Intermittent Fasting

July 5, 2026 · John Bergeron, MD

Healthy meal beside a clock representing timed eating windows

Intermittent fasting is different from most diets in one basic way: it is about when you eat, not what you eat. Instead of restricting particular foods, you limit eating to a set window of time and fast the rest. It has been popular for years, and it comes up more than ever now, partly because people taking GLP-1 medications often find their appetite drops so much that they are already eating in a narrower window without trying.

That overlap is exactly why fasting is worth understanding clearly rather than just following a trend. Below is what intermittent fasting is, the common schedules, an honest look at whether it works, and how it fits, or does not fit, alongside GLP-1 therapy.

What is intermittent fasting?

Water, black coffee, and herbal tea for the fasting window

Intermittent fasting is a pattern of eating where you deliberately go without food for part of your day or week. In most versions there are no rules about which foods you eat, only about the hours during which you eat them. During the fasting window, water, black coffee, unsweetened tea, and other zero-calorie drinks are generally fine.

The idea is not new. Fasting has been part of human life for centuries, from religious observances like Ramadan and Lent to its early-1900s use in managing conditions like epilepsy. What is new is the recent interest in using timed eating specifically for weight loss.

The common intermittent fasting schedules

Protein-forward balanced meal to break a fast

There are several ways to structure a fast. These are the four you will hear about most.

16/8 (the Leangains protocol). You fast for 16 hours a day and eat within an 8-hour window, often something like noon to 8 p.m. It is usually considered the easiest to sustain, since you are mostly just skipping breakfast, and it is the version most people start with.

5:2. You eat normally five days a week and cut way back on the other two, typically to around 500 to 600 calories on each fasting day. The two low days should not be back to back.

Eat Stop Eat. A full 24-hour fast, from dinner one day to dinner the next, done no more than once or twice a week and never on consecutive days.

The Warrior Diet. The most demanding of the four, with a 20-hour fast and one large meal in a 4-hour evening window. The main risk here is overeating in that short window.

Whatever the schedule, going straight from a long fast into a large, unplanned meal tends to undo the point, so a planned, balanced meal to break the fast matters.

Does intermittent fasting work for weight loss?

It can, but not for the reasons it is sometimes sold on. Fasting mostly works by reducing the total calories you take in over the week and by extending the time your body spends between meals. There is no magic to it, and the popular idea that skipping meals throws your metabolism into “starvation mode” is overstated. Short-term fasting does not shut your metabolism down.

The honest picture is that fasting works well for some people and poorly for others. Some adjust easily. Others feel weak, foggy, or irritable on fasting days, or find they overeat once the window opens, which cancels out the deficit. If fasting leaves you bingeing later or unable to function, it is not the right tool for you, and that is not a failure of willpower, just a poor fit.

The possible benefits

Weight loss is the headline, but researchers are studying other effects too. The most supported so far is that intermittent fasting may improve insulin sensitivity and blood sugar. Some evidence also points to modest improvements in cholesterol.

It is worth being honest about the state of the science: much of the research is still early, and a lot of it has been done in animals rather than people. So the reasonable takeaway is that fasting may offer some metabolic benefits for some people, not that it is a proven treatment for any condition.

Is fasting safe?

For healthy adults, moderate intermittent fasting is generally considered safe, and no reputable schedule recommends fasting more than a day at a time. But it is not right for everyone, and some people should not fast without medical guidance, including those who are pregnant or nursing, many people taking diabetes medication, and anyone with a history of disordered eating. If you have any health condition, check with your doctor before starting.

Intermittent fasting and GLP-1 medications

This is where fasting needs the most care. GLP-1 medications, the class that includes semaglutide, tirzepatide, and liraglutide, already reduce hunger substantially. Adding a formal fasting schedule on top of that can push your intake too low, and the risks are real, which is why we treat this combination cautiously.

You may already be fasting without calling it that. Many people on a GLP-1 naturally eat within a shorter window because they are simply not hungry in the morning. That is usually fine on its own, but deliberately stacking a strict fast on top often is not necessary and can tip you into eating too little.

Protein and muscle come first. When appetite is already low, cutting your eating window further makes it easy to fall short on protein, which risks losing muscle along with fat. Getting adequate protein is more important than extending a fast, and it is one of the first things we monitor.

Blood sugar and hydration. Both fasting and GLP-1 medications lower blood sugar, and for anyone on diabetes medication that combination can go too low, so it needs monitoring. Fasting also makes it easy to under-hydrate when you are already eating and drinking less.

For most people on a GLP-1, a structured, protein-forward eating pattern is a safer and more effective choice than aggressive fasting. If timed eating appeals to you, the right move is to do it with your provider so your intake, protein, and any medications stay in a safe range.

Is intermittent fasting right for you?

Fasting can work for the right person, but it is demanding, it does not suit everyone, and it is easy to get wrong, especially alongside a medication. We tend to lean toward a structured, sustainable eating plan with adequate protein rather than prescribing fasting, though we recognize it helps some people. As with any approach, the eating pattern is only one piece. What decides whether the weight stays off is having medical support that adapts the plan to your body over time.

At Houston Weight Loss Center, we have practiced metabolic medicine since 1996, and today that means pairing sound nutrition with GLP-1 therapy and physician oversight, not one or the other. If you are curious about intermittent fasting, or how timed eating fits with a GLP-1 medication, talk with our team in Houston, Katy, and Webster about a plan that is both effective and safe.