Fasting and Cholesterol: Why Blood Lipids Sometimes Get Worse at First

Maybe you know this moment.

If you recognize yourself here, you are not alone. Hardly any lab value causes as much uncertainty during fasting. And hardly any is interpreted in such opposing ways.

I am writing this as a physician who values fasting and takes blood lipids seriously. The two go together. I am interested in four questions: What do studies show about fasting and cholesterol? Why can LDL rise at first when you lose weight? How do you read your blood lipids without looking at just a single line? And when does a value belong in a physician's hands?

By fasting, I mean intermittent fasting, longer fasting periods such as therapeutic fasting, and any phase in which you lose a significant amount of weight. You can read the basics of how it works in Intermittent fasting 16:8 and Buchinger therapeutic fasting.

Intermittent fasting and cholesterol: what studies in many people show

In short: On average, LDL, total cholesterol and triglycerides fall slightly with intermittent fasting. One meta-analysis found about 6 mg/dl less LDL. An overview of 57 meta-analyses saw the clearest effect on triglycerides, while the results for LDL were inconsistent. In a one-year study, LDL was even higher with alternate-day fasting than with daily calorie restriction.

Maybe one reason you started fasting is that you had read it would improve your blood lipids. And then your lab report points in the other direction. That feels like a small breach of trust.

Let's look at the big picture first. Studies with many people are like a map. They show the direction, but not exactly where you are standing right now.

A smaller study by the same research group painted a different picture. After ten weeks of alternate-day fasting, LDL fell by 18 to 25 percent in 35 people with obesity. On average, the LDL particles became somewhat larger, and the proportion of small particles decreased. Same form of fasting, different time frame, different result, though without a comparison group that did not fast.

With Ramadan fasting, too, the picture is more reassuring than dramatic. A team from Bahrain and the United Arab Emirates pooled 91 studies with 4,431 healthy adults. LDL fell slightly and HDL rose slightly, and both effects were small. You can read more about this in the article Fasting during Ramadan.

What does this mean for you? If you fast to improve your blood lipids, you need a baseline value before you start. Without it, you have nothing to compare against. And try not to measure in the middle of the phase when the kilos are dropping fastest. You will read why in the next section.

And now you know why the evidence on intermittent fasting and cholesterol is encouraging, but no promise for your own lab results.

Why LDL can rise temporarily during weight loss and fasting

In short: Your fat tissue stores not only fat but also cholesterol. When it is broken down, this cholesterol can travel through your blood for a while. In one study, total cholesterol rose above the starting value during major weight loss and fell again once the weight was maintained. After seven days without calories, LDL and ApoB rose by about two thirds in normal-weight adults.

Imagine you are finally clearing out the basement. For a few days, all the junk sits in the hallway. The apartment looks messier than before. And yet this is exactly where order begins.

Your fat tissue is a basement like that. It mainly stores triglycerides, meaning fat as an energy reserve. But it also stores cholesterol. As early as 1984, Krause and Hartman described fat tissue as an important cholesterol store in humans. In obesity, according to a joint expert review by two US professional societies, more than half of the body's free cholesterol can be found there.

Schematic illustration. That this pathway explains the rise is mechanistically plausible, but not fully proven in humans.

Why the results differ so much has not been conclusively explained. The form of fasting, starting weight, duration and timing of the measurement vary from study to study. What stands out: the people with the large rise were lean and took in no energy at all. We will come back to this point with the Lean Mass Hyper-Responder.

Another possible mechanism runs through the thyroid. During fasting, the active thyroid hormone T3 falls. In an older study, it dropped by 24 to 55 percent in normal-weight women and men after 60 hours without food. T3 keeps the LDL receptors in the liver active. These are docking sites where the liver fishes LDL out of the blood. When T3 is lacking, as in an underactive thyroid, LDL often rises, as a review from Athens and Buenos Aires describes. That this pathway also plays a role during fasting is biologically plausible, but not directly proven in humans.

What does this mean for you? For every lab report, make a note of the phase in which you were tested: before you started, in the middle of losing weight, right after a fasting program or at a stable weight. Only with this information can a value be put into context. You can read how to tell that your body has switched to burning fat in Recognizing and measuring ketosis during fasting.

And now you know why your LDL can rise precisely when your body is breaking down the most fat.

Where else the cholesterol goes: your bile

In short: Part of the mobilized cholesterol ends up in the bile. During rapid weight loss, bile can therefore be more saturated with cholesterol. Together with a sluggish gallbladder, this can promote gallstones. In studies, gallstones formed considerably more often on a very low-fat weight loss diet than on a diet with some fat.

