SlimFast: one trial under its own name, and a category strongest at three months

The only verdict published on SlimFast by name comes from a 2015 review of 45 studies of 11 commercial weight-loss programs, 39 of them randomized trials, and its word for SlimFast is mixed, which means the trials disagreed with each other. In that same review Weight Watchers came out at least 2.6 percent better at 12 months than control or education and Jenny Craig at least 4.9 percent, so a verdict was possible for other programs and not for this one. One randomized trial names the product. In 113 obese adolescents, mean age 15.0 years and mean BMI 37.1, three SlimFast shakes a day inside a 1300 to 1500 calories plan cut BMI by 6.3 percent at month 4. On ordinary food the reduction was 3.8 percent (P = 0.01), an effect size of 0.52. By month 12 the two groups were level (P = 0.39), at a mean reduction of 3.4 percent across both. The shakes in that trial were supplied free by the manufacturer, retention had fallen to 67 percent by month 12, and 81 percent of the participants were girls, so the sample is narrow as well as young. Nothing in it describes an adult buying shakes in a supermarket. Everything else strong on this page is about meal replacement as a category rather than about this brand. Pooling 6 randomized trials in overweight and obese adults on 800 to 1600 calories a day, replacing one or two meals with a fortified liquid product gave 5.6 lb (2.54 kg) more weight loss at 3 months (P < 0.01). At one year the difference was 5.4 lb (2.43 kg) and did not reach significance (P = 0.14), which is a different statement from the product ceasing to work. In 10 randomized trials with 1254 adults who had pre-diabetes or features of metabolic syndrome, meal replacements beat food-based diets by 3 lb (95% CI -4 to -2.1; metric: 1.38 kg, -1.81 to -0.95) and by 2.22 mmHg of systolic pressure (95% CI -4.20 to -0.23). The authors rated that certainty low to moderate on GRADE. The 3 lb (1.38 kg) is the size of the advantage over another diet and not the size of the weight loss. Replacing the whole diet with shakes is a separate product in EU law, with a floor of 600 calories a day. In one cohort a 500 calories a day diet carried 3.4 times the risk of gallstones needing hospital care compared with 1,200 to 1,500 calories a day.

How much is actually in it

• When 9 very low energy diets were checked against nutrition benchmarks, none met all of them, and the formula shake products missed adequate fiber intake just as the food-based plans did. — 9 balanced food-based very low energy diets under 800 calories a day, compared with formula meal replacement products and with Codex standards and Australian requirements for adults 19 to 50 years (n = 9 diets) Evidence C source
  None of the diets met all nutritional benchmarks; all including the formula comparator failed adequate intake for dietary fiber except one; one food-based diet was adequate for everything except thiamine, magnesium and zinc in men and iron in women

What your body absorbs

• A nutritionally complete liquid meal left the stomach in a median of 180 minutes in 25 healthy adults, against 150 minutes for a plain glucose drink of the same energy, so a shake is handled as food and not as a drink. — 25 healthy adults, double-blind crossover randomized study, three sessions with a 75 g glucose solution, a fat-free liquid meal and a nutritionally complete liquid meal of equal volume and energy (n = 25) Evidence B source
  Median gastric emptying time 150 min for the glucose solution and for the fat-free high-calorie drink, 180 min for the nutritionally complete liquid meal; emptying complete in everyone within 6 hours

What it does to your health

• Pooling 6 randomized trials of replacing one or two meals a day with a fortified liquid product, the extra weight loss was 5.6 lb (2.54 kg) at 3 months and 5.4 lb (2.43 kg) at one year, and only the 3-month figure reached significance. — overweight and obese adults aged 18 or older with BMI 25 or above, 6 randomized trials of at least 3 months on 800 to 1600 calories a day (n = 6 trials) Evidence A source
  5.6 lb (2.54 kg) greater loss at 3 months (P < 0.01) and 5.4 lb (2.43 kg) at 1 year (P = 0.14) in the partial meal replacement group; the plan group lost about 7 to 8% of body weight against about 3 to 7% on a conventional reduced-calorie diet
• In 10 randomized trials with 1254 people who had pre-diabetes or metabolic syndrome, meal replacements beat food-based diets by 3 lb (1.38 kg) of weight and 2.22 mmHg of systolic blood pressure, with certainty rated low to moderate. — 1254 adults with pre-diabetes and features of metabolic syndrome in 10 randomized controlled trials (n = 1254) Evidence A source
  Body weight -3 lb (95% CI -4 to -2.1; metric: -1.38 kg, -1.81 to -0.95), BMI -0.56 kg/m2, waist -0.46 in (-1.17 cm), HbA1c -0.11%, LDL-c -7 mg/dL (-0.18 mmol/L), non-HDL-c -6.6 mg/dL (-0.17 mmol/L), systolic blood pressure -2.22 mmHg (95% CI -4.20 to -0.23); GRADE certainty low to moderate

