Does Bone Broth Break a Fast? Dr. Fung Explains

Bone broth does technically break a strict water-only fast because it contains calories, protein, and amino acids that provoke at least a mild metabolic response. But Dr. Jason Fung, the Canadian nephrologist who helped popularize therapeutic fasting, has consistently argued that bone broth is not just acceptable during fasting periods but actively helpful for many people. His reasoning is pragmatic rather than purist, and understanding it requires looking at what fasting is actually supposed to accomplish.

Dr. Fung’s Position and Why It Surprises People

In his book The Complete Guide to Fasting and in numerous public talks, Dr. Fung recommends bone broth as a fasting aid, particularly during longer fasts that extend beyond 24 hours. His argument boils down to a cost-benefit analysis: bone broth delivers electrolytes, a small amount of protein, and enough flavor to keep people from abandoning their fasts altogether. The modest insulin response it triggers is, in his view, trivial compared to the metabolic disruption of eating a full meal. He frames fasting not as an all-or-nothing metabolic switch but as a spectrum, and bone broth sits close enough to the fasting end of that spectrum to be useful.

This surprises people who treat fasting as a binary state. In strict biochemical terms, anything with calories moves you out of a pure fasted state. But Fung’s clinical experience led him to a more flexible view: the goal is sustained periods of low insulin, and a cup of bone broth containing roughly 40 to 50 calories doesn’t meaningfully interfere with that. He’s also noted that the electrolytes in bone broth (sodium, potassium, magnesium) help prevent the headaches, dizziness, and cramping that cause many people to quit fasting early. If the broth keeps someone fasting for 36 hours instead of 16, the net metabolic benefit is greater than what was lost from those few calories.

What Bone Broth Actually Does to Insulin

The concern about “breaking a fast” almost always comes back to insulin. During a water-only fast, insulin drops to its baseline, which allows the body to shift toward burning stored fat and eventually producing ketones. Any food that raises insulin could, in theory, slow or reverse that process. So the question becomes: how much does bone broth raise insulin?

Bone broth is rich in amino acids like glycine, proline, and glutamine, which come from the collagen and connective tissue that dissolves during cooking. Research on glycine, one of the most abundant amino acids in bone broth, shows that ingesting it alone does produce a statistically measurable rise in insulin, but the increase is modest compared to what glucose triggers.¹ In that study, glycine actually attenuated the glucose response when the two were consumed together, suggesting it may even have a stabilizing effect on blood sugar.

The broader picture of protein and insulin is more nuanced. When researchers fed diabetic subjects meals combining glucose with various protein sources, all the protein-containing meals raised insulin more than glucose alone, but the magnitude varied wildly depending on the protein type. Cottage cheese produced the largest insulin response, while egg white produced the smallest.² Gelatin, which is essentially what bone broth delivers, fell somewhere in the middle. But keep in mind that those meals contained 25 grams of protein alongside 50 grams of glucose. A typical cup of bone broth has around 6 to 10 grams of protein and almost no carbohydrate, so the insulin spike from a cup of broth would be substantially smaller than what those study meals produced.

Fung’s point lands here: the insulin response from bone broth exists, but it’s small and transient. It doesn’t resemble the sustained insulin elevation you’d see from eating a sandwich or a bowl of oatmeal. For someone whose fasting goal is to keep insulin low enough to remain in a fat-burning state, bone broth barely registers.

It Depends on Why You’re Fasting

Not everyone fasts for the same reason, and the answer to whether bone broth “breaks” a fast genuinely changes depending on the goal.

If you’re fasting for weight loss, bone broth is unlikely to interfere. The calorie content is minimal, and the amino acids may actually support the process by reducing hunger. A systematic review and meta-analysis of randomized trials found that protein consumption reduced self-reported hunger, desire to eat, and prospective food intake while increasing feelings of fullness and satiety.³ Protein also lowered ghrelin (the hunger hormone) and raised GLP-1 and cholecystokinin, both of which signal satiety. Even a small amount of protein from broth could help curb the appetite swings that derail fasting attempts.

If you’re fasting for metabolic health benefits like improved insulin sensitivity, the evidence is again on bone broth’s side. A review of intermittent fasting protocols found that the core metabolic benefits come from sustained periods of low caloric intake that trigger a shift from glucose to fatty acid and ketone utilization, improving glucose tolerance and insulin sensitivity.⁴ A cup of bone broth doesn’t meaningfully interrupt that shift.

If you’re fasting specifically to maximize autophagy, the answer gets murkier. Autophagy, the cellular recycling process that clears out damaged components, is suppressed by amino acids and insulin through the mTOR signaling pathway. Even a small influx of amino acids could theoretically slow autophagy. This is the one fasting goal where bone broth might genuinely compromise the outcome, though how much amino acid intake it takes to measurably blunt autophagy in a living human (as opposed to cell cultures in a lab) remains unclear. If autophagy is your primary objective, water, plain tea, and black coffee are safer bets.

