Losing weight is hard because the human body defends its fat stores. When you cut calories, hunger signals rise and the body works to conserve energy. That response protected our ancestors from starvation, and today it works against many people living with obesity.
For most of human history, the main threat to survival was too little food. Bodies that held on to energy had an edge when winters were long and harvests failed. Today, in many countries, the bigger health threat is too much food, and our biology hasn’t caught up. We are built for scarcity but live surrounded by abundance.
This article covers the famous “thrifty gene” idea (and why scientists now debate it), the dangerous history of diet drugs from DNP to Fen-Phen, and how modern GLP-1 medicines such as Ozempic, Wegovy and Mounjaro work differently.
Why is losing weight so hard? The thrifty gene hypothesis
The idea is usually traced to geneticist James Neel, who proposed the “thrifty genotype” hypothesis in a 1962 paper. Neel was originally trying to explain diabetes: why would genes that raise the risk of a harmful disease stay common? His answer was that such genes once helped our ancestors survive cycles of feast and famine, and only became harmful when food turned plentiful. The idea was later extended to obesity.
In a hunter-gatherer world, food was unpredictable. People whose bodies stored surplus calories efficiently as fat, and then held on to that energy, would have been more likely to survive lean periods.
Whatever its evolutionary origin, the body’s reaction to weight loss is well documented. Researchers describe compensatory biological changes that make long-term weight loss hard to maintain. Appetite hormones such as ghrelin tend to rise, and the body tends to burn less energy than before. To your body, a deliberate diet looks like a food shortage. It doesn’t know the supermarket is down the street.
Is the thrifty gene hypothesis actually true?
It is influential, but contested. Neel himself later revised and broadened the idea. Critics such as obesity researcher John Speakman argue that if fat-storing genes were so advantageous, they would have spread to everyone, and they haven’t. He lays out this case, along with an alternative “drifty gene” explanation based on random genetic drift, in a 2008 paper. A 2016 genetic analysis found signs of positive selection in only 9 of 115 genes linked to obesity, which the authors read as evidence against the hypothesis.
Other explanations include the “thrifty phenotype” idea, in which conditions early in life shape later metabolism. The takeaway: researchers still argue about why our biology evolved this way, but the evidence that the body resists weight loss does not depend on winning that argument.
A short history of dangerous diet pills
Because the body resists weight loss, the search for a shortcut has a long and often deadly history.
| Era | Drug | How it worked | What went wrong |
|---|---|---|---|
| 1930s | DNP (2,4-dinitrophenol) | Made cells release energy as heat instead of storing it | Dangerous overheating; deemed unfit for human consumption in 1938 |
| 1950s–60s | Amphetamines and “rainbow pills” | Suppressed appetite; rainbow pills added diuretics and thyroid hormone | Addiction, heart problems, psychosis and deaths |
| 1990s | Fen-Phen (fenfluramine + phentermine) | Suppressed appetite | Heart valve disease and pulmonary hypertension; fenfluramine and dexfenfluramine withdrawn in 1997 |
| 2020s | GLP-1 drugs (semaglutide, tirzepatide) | Amplify fullness signals and slow stomach emptying | Digestive side effects, cost, and weight regain after stopping |
The 1930s: DNP, burning from the inside out
The first widely used weight-loss drug was 2,4-dinitrophenol (DNP), an industrial chemical used in munitions manufacturing. Factory workers began losing weight rapidly, and doctors started prescribing it.
DNP works by uncoupling the body’s energy production, releasing the energy from food as heat instead of storing it. It was effective, but it could cause fatal overheating (hyperthermia). By 1938, DNP was deemed unfit for human consumption.
DNP is still sold illegally online as a “fat burner,” and it still kills people. Public Health England and the UK Food Standards Agency strongly advise against taking it, noting that severe harm can occur even at the doses suppliers recommend. Anyone who has taken DNP needs urgent medical attention.
The 1950s and 1960s: the amphetamine era
By the mid-20th century, doctors turned to stimulants. Amphetamines (commonly called speed) were widely prescribed to suppress appetite and boost energy, often marketed to women. They worked temporarily, but they were highly addictive and caused serious heart problems and psychosis.
This era also produced “rainbow pills,” cocktails of amphetamines, diuretics and thyroid hormone that were linked to numerous deaths.
The 1990s: the Fen-Phen disaster
In the 1990s, the “miracle cure” was a combination of fenfluramine and phentermine, known as Fen-Phen. Then came reports of heart valve disease and pulmonary hypertension. On September 15, 1997, the U.S. FDA asked manufacturers to withdraw fenfluramine and its close chemical relative dexfenfluramine, citing an “unacceptable risk” to patients.
Phentermine on its own was not implicated in the valve problems and is still prescribed today. The withdrawal remains one of the most high-profile in modern drug history.
How GLP-1 drugs work: turning up the “I’m full” signal
For a long time, the focus was on burning more energy. The breakthrough came from a different direction: working with the signals that regulate appetite.