Safety, limits and interactions

• Under EU rules a meal replacement may carry a weight claim only if each meal gives 200 to 250 calories. The maintenance claim needs one main meal replaced a day and the weight loss claim two, and the label must say other foods are a necessary part of the diet. — meal replacement products bearing the EU weight control claims Evidence E source
  200 to 250 calories per meal; one main meal replaced daily for the maintenance claim, two for the weight loss claim
• EU law requires a product that replaces the whole daily diet for weight control to give at least 600 and at most 1,200 calories a day. It may be called a very low calorie diet only below 800 calories, and may not promise any rate or amount of weight loss. — total diet replacement for weight control products sold in the EU Evidence E source
  600 to 1,200 calories for the whole daily ration; 'very low calorie diet' only below 800 calories a day; no claims about rate or amount of weight loss
• In 6,640 adults on a commercial program, a 500 calories a day very low calorie diet carried 3.4 times the risk of gallstones needing hospital care compared with 1,200 to 1,500 calories a day over one year. One extra case occurred for every 92 people on the stricter diet. — adults in a Swedish commercial weight loss program, VLCD 500 calories a day or LCD 1200 to 1500 calories a day for 3 months, matched 3320:3320 (n = 6640) Evidence C source
  Gallstones needing hospital care 152 vs 44 per 10,000 person-years; HR 3.4 (95% CI 1.8 to 6.3); number needed to harm 92
• A meta-analysis of randomized trials found that 19% of people on very low energy diets stopped early, against 20% on comparators. One serious adverse event, a hospital stay for gallbladder inflammation, was reported in the diet groups. — overweight adults in randomized trials of very low energy diets with a behavioral program Evidence A source
  Early discontinuation 19% vs 20% (RR 0.96, 0.56 to 1.66); one serious adverse event, hospitalization with cholecystitis, in the VLED groups
• An NIH task force concluded in 1993 that very low calorie diets of 800 calories a day or less are generally safe under proper medical supervision in people with a BMI above 30. Gallstones were the most common serious complication. — adults with obesity, BMI above 30, on diets of 800 calories a day or less Evidence E source
  Gallstones the most common serious complication; VLCDs generally safe under proper medical supervision
• Across 17 randomized trials in 2112 people with type 2 diabetes, both partial and total meal replacement were well tolerated, which is the strongest safety statement the evidence supports. — 2112 participants with type 2 diabetes in 17 randomized controlled trials of 2 weeks or longer (n = 2112) Evidence A source
  Both partial and total meal replacement well tolerated; no separate rate of adverse events reported in the abstract

What people believe that the data does not support

• A review of 45 studies of commercial weight-loss programs, 39 of them randomized trials, found the results for SlimFast mixed, while Weight Watchers and Jenny Craig had consistent 12-month advantages. — overweight and obese adults in 45 studies of 11 commercial programs, trials of at least 12 weeks (n = 45 studies) Evidence A source
  Weight Watchers at least 2.6% greater weight loss at 12 months than control or education, Jenny Craig at least 4.9%, Nutrisystem at least 3.8% at 3 months; results for SlimFast mixed
• The European Union allows a label to say that substituting two daily meals of an energy restricted diet with meal replacements contributes to weight loss, while the United States has authorized no such claim at all. — food labels in the European Union; claim POL-HC-6416, snapshot of the register taken 2026-09-21 Evidence E source
  Authorized claim for substituting two daily meals; a second authorized claim POL-HC-6415 covers substituting one daily meal and speaks of maintenance of weight after weight loss