If you’re fasting for religious or spiritual reasons, the definition is set by your tradition, not by metabolic science. Most religious fasts that prohibit food would consider bone broth a food. No amount of insulin data changes that.

Modified Fasting Versus True Fasting

Researchers have actually compared modified fasting (consuming small amounts of food) to true water-only fasting, and the results are illuminating. In a randomized crossover trial, participants either fasted completely or consumed small amounts of food spread throughout the day. The modified fasting conditions did produce measurably higher blood glucose levels at several time points compared to true fasting.⁵ But here’s what’s interesting: the modified fasting groups reported significantly lower hunger, greater fullness, and fewer food cravings, with no difference in cognitive performance across conditions.

That trade-off is essentially Fung’s argument in data form. You give up a small degree of metabolic “purity” and get a substantially easier fasting experience in return. For most people who aren’t fasting in a clinical research setting, the practical benefits of being able to stick with the fast outweigh the marginal metabolic cost of a few grams of protein and minerals.

Gut Health During Fasting

One of the underappreciated arguments for bone broth during fasting is what it does for the gut lining. Fasting itself can stress the intestinal barrier. When you stop eating, the gut’s mucosal layer doesn’t get the amino acids it normally uses for repair and maintenance. Bone broth delivers several of the amino acids that the intestinal lining specifically needs.

A review of animal and human studies found that the amino acids in bone broth, particularly glutamine, glycine, proline, and arginine, along with minerals like calcium, magnesium, and zinc, support gut barrier function, reduce intestinal inflammation, and improve nutrient absorption.⁶ The researchers highlighted these effects as especially relevant for people with inflammatory bowel conditions, but the gut-supportive properties apply broadly.

Glutamine, which is abundant in bone broth, has shown particular promise for gut barrier integrity. In a study on chemotherapy-induced intestinal damage in rats, glutamine supplementation prevented gut barrier disruption by regulating tight junction proteins.⁷ While chemotherapy-induced damage is obviously more severe than anything fasting causes, the underlying mechanism of tight junction maintenance is relevant whenever the gut is under stress. Fung has acknowledged this benefit as one reason he favors bone broth over simply adding a pinch of salt to water: the broth delivers building blocks the gut can use immediately.

Not All Bone Broths Are Created Equal

This is where things get practical and a bit messy. The bone broth you buy in a carton at the grocery store can be radically different from the stuff you simmer at home for 24 hours, and both can differ from concentrated bone broth products marketed to the fasting community.

When researchers compared a commercially enriched chicken broth to homemade broth, the commercial product had substantially higher collagen content (based on hydroxyproline levels) and roughly seven times more polyphenol content measured in gallic acid equivalents.⁸ The commercial broth also had higher chondroitin sulfate levels. These differences matter because more collagen means more amino acids per serving, which means a bigger insulin response and more calories.

A basic store-bought bone broth might contain 30 to 50 calories per cup. A concentrated collagen-rich broth powder reconstituted into a cup could pack 80 to 100 calories or more. Some products marketed as “bone broth” are essentially flavored water with minimal protein. Others are closer to a liquid meal. Reading the nutrition label matters. For fasting purposes, you want something on the lower end of the calorie and protein spectrum, ideally under 50 calories per serving with a few grams of protein.

Homemade broth is harder to standardize. Cooking time, the type and amount of bones used, whether you add vegetables or vinegar (which helps leach minerals from bones), and even the animal’s diet all affect the final nutritional profile. If you’re making your own, a lighter simmer of about 8 to 12 hours with a modest amount of bones will give you a less calorie-dense broth than a thick, gelatinous 24-hour simmer that wobbles like jello when refrigerated. That jiggly texture means high gelatin, which means more protein and more potential for an insulin response.

The Heavy Metal Question

A persistent concern about bone broth, whether during fasting or not, is lead contamination. Bones do accumulate heavy metals over an animal’s lifetime, and the long simmering process can leach those metals into the broth. A small controlled study found that organic chicken bone broth contained several times the lead concentration of the water used to make it, with levels reaching up to about 9.5 micrograms per liter for broth made from cooked skin and cartilage.⁹

That sounds alarming until you put the numbers in context. A separate analysis of various animal bone broths found that lead and cadmium levels were in the range of a few micrograms per serving, which the researchers concluded posed minimal risk.¹⁰ To put it plainly: the lead levels in bone broth are detectable but low enough that occasional consumption isn’t a meaningful health hazard for most adults. If you’re drinking multiple cups daily as a long-term habit, it’s worth being aware of, but for someone having a cup or two during a fasting window, the exposure is negligible. Using bones from pasture-raised animals and shorter cooking times can also help keep metal levels down.