Maybe you have felt a dull pressure on the right side below your rib cage after a fasting program. Often this is harmless. But sometimes the gallbladder is behind it.

Bile is an important route by which your body excretes cholesterol. If a lot of cholesterol is released during weight loss, part of it ends up there. At the same time, the gallbladder has little reason to contract during fasting. An important trigger for emptying it is fat in a meal. Without it, the bile sits like water in a still pond, where sediment settles more easily.

A meta-analysis from Homburg in Saarland pooled 13 randomized trials with 1,836 people who were losing weight. It describes that rapid weight loss also increases the risk of gallstones. Weight loss diets higher in fat were associated with considerably fewer gallstones than very low-fat diets. The medication ursodeoxycholic acid also lowered the risk. Whether it is an option for you is something you decide together with your physician.

What does this mean for you? If you are losing weight quickly or fasting for longer, watch for warning signs in the upper abdomen. Don't plan your meals after fasting to be entirely fat-free. You can read how to end a fasting phase gently in Breaking a Fast Properly. The risks of longer fasting programs are described in the article Water Fasting: Potential Benefits and Risks.

And now you know why rising cholesterol in your blood and a sluggish gallbladder can be two sides of the same coin.

Reading your blood lipids functionally: more than one line on the lab report

In short: LDL is a causal risk factor for vascular disease, and what matters is how high it is and for how long. ApoB counts the particles that can bring cholesterol into the vessel wall, and it is hardly affected by meals. The ratio of triglycerides to HDL can point to insulin resistance. Lp(a) is largely inherited and, according to European consensus, should be measured once in a lifetime.

A lab report with six or seven lines, and every eye goes to just one. That is understandable, because LDL is what gets talked about most. But a lipid profile is more of an orchestra than a solo.

Let's start with LDL, because clarity matters to me here.

Imagine the particles in your blood as trucks. LDL cholesterol measures how much cargo is on the road. ApoB counts the trucks, because each of them carries exactly one ApoB molecule. And it is the trucks that drive into the vessel wall, not the cargo alone.

A team led by Sniderman at McGill University in Montreal therefore describes ApoB as a more accurate measure of the risk from these particles than LDL cholesterol. According to a European consensus paper, meals practically do not change ApoB. For labs, this paper recommends flagging ApoB of 1.0 g/l and above as abnormal.

Triglycerides and HDL tell a different story, the one about sugar and insulin metabolism.

With overweight, a typical pattern often appears: high triglycerides, low HDL, elevated ApoB and many small, dense LDL particles. This is how the joint expert review from the US describes it. In many fasting studies, it was above all the triglycerides that fell. Your profile as a whole can therefore improve, even if LDL rises temporarily. You can read how fasting and insulin are connected in Fasting, blood sugar and insulin resistance.

Flagging cutoffs based on the 2016 joint consensus paper of the European Atherosclerosis Society and the EFLM. They are not personal target values. Your target values depend on your overall risk.

What does this mean for you? At your next blood test, ask whether ApoB can be measured in addition to the standard profile. Have Lp(a) measured once if that has never been done. And calculate the ratio of triglycerides to HDL yourself. Since the thyroid can also affect blood lipids, it is worth taking a look at it if there is any suspicion. You can read more about this in Fasting with Hashimoto's.

And now you know why a lab report with rising LDL can still look more favorable overall, and why LDL should not slip out of view in the process.

The right timing: when a blood test is meaningful

In short: As a rule, you do not need to fast for a lipid profile, because normal meals change the values only a little. Longer fasting, on the other hand, can shift the values markedly. In one study, even a single day of water only raised total cholesterol. The most meaningful comparison is at a stable weight and under conditions similar to those of the baseline value.

Maybe you went to your blood test being especially conscientious. Nothing to eat for a day and a half, extra fasted. And that, of all things, can distort the result.

Orientation from a physician's perspective, derived from the studies mentioned. A fixed time frame cannot be derived from the data.

What does this mean for you? Plan your follow-up test deliberately. Don't measure right after a fasting day, and ideally not during the phase of fastest weight loss. If a value is very high, however, don't wait for months, but have a physician interpret it promptly.

And now you know why the timing of your blood test helps determine how meaningful your lab results are.