Who this works differently for

• EU law makes whole-diet replacement products for weight control state that they are only for healthy overweight or obese adults. The label must say they should not be used by pregnant or breastfeeding women, adolescents or people with a medical condition without a health professional's advice. — pregnant or lactating women, adolescents, people with a medical condition Evidence E source
  Mandatory label: only for healthy overweight or obese adults; not for pregnant or lactating women, adolescents or people with a medical condition without a health professional's advice
• In 113 obese adolescents, three SlimFast shakes a day as part of a 1300 to 1500 calories plan cut BMI by 6.3% in 4 months against 3.8% on ordinary food, and by month 12 the two groups were level. — 113 obese adolescents, mean age 15.0 years and BMI 37.1 kg/m2, 12-month trial; 65 on meal replacement and 37 on conventional diet at month 4 (n = 113) Evidence B source
  BMI reduction at month 4: 6.3 +/- 0.6% with meal replacement against 3.8 +/- 0.8% with conventional diet (P = 0.01), effect size 0.52; no significant difference between groups at month 12 (P = 0.39); mean BMI reduction across groups at month 12 was 3.4 +/- 0.7%
• In 17 randomized trials with 2112 people who had type 2 diabetes, meal replacement lowered HbA1c by 0.46% and weight by 5.4 lb (2.43 kg) against a conventional diabetic diet, and replacing every meal worked better than replacing some. — 2112 participants with type 2 diabetes in 17 randomized controlled trials of 2 weeks or longer, with and without insulin treatment (n = 2112) Evidence A source
  HbA1c -0.46% (P < 0.001), fasting glucose -11 mg/dL (-0.62 mmol/L), body weight -5.4 lb (-2.43 kg), BMI -0.65; total replacement against partial: HbA1c -0.72% against -0.32% (P = 0.01) and weight -14 lb (-6.57 kg) against -3.5 lb (-1.58 kg) (P < 0.001)

What is still unknown

• In a meta-analysis of 24 studies in children and teenagers with obesity, very low energy diets cut weight by 22 lb (10.1 kg) on average over 3 to 20 weeks. The authors say safety cannot be judged because adverse events were poorly reported. — children and adolescents aged 5 to 18 with obesity on very low energy diets of 800 calories a day or less (n = 24 studies) Evidence C source
  Mean weight loss 22 lb (95% CI 19 to 25; metric: 10.1 kg, 8.7 to 11.4) after 3 to 20 weeks; adverse event reporting too limited to judge safety
• The 2015 review that covered 45 studies of commercial programs found too little evidence to judge how well people stick with them or what harms they cause, and no trial since has fixed that for SlimFast. — 11 commercial weight-loss programs with eligible trials, including SlimFast (n = 45 studies) Evidence A source
  Limited evidence on adherence and harms for every program reviewed; the corpus holds no randomized trial of SlimFast after the 2011 adolescent trial

