Vitamin B1 (thiamine) is not a proven weight loss supplement in humans. No clinical trials have shown that taking B1 leads to meaningful fat loss in people at a normal baseline. That said, B1 plays a real role in how your body converts food into energy, and being low in it can slow your metabolism and make exercise feel harder. The connection between B1 and weight is more nuanced than a simple yes or no.
What B1 Actually Does in Your Body
Thiamine is essential for turning the carbohydrates you eat into usable energy. It acts as a helper molecule for several key enzymes that sit at critical junctions in your metabolism. One of these enzymes converts pyruvate (the end product of breaking down glucose) into the fuel that enters your cells’ main energy-producing cycle. Another keeps that cycle running by processing intermediates along the way. Without enough B1, these steps stall, and your cells produce less energy from the same amount of food.
When B1 levels drop inside cells, enzyme activity slows, energy production decreases, and fatigue sets in. A study on exercise performance found that people deficient in B1 and other B vitamins saw their maximum oxygen uptake drop by about 11.6% and lactate buildup increase by 7% over 11 weeks. That translates to feeling exhausted sooner during workouts and recovering more slowly, both of which undermine any weight loss effort.
The Mouse Study That Got Attention
A 2025 study published in Frontiers in Nutrition found that mice fed a high-fat, high-fructose diet gained significantly less weight when given high-dose thiamine supplements. The effect kicked in after about four weeks and wasn’t due to eating less, since food intake was the same across groups. The supplemented mice also had smaller fat cells and less abdominal fat tissue. Researchers linked the effect partly to changes in gut bacteria: beneficial species like Bifidobacterium increased, and these bacteria were negatively correlated with body mass, cholesterol, and fasting blood sugar.
The catch is the dosage. The mice received 50 to 100 mg per kilogram of body weight daily, which the researchers noted is roughly 250 to 500 times the recommended dietary allowance for humans. That’s not a dose anyone would casually take. And mouse metabolism differs from human metabolism in ways that make direct translation unreliable. Promising, but far from proof that B1 supplements will help you lose weight.
B1 Deficiency Is Common in Obesity
Here’s where the story gets more relevant for many readers. Between 15.5% and 29% of people with obesity who seek bariatric surgery are thiamine deficient. That’s a striking number for a vitamin found in common foods. The reasons likely include a diet heavy in processed carbohydrates (which require B1 to metabolize but don’t contain much of it) and possibly impaired absorption.
If you’re carrying extra weight and feeling unusually fatigued, sluggish during exercise, or foggy, low B1 could be a contributing factor. Correcting a deficiency won’t melt fat on its own, but it can restore the metabolic machinery that makes calorie burning more efficient and exercise more tolerable.
B1 and Blood Sugar Stability
Blood sugar control is closely tied to weight management, and B1 appears to play a role here. A randomized, double-blind trial in people with high blood sugar found that high-dose thiamine supplementation significantly lowered two-hour glucose levels compared to baseline. In the placebo group, fasting glucose, insulin, and insulin resistance all worsened over the six-week study period. In the thiamine group, those markers held steady.
Stable blood sugar means fewer energy crashes and less of the hunger-insulin cycle that drives overeating. This indirect effect on appetite regulation may be one of the more practical ways B1 supports weight management, even if it doesn’t directly burn fat.
B1 and Exercise Performance
If you’re trying to lose weight through exercise, adequate B1 matters. Research on thiamine supplementation during exercise found that it reduced lactate (the compound that makes muscles burn during hard effort) and lowered ammonia levels, both markers of fatigue. Participants also reported lower perceived effort during workouts. The researchers concluded that thiamine intake during exercise provided benefits comparable to endurance training itself in terms of reducing fatigue markers.
This doesn’t mean B1 replaces exercise. It means that if you’re deficient, your workouts are harder than they need to be. Bringing your levels up to normal can help you train longer, recover faster, and stick with a routine.
How Much You Need and Where to Get It
The recommended daily intake is 1.2 mg for men and 1.1 mg for women. Most people can hit this through food, though the margin is thinner than you might expect. The richest common sources include:
- Fortified breakfast cereals: 1.2 mg per serving (100% of daily needs)
- Enriched egg noodles: 0.5 mg per cup cooked
- Pork chop: 0.4 mg per 3 ounces
- Trout: 0.4 mg per 3 ounces
- Black beans: 0.4 mg per half cup
- Mussels: 0.3 mg per 3 ounces
Notably, chicken, cheese, apples, and most dairy contain little to no thiamine. If your diet leans heavily on these foods while avoiding grains and legumes, you could fall short. Brown rice, whole wheat bread, and sunflower seeds each provide about 0.1 to 0.2 mg per serving, so they help but aren’t powerhouses on their own.
Benfotiamine: A More Absorbable Form
Standard thiamine is water-soluble, meaning it needs specific transport molecules to cross your intestinal wall. This creates a bottleneck: no matter how much you take, absorption is limited by how many transporters are available. Benfotiamine is a fat-soluble form that bypasses this limitation entirely, diffusing freely through intestinal cells and into the bloodstream. It produces at least five times higher blood levels of thiamine than an equivalent dose of regular B1.
Benfotiamine has been studied more for blood sugar complications than for weight loss specifically, but its superior absorption makes it worth knowing about if you suspect you’re deficient or if standard supplements haven’t made a noticeable difference in your energy levels.
The Bottom Line on B1 and Weight
B1 is not a fat burner. No human trial has demonstrated that taking thiamine supplements causes weight loss in people who aren’t deficient. What B1 does is keep your energy metabolism running properly, support stable blood sugar, and help your body handle exercise without premature fatigue. If you’re deficient, and a significant percentage of people with obesity are, correcting that deficiency removes a metabolic obstacle. It won’t replace a calorie deficit or an exercise habit, but it can make both of those things work the way they’re supposed to.