Lean Mass Hyper-Responder: when lean people develop very high LDL levels on low carb

In short: In some lean, metabolically healthy people, LDL rises sharply on a very low-carbohydrate diet, alongside high HDL and low triglycerides. In one survey, the lower the BMI, the larger the rise. Whether this pattern raises the risk to the blood vessels less is unclear. A follow-up study on this was retracted in 2026, and experts advise against regarding such values as harmless.

You are lean, have been eating very low carb for months and feel good. Your triglycerides are low, your HDL is high, your blood sugar is steady. And then an LDL that is higher than ever. How does that fit together?

The researchers explain this with a hypothesis, the so-called lipid energy model. Someone who has little body fat and eats hardly any carbohydrates might send more fat through the body as fuel via lipoproteins. A small crossover study with ten lean, healthy women fits this: body composition and the thyroid hormone T3 were linked to the change in LDL, whereas the amount of saturated fat was not. This is biologically plausible, but not yet proven.

What does this mean for you? If you are lean, eat a very low-carbohydrate diet and your LDL has risen sharply, talk to your physician about three things. First: measuring ApoB and Lp(a) and considering familial hypercholesterolemia. Second: whether adjusting your diet is an option, for example more carbohydrates, because in case reports LDL fell markedly as a result. Third: whether, in your situation, imaging of the coronary arteries could help assess your risk better. You can read more about ketosis in Recognizing and measuring ketosis during fasting.

And now you know why a lean, metabolically healthy body with very high LDL is a fascinating research question, but not an all-clear.

When you should have your LDL checked by a physician

In short: An LDL above 190 mg/dl, early heart attacks or strokes in the family, a high Lp(a), existing vascular disease or diabetes are reasons for a prompt medical evaluation, regardless of fasting. About one in 313 people has familial hypercholesterolemia. If LDL stays high after your weight has stabilized, it also needs to be assessed by a physician.

Maybe you are now asking yourself: how do I know whether my value is just a snapshot or a signal I should not ignore? That is a fair question. And it deserves an honest answer.

Some people have a very high LDL from birth. In familial hypercholesterolemia, the uptake of LDL into the liver is genetically impaired, often because the LDL receptors work less well. LDL is then elevated for a lifetime, and over the years the burden on the blood vessels adds up.

The European consensus paper on lipid testing recommends a separate evaluation for an LDL above 5 mmol/l, that is, above 190 mg/dl. Triglycerides above 10 mmol/l, that is, above 880 mg/dl, are also considered a reason for a separate referral there, because of the risk of pancreatitis. According to this paper, an Lp(a) above 150 mg/dl indicates a very high risk.

Other causes can also raise LDL, such as an underactive thyroid. According to a review, even subclinical hypothyroidism can affect blood lipids unfavorably. When a mild underactive thyroid should be treated is described in the article Subclinical hypothyroidism: treat or wait?

Orientation from a physician's perspective, not a diagnosis. Both columns can apply at the same time. In that case, the right-hand one applies.

What does this mean for you? If any of the points in the right-hand column apply to you, don't wait for your next fasting program. Have a physician assess your lab results. Bring your older values, your family history and information about your diet.

And now you know why the most important question is not whether your LDL rises during fasting, but whether it falls again afterwards and what else your profile shows.

Blood lipids through the KPNI lens: four systems in conversation

Back to your lab report with the small arrow pointing up. Why does one body respond to fasting with falling blood lipids and another with rising ones? This question leads straight into Clinical Psychoneuroimmunology, KPNI for short.

KPNI does not look at the nervous system, immune system, metabolism and hormonal system separately, but as one conversation. When it comes to blood lipids, all four have a say.

You can read how menopause can affect fasting in Fasting during menopause.

I see a fifth level, too: your attitude. In Nonviolent Communication, Marshall Rosenberg coined a principle that fits well here: observing is something different from evaluating. “My LDL went up from one lab report to the next” is an observation. “I'm doing everything wrong” is an evaluation. Behind the observation there is often a need, for safety and for a clear next step.

And now you know why the same LDL value can read like a transitional stage or like a warning signal, depending on the context.

Three levers if your blood lipids rise during fasting

You don't need to rush anything. These three directions can give you orientation. They do not replace medical advice.

If you would rather not interpret your values on your own: below this article you will find the option to book an appointment.

And now you know why a rising value during fasting is not a stop sign, but an invitation to take a closer look.

Frequently asked questions about fasting and cholesterol

Where this topic touches other areas

Fasting and blood lipids touch on insulin, the thyroid, hormones, the liver and exercise. If you want to go deeper, these neighboring topics from other guides are worth a look.