months on the diet.
Other important species are also affected. Faecalibacterium prausnitzii, Eubacterium rectale, and Roseburia, all of which produce butyrate (a short-chain fatty acid that feeds the cells lining your colon), decline in the first weeks of keto. There’s a silver lining here: some research suggests these butyrate-producing populations may recover after about six months as the microbiome adapts. One theory is that ketone bodies themselves partially compensate for reduced microbial butyrate production. Still, the sustained loss of Bifidobacterium is a concern, given its role in immune function and gut barrier integrity.
Bone Density and Nutrient Gaps
The bone density news is more reassuring than you might expect. Studies tracking people on low-carb diets for one to two years found no significant changes in bone mineral density compared to control groups eating higher-carb diets. One 52-week study observed a small decrease in total body bone density, but it wasn’t meaningfully different from what happened in the comparison group eating a low-fat diet. So while the acidosis and calcium loss from keto are real, they don’t appear to translate into measurable bone loss over one to two years. Whether that holds over five or ten years is unknown.
Micronutrient deficiencies are a more practical concern. Low-carb diets tend to be low in thiamin, folate, vitamins A, E, and B6, along with calcium, magnesium, iron, and potassium. Many of these come primarily from foods that keto restricts: whole grains, legumes, fruits, and starchy vegetables. If you’re on keto long-term, targeted supplementation or very careful food selection becomes important rather than optional.
What the Mortality Data Shows
A large observational study published in Nature found that higher adherence to a ketogenic dietary pattern was associated with a 24% reduction in all-cause mortality risk after adjusting for confounding variables. Cardiovascular mortality did not increase. This sounds encouraging, but observational studies can’t prove cause and effect, and the people who maintain keto long-term may differ from the general population in ways that independently affect their health, such as being more health-conscious overall, exercising more, or avoiding processed food.
The mouse data tells a less optimistic story. A long-term ketogenic diet study in mice, published in Science Advances, found that sustained ketosis led to fatty liver disease, elevated blood lipids, and impaired glucose tolerance from reduced insulin secretion. Animal models don’t translate directly to humans, but they do suggest that the metabolic stress of permanent ketosis may have consequences that short-term human trials haven’t captured yet.
Who Has the Most to Lose
The risks of long-term keto aren’t distributed equally. If you have a family history of high cholesterol or heart disease, the potential for dramatic LDL increases is a serious consideration. If you’ve had kidney stones before, the 6% incidence rate on keto is particularly relevant to you. People with thyroid conditions should be aware that keto will likely lower their active thyroid hormone levels, potentially complicating management of an existing condition.
For people using keto to manage epilepsy, the risk-benefit calculation is different. Seizure control can be life-changing, and many of these risks are manageable with medical monitoring. For people using keto primarily for weight loss, the equation is harder to justify given that less restrictive dietary patterns achieve similar long-term weight loss without the same metabolic trade-offs. The adherence data supports this: nearly two-thirds of people have abandoned the diet by three years, suggesting that for most people, keto is a temporary intervention regardless of their intentions.