The bottom line

The composition table on this page holds a single USDA reference entry, for one ready-to-drink high protein shake on the 3-2-1 plan, and it is a legacy record rather than a measured one, so we build no numbers on it. There is no entry in the USDA database used for dietary studies either, so no household portion with a weight exists in our data, and what counts as a serving comes off the pack rather than out of a reference table. One nutrient gap in this kind of product is structural. When 9 very low energy diets under 800 calories a day were checked against Codex standards and Australian requirements for adults 19 to 50, none met every benchmark, and all of them including the formula product missed the adequate intake for fiber except one. The formula comparator there was Optifast rather than SlimFast, and 800 calories a day is stricter than a plan in which you still eat one real meal. The mechanism carries over regardless, because a fortified shake fixes vitamins and minerals by design and does not fix fiber. A shake is handled by the body as food. In 25 healthy adults given three drinks of equal volume and energy in a randomized double-blind crossover, a nutritionally complete liquid meal took a median 180 minutes to leave the stomach. A 75 g glucose solution and a fat-free high-calorie drink both took 150 minutes, and everyone had emptied within 6 hours. Those were drinks made for surgical patients rather than SlimFast, and the study measured how long the liquid sat in the stomach and not the absorption of any nutrient. The largest evidence base sits in diabetes, where meal replacement is a clinic tool. Across 17 randomized trials with 2112 people who had type 2 diabetes, it lowered HbA1c by 0.46 percent and weight by 5.4 lb (2.43 kg) against a conventional diabetic diet. Replacing every meal did more than replacing some, at 0.72 against 0.32 percent of HbA1c (P = 0.01) and 6.57 against 3.5 lb (1.58 kg) of weight (P < 0.001). No authorized label wording goes that far: the two claims on the register cover substituting one or two daily meals, not every meal of the day. That last comparison is between subgroups inside a meta-analysis, so it can reflect who was prescribed what. These trials ran in clinics with supervision, which makes them a description of a managed program rather than of a purchase. Replacing every meal is also a different product in law. In the EU a total diet replacement must give at least 600 and at most 1,200 calories a day, may be called a very low calorie diet only below 800 calories, and may not promise any rate or amount of weight loss. The harm that has been counted at the strict end is gallstones. In 6,640 adults on a Swedish commercial program, a 500 calories a day diet carried 3.4 times the risk of gallstones needing hospital care compared with 1,200 to 1,500 calories a day over one year, one extra case for every 92 people. Both groups used meal replacement products, and 500 calories is below the EU floor. An NIH task force in 1993 called diets of 800 calories a day or less generally safe under proper medical supervision in people with a BMI above 30, and named gallstones as the most common serious complication. In a meta-analysis of randomized trials of very low energy diets, 19 percent stopped early against 20 percent on comparators, and one serious adverse event was reported, a hospital stay for gallbladder inflammation. Outside that strict end the honest ceiling is low. The 17 diabetes trials report that both partial and total meal replacement were well tolerated, which is the authors’ wording and not a count of adverse events, and trials of this kind exclude people with complications. We found nothing about medicines and no report of contamination or allergy under the brand name. For pregnancy and breastfeeding we have no study, only a rule. EU law makes a product that replaces the whole daily diet state that it is only for healthy overweight or obese adults. Its label must also say that it should not be used by pregnant or breastfeeding women, adolescents or people with a medical condition without a health professional’s advice. That rule binds whole-diet products and not a single shake inside ordinary meals, and it is a precaution rather than a measured harm. The one finding we have about a specific group is the adolescent trial above, and it is a caution as much as a result. Three shakes a day in 15-year-olds happened inside a 12-month study with a prescribed 1300 to 1500 calories plan and shakes supplied free by the manufacturer, and our data says nothing about what happens when that is done without it. The only pooled data on teenagers is about very low energy diets of 800 calories a day or less in children and adolescents with obesity. Across 24 studies, mostly without a control group and run as clinical programs, weight fell by 22 lb (10.1 kg) over 3 to 20 weeks, and the authors say safety cannot be judged because adverse events were poorly reported. Regulators disagree about the whole category. The European Union authorizes a label saying that substituting two daily meals of an energy restricted diet with meal replacements contributes to weight loss, and a second authorized claim covers substituting one daily meal for keeping weight off after losing it. The United States has authorized no such claim at all. An authorization records that the regulator judged the evidence sufficient at the time of assessment, and it carries no number about size. To use either wording, each replacement meal must give 200 to 250 calories, and the label must say that other foods are a necessary part of the diet. The gap is wider than the findings. That 2015 review found too little evidence to judge adherence or harms for every program it examined, including this one, and no randomized trial of SlimFast has appeared since the adolescent trial of 2011. What the evidence supports is narrow. Replacing meals with a fortified shake produces a few kilograms more loss than a food diet while you keep doing it, clearest at 3 months, thinner and unproven at a year, and measured mostly in people who had pre-diabetes or diabetes. Whether this brand does it better or worse than any other has not been measured.