The story starts with an unlikely source. In the early 1990s, endocrinologist John Eng isolated a peptide called exendin-4 from Gila monster venom. It closely resembled the human hormone GLP-1 but lasted far longer in the body. A synthetic version, exenatide (Byetta), was approved for type 2 diabetes in 2005 and became the first GLP-1 drug. Newer GLP-1 medicines were later developed for diabetes and for weight management.
GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It helps regulate blood sugar, acts on appetite circuits in the brain (directly and indirectly) to promote a feeling of fullness, and slows how quickly the stomach empties. Natural GLP-1 is broken down rapidly, so the medicines are engineered to last much longer, which is why semaglutide can be taken once a week.
Semaglutide (sold as Ozempic and Wegovy) is a GLP-1 receptor agonist, meaning it binds to GLP-1 receptors and switches them on. Tirzepatide (Mounjaro and Zepbound) activates both GLP-1 and GIP receptors. Researchers are still working out how much natural GLP-1 signaling differs in people with obesity, and the evidence is mixed. What is clear is that the drugs strongly reduce appetite. Many patients describe the “food noise,” the constant mental chatter about what to eat next, going quiet.
What the evidence shows
- Semaglutide: In the STEP 1 trial, 1,961 adults with overweight or obesity (without diabetes) took semaglutide 2.4 mg or placebo, plus lifestyle support. After 68 weeks, average weight loss was 14.9% with semaglutide versus 2.4% with placebo.
- Tirzepatide: In the SURMOUNT-1 trial, 2,539 adults took tirzepatide or placebo for 72 weeks. Average weight loss was 15.0%, 19.5% and 20.9% at the 5 mg, 10 mg and 15 mg doses, versus 3.1% with placebo.
These are averages, and individual results vary. Both trials were funded by the drug makers (Novo Nordisk and Eli Lilly), which is standard for this stage of drug development but worth knowing.
Limits and risks of GLP-1 drugs
- Side effects: The most common are digestive (such as nausea and diarrhea) and occur mainly while the dose is being increased. In STEP 1, 4.5% of people on semaglutide stopped treatment because of gastrointestinal events, versus 0.8% on placebo. Product labels list additional warnings, so these medicines need a clinician’s oversight.
- Weight regain after stopping: One year after treatment ended, participants in a STEP 1 follow-up study had regained about two-thirds of their lost weight, and most health improvements drifted back toward baseline.
- Cost and access: Prices, insurance coverage and availability vary widely by country and provider.
- Open questions: Long-term data beyond a few years is still accumulating.
Why obesity is biology, not a willpower failure
The success of these drugs adds to a growing body of evidence that body weight is governed by powerful biological systems, not just character or discipline. The researchers behind the STEP 1 follow-up said their results confirm the chronic nature of obesity and suggest that ongoing treatment is needed to keep the benefits.
That doesn’t make lifestyle irrelevant. The trials combined medication with lifestyle support. But it changes how we should talk about weight: as a condition with biological drivers, not a moral failing.
We have moved from industrial chemicals that forced the body into thinness to medicines that work with its hormonal signals. They are not a cure, and important questions remain, but our understanding of the biology has finally begun to catch up.
What this means for you
If you are struggling with weight, talk to a qualified healthcare professional about the options that fit your health, goals and budget. Want more explainers like this? Get The Morning Lodge brief or browse our GLP-1 stories.
This article is for general information and is not medical advice. Talk to a qualified healthcare professional before starting, stopping or changing any medication or weight-loss plan.
Sources
- Neel JV. Diabetes mellitus: a “thrifty” genotype rendered detrimental by “progress”? American Journal of Human Genetics, 1962;14(4):353–362.
- Neel JV. The “thrifty genotype” in 1998. Nutrition Reviews, 1999;57(5):S2–S9.
- Speakman JR. Thrifty genes for obesity, an attractive but flawed idea, and an alternative perspective: the ‘drifty gene’ hypothesis. International Journal of Obesity, 2008;32:1611–1617.
- Wang G, Speakman JR. Analysis of positive selection at single nucleotide polymorphisms associated with body mass index does not support the “thrifty gene” hypothesis. Cell Metabolism, 2016.
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine, 2021;384:989–1002.
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 2022;387:205–216.
- Wilding JPH, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes, Obesity and Metabolism, 2022;24(8):1553–1564.
- U.S. Food and Drug Administration. FDA announces withdrawal of fenfluramine and dexfenfluramine (Fen-Phen). September 15, 1997 (archived).
- Elliott WT, Chan J. Fenfluramine and dexfenfluramine withdrawn from market. Clinician Reviews, November 1997.
- Public Health England. PHE supports FSA warnings over deadly weight loss supplement. October 2013.
- UNC Endeavors. The Gila monster, exendin-4 and the development of exenatide. Fall 2007.
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