Fish oil tablets do not directly cause weight gain, but they contain calories that count toward your daily total
Fish oil tablets are calorie-dense. A standard 1,000 mg softgel contains roughly 9 calories, and most people take one to three per day. If you add three tablets daily without cutting calories elsewhere, you are adding about 27 calories to your diet. Over a year, that is roughly 10,000 extra calories — enough to gain three pounds if nothing else changes.
The real issue is not the fish oil itself but whether you account for those calories. Fish oil does not speed up metabolism or trigger weight gain the way some supplements claim to do. It straightforward adds to your calorie count. If you are already eating at maintenance or above, adding fish oil without adjustment will shift the balance toward weight gain.
Some people report bloating or water retention when they start fish oil, which can show on the scale within days. This is usually temporary and related to digestive adjustment, not fat gain. It typically resolves within one to two weeks as your body adapts.
Key Takeaways
- A typical fish oil tablet contains 9 calories, so three daily tablets add about 27 calories to your diet.
- Weight gain happens only if those calories push you above your daily total without offsetting cuts elsewhere.
- Temporary bloating or water retention can appear on the scale in the first week or two, but this is not fat gain.
- Fish oil does not speed metabolism or cause weight gain on its own — the calories are what matter.
- If you are concerned about the calorie load, fish oil capsules with lower oil content or liquid fish oil measured by the teaspoon may offer more control.
How many calories are actually in fish oil tablets
The calorie content depends on the size and type of tablet. A 1,000 mg softgel typically holds about 9 calories. A 500 mg tablet holds roughly 4 to 5 calories. Some brands pack more oil into the same capsule size and deliver 10 to 12 calories per tablet.
Check the nutrition label on your bottle. Most list calories per serving, and one serving is usually one or two tablets. If the label does not show calories, the amount is so small the manufacturer is not required to list it — but it is still there. Liquid fish oil is measured differently: a teaspoon usually contains 40 to 50 calories, so it is a heavier calorie load per dose than tablets.
The oil itself is what carries the calories. Fish oil is fat, and fat contains 9 calories per gram. A 1,000 mg tablet does not hold a full gram of pure oil — the rest is the softgel capsule and any filler — but the oil portion is what adds up.
When bloating happens and why it feels like weight gain
Many people report bloating, gas, or a heavier feeling in the stomach within the first few days of starting fish oil. This is not fat gain. It is your digestive system responding to a new substance. Fish oil can slow gastric emptying — the rate at which food leaves your stomach — which creates a sensation of fullness and can cause temporary water retention in the gut.
This bloating usually peaks around day three to day five and fades by day ten to day fourteen as your body adjusts. If you weigh yourself during this window, the scale may show a gain of one to three pounds. Once the bloating resolves, that weight drops back off. Taking fish oil with food rather than on an empty stomach can reduce this effect.
If bloating persists beyond two weeks, it may signal that fish oil is not sitting well with your digestive system. Some people have ongoing sensitivity to fish oil, and switching to a different brand, a lower dose, or a plant-based omega-3 source may help.
What happens if you do not account for the extra calories
If your diet is already at maintenance — the calorie level where you neither gain nor lose weight — adding fish oil without cutting anything else will tip the balance toward weight gain. The math is straightforward: three tablets daily at 9 calories each equals 27 calories per day, or 189 calories per week, or roughly 9,800 calories per year. At 3,500 calories per pound of body fat, that is close to three pounds of potential gain annually.
This is not unique to fish oil. Any food or supplement you add without adjustment will have the same effect. The difference is that fish oil is straightforward to overlook because the tablets are small and the calories are not obvious. A person might remember that they ate an extra snack but forget they started taking three fish oil tablets daily.
If you are trying to lose weight or maintain your current weight, the solution is straightforward: either count the fish oil calories as part of your daily total, or reduce calories elsewhere by the same amount. If you are not tracking calories at all, fish oil is unlikely to be the reason for weight gain — but it is not helping either.
How to take fish oil without the calorie concern
If you want the omega-3 benefits of fish oil without adding calories, you have a few options. The most direct is to reduce calories elsewhere in your diet by the amount fish oil adds. If you take three tablets daily at 27 calories, cut 27 calories from your food intake — skip a small piece of bread, reduce oil in cooking by half a teaspoon, or drink one fewer ounce of juice.
You can also choose a lower-dose tablet. A 500 mg tablet contains roughly half the calories of a 1,000 mg tablet. If you take two 500 mg tablets instead of one 1,000 mg tablet, you are cutting the calorie load in half while still getting omega-3s, though you may need to take more tablets to reach the same total dose.
Another option is to get omega-3s from food instead of supplements. Fatty fish like salmon, mackerel, and sardines contain omega-3s and are part of a meal, so the calories are already part of your eating plan. Plant-based sources like flaxseeds, chia seeds, and walnuts also provide omega-3s, though in a different form that your body converts less efficiently than fish oil.
Fish oil and metabolism: what the research shows
Fish oil does not speed up your metabolism or burn extra calories. Some older studies suggested omega-3 fatty acids might slightly increase fat burning, but the effect is small and inconsistent. A person taking fish oil does not burn significantly more calories at rest than someone who does not.
What fish oil may do is reduce appetite slightly in some people, which could lead to eating fewer calories overall. This is not a may provide effect — some people notice no change in hunger — but if it happens, it works in the opposite direction of weight gain. The calories in the fish oil itself still count, though, so any appetite reduction would need to offset the tablet calories to prevent weight gain.
Fish oil also does not cause your body to store fat differently or preferentially. Weight gain from fish oil, like weight gain from any source, comes from consuming more calories than you burn. There is no special mechanism that makes fish oil calories worse than calories from other sources.
Frequently Asked Questions
Can fish oil tablets cause water weight gain?
Yes, temporarily. Fish oil can slow stomach emptying and cause bloating in the first one to two weeks, which shows on the scale as water weight and digestive content, not fat. This usually resolves as your body adjusts. Taking fish oil with food can reduce this effect.
Do I need to cut calories if I start taking fish oil?
Only if you are trying to maintain your current weight or lose weight. Three tablets daily add about 27 calories. If your diet is already at the calorie level you want, you will need to cut 27 calories elsewhere or accept a small gain. If you are not tracking calories, the effect will be minor.
Is liquid fish oil worse for weight than tablets?
Liquid fish oil contains more calories per dose — roughly 40 to 50 per teaspoon versus 9 per tablet — so it adds more to your daily total. Tablets are easier to control if calories are a concern. However, some people find liquid easier to digest and less likely to cause bloating.
Will fish oil make me gain weight if I exercise?
No more than it would without exercise. The calories still count the same way. If you exercise and eat at maintenance, adding fish oil without cutting calories elsewhere will still shift you toward weight gain. Exercise does not exempt you from calorie balance.
What if I stop taking fish oil — will I lose the weight?
If the weight gain was from the extra calories, stopping fish oil will allow you to return to your previous weight, assuming you do not replace those calories with something else. If the gain was bloating, it will resolve within days. If you gained fat, you will need to cut calories overall to lose it.