Bone Broth for Breaking Extended Fasts

There’s a separate and often overlooked use for bone broth that Fung has discussed: using it to break a prolonged fast gently. After fasting for several days, the digestive system has been largely dormant. Jumping straight into a full meal can cause nausea, cramping, and digestive distress. In rare cases involving severely malnourished individuals, aggressive refeeding after prolonged calorie restriction can cause dangerous electrolyte shifts. While this extreme scenario applies mainly to hospitalized patients, the gentler principle holds for anyone ending a multi-day fast.

Bone broth serves as an ideal bridge food. It reintroduces calories and amino acids in a form that the gut can absorb without much mechanical digestion. The electrolytes help stabilize fluid balance. The warmth and saltiness satisfy the psychological craving for food without overwhelming a sluggish digestive system. Fung has recommended starting with bone broth for the first meal after an extended fast, then gradually introducing solid foods over the next few hours.

Hunger, Cravings, and Sticking With It

A clinical trial investigating a bone broth-based diet in adults with obesity found that participants experienced decreases in hunger and cravings over the study period, with no reduction in their satisfaction after eating.¹¹ While this study looked at a broader bone broth diet rather than bone broth during fasting specifically, the appetite-related findings align with the broader protein-satiety research and support the practical case for bone broth as a fasting tool.

The adherence angle is worth taking seriously. Fasting research consistently shows that the biggest predictor of whether someone gets metabolic benefits from fasting is whether they actually fast for the intended duration. Anything that makes fasting more tolerable without substantially undermining the metabolic state is a net positive. Bone broth fits that description for most people pursuing most fasting goals. It provides warmth, flavor, a sense of eating something, electrolyte support, and gut-protective amino acids, all for a trivial caloric cost.

What About Coffee, Tea, and Other “Gray Area” Drinks

Bone broth isn’t the only drink that occupies the fasting gray zone. Black coffee contains essentially zero calories but does provoke small metabolic effects. Tea is similar. Even sparkling water with a squeeze of lemon has been debated by fasting purists. Fung has generally grouped all of these as acceptable during fasting, drawing the line at anything containing sugar, significant calories, or artificial sweeteners (which may trigger an insulin response in some people despite containing no calories).

Where bone broth differs from coffee and tea is in its amino acid and mineral content. Coffee and tea are essentially calorie-free plant infusions with caffeine and polyphenols. Bone broth is an animal-derived liquid with measurable protein and micronutrients. This makes bone broth more nutritionally supportive but also more likely to produce a metabolic response. For a 16:8 intermittent fasting protocol where you’re fasting for 16 hours and eating in an 8-hour window, black coffee or tea during the fasting window is closer to “doing nothing” metabolically. Bone broth is doing something, but something quite small.

For longer fasts of 24 to 72 hours, Fung’s calculus shifts. The longer the fast, the more your body needs electrolyte support, and the more likely you are to quit from discomfort. In that context, bone broth’s slight metabolic cost becomes an even better trade-off. You’re already deep into ketosis and fat oxidation. A cup of broth isn’t going to reverse 30 hours of metabolic adaptation. But the sodium, potassium, and magnesium it provides might be the difference between completing the fast and breaking it early with a pizza.

Practical Guidelines for Using Bone Broth While Fasting

If you want to incorporate bone broth into a fasting routine in a way that preserves most of the fasting benefits, a few practical considerations help:

Keep servings small: One to two cups per fasting day, ideally under 50 calories per cup. This isn’t meant to be a meal replacement but a supportive sip.
Read labels carefully: Many commercial bone broths add sugar, starch, or cream that significantly increase the calorie and carbohydrate content. Choose plain broth with minimal ingredients.
Time it when you need it: If you notice a slump in energy or a spike in hunger during your fasting window, that’s a good moment for a cup. Some people find the late afternoon or early evening the hardest stretch.
For autophagy-focused fasts, skip it: If cellular cleanup is your specific objective, the amino acids in bone broth activate pathways that work against autophagy. Stick to water, black coffee, or plain tea.
For multi-day fasts, use it to refeed: When ending a fast of 48 hours or longer, start with bone broth before moving to solid food. Your gut will handle the transition better.

The fasting community tends to argue about bone broth with a certainty the science doesn’t support. There’s no study that definitively shows drinking a cup of bone broth at hour 18 of a fast erases all the metabolic benefits accumulated during the preceding hours. And there’s no study that proves it doesn’t affect them at all. What we do know is that the metabolic impact is small, the practical benefits for adherence are real, and the electrolyte and gut support serve a genuine physiological purpose. Fung’s recommendation sits comfortably within that evidence, even if it doesn’t satisfy the fasting perfectionists.