Sources

1. sfr5-07 · systematic review with nutritional analysis
   None of the VLEDs met all nutritional benchmarks...All VLEDs, including Optifast®, did not meet AI for dietary fiber, except one.
   Nutrients, 2024 · checked 2026-09-24
2. sfr5-08 · randomized crossover trial
   The glucose solution and the standard fat-free high-calorie drink with the same caloric content had a median gastric emptying time of 150 min. The nutritionally complete liquid meal had a median gastric emptying time of 180 min.
   Clin Nutr ESPEN, 2024 · checked 2026-09-24
3. sfr5-03 · meta-analysis of randomized controlled trials
   A random effects meta-analysis estimate indicated a 2.54 kg (P<0.01) and 2.43 kg (P=0.14) greater weight loss in the PMR group for the 3-month and 1-y periods, respectively.
   Int J Obes Relat Metab Disord, 2003 · checked 2026-09-24
4. sfr5-04 · meta-analysis of randomized controlled trials
   Ten trials (n = 1254) met the eligibility criteria. MRs led to greater reductions in body weight (-1.38 kg [-1.81, -0.95])
   Obes Rev, 2024 · checked 2026-09-24
5. hf2-sfr-1 · EU regulation
   The energy content shall not be less than 200 kcal (840 kJ) and shall not exceed 250 kcal (1 046 KJ) per meal ... In order to bear the claim, information shall be provided to the consumer on the importance of maintaining an adequate daily fluid intake and on the fact that the products are useful for the intended use only as part of an energy-restricted diet and that other foodstuffs should be a necessary part of such diet. In order to achieve the claimed effect, one main meal should be substituted with one meal replacement daily.
   Commission Regulation (EU) 2016/1413 of 24 August 2016 amending Regulation (EU) No 432/2012, meal replacement for weight control, OJ L 230/8 · checked 2026-10-08
6. hf2-sfr-2 · EU regulation
   The energy provided by total diet replacement for weight control products shall not be less than 2 510 kJ (600 kcal) and shall not exceed 5 020 kJ (1 200 kcal) for the total daily ration. ... The statement ‘very low calorie diet’ may be used for total diet replacement for weight control products provided that the energy content of the product is below 3 360 kJ/day (800 kcal/day). ... The labelling, presentation and advertising of total diet replacement for weight control products shall not make any reference to the rate or amount of weight reduction which may result from its use.
   Commission Delegated Regulation (EU) 2017/1798 of 2 June 2017, total diet replacement for weight control, OJ L 259/2 · checked 2026-10-08
7. hf2-sfr-4 · matched cohort study
   During 6361 person-years, 48 and 14 gallstones requiring hospital care occurred in the VLCD and LCD groups, respectively, (152 versus 44/10 000 person-years; hazard ratio, 3.4, 95% CI 1.8-6.3; P<0.001; number-needed-to-harm, 92, 95% CI 63-168; P<0.001).
   Int J Obes (Lond), 2014 · checked 2026-10-08
8. hf2-sfr-5 · meta-analysis of randomized trials
   Nineteen per cent of the VLED group discontinued treatment prematurely compared with 20% of the comparator groups, relative risk 0.96 (0.56 to 1.66). One serious adverse event, hospitalization with cholecystitis, was reported in the VLED group and none in the comparator group.
   Obes Rev, 2016 · checked 2026-10-08
9. hf2-sfr-7 · expert panel guideline
   Serious complications of modern VLCDs are unusual, cholelithiasis being most common. CONCLUSIONS: Current VLCDs are generally safe when used under proper medical supervision in moderately and severely obese patients (body mass index [weight in kilograms divided by height in meters squared] > 30)
   JAMA, 1993 · checked 2026-10-08
10. sfr5-06 · meta-analysis of randomized controlled trials
    Both partial and total MR were well tolerated.
    J Clin Endocrinol Metab, 2023 · checked 2026-09-24
11. sfr5-02 · systematic review of randomized controlled trials
    We included 45 studies, 39 of which were RCTs...Results for SlimFast were mixed.
    Ann Intern Med, 2015 · checked 2026-09-24
12. sfr5-09 · EU health claim register entry
    POL-HC-6416 Meal replacement for weight control Substituting two daily meals of an energy restricted diet with meal replacements contributes to weight loss Authorised
    EU Register on nutrition and health claims, claim POL-HC-6416, snapshot 2026-09-21 · checked 2026-09-24
13. hf2-sfr-3 · EU regulation
    (a) a statement that the product is only intended for healthy overweight or obese adults who intend to achieve weight reduction; (b) a statement that the product should be not used by pregnant or lactating women, adolescents or by individuals suffering from a medical condition without the advice of a healthcare professional;
    Commission Delegated Regulation (EU) 2017/1798 of 2 June 2017, total diet replacement for weight control, OJ L 259/2 · checked 2026-10-08
14. sfr5-01 · randomized controlled trial
    At month 4, participants assigned to MR (N = 65) achieved a mean (±s.e.) 6.3 ± 0.6% reduction in BMI, compared to a significantly (P = 0.01) smaller 3.8 ± 0.8% for CD participants (N = 37)...Use of MR significantly improved short-term weight loss, compared with CD, but its continued use did not improve maintenance of lost weight.
    Obesity (Silver Spring), 2011 · checked 2026-09-24
15. sfr5-05 · meta-analysis of randomized controlled trials
    A total of 17 RCTs involving 2112 participants were ultimately included. Compared with CDs, MR significantly reduced glycated hemoglobin A1c (HbA1c) (MD -0.46%; P < .001), fasting blood glucose (FBG, -0.62 mmol/L; P < .001), body weight (-2.43 kg; P < .001)
    J Clin Endocrinol Metab, 2023 · checked 2026-09-24
16. hf2-sfr-6 · systematic review and meta-analysis
    Meta-analysis of 20 studies indicated a mean 10.1 kg (95% confidence interval [CI], 8.7-11.4 kg, P < 0.001; I2 = 92.3%) weight loss following interventions lasting 3 to 20 weeks. ... Details of adverse events were limited. VLED programs are effective for treating children and adolescents with obesity. However, conclusions on their safety cannot be drawn from the existing literature at this time.
    Obes Rev, 2019 · checked 2026-10-08
17. sfr5-10 · systematic review of randomized controlled trials
    We found limited evidence to evaluate adherence or harms for all programs and weight outcomes for other commercial programs.
    Ann Intern Med, 2015 · checked 2026